Synthesis and Structure of the Double-Layered Sillén-Aurivillius Perovskite Oxychloride La2.1Bi2.9Ti2O11Cl as a Potential Photocatalyst for Stable Visible Light Solar Water Splitting

Exploring photocatalysts for solar water splitting is a relevant step toward sustainable hydrogen production. Sillén-Aurivillius-type compounds have proven to be a promising material class for photocatalytic and photoelectrochemical water splitting with the advantage of visible light activity coupled to enhanced stability because of their unique electronic structure. Especially, double- and multilayered Sillén-Aurivillius compounds [A_n-1B_nO_3n+1][Bi₂O₂]₂X_m, with A and B being cations and X a halogen anion, offer a great variety in material composition and properties. Yet, research in this field is limited to only a few compounds, all of them containing mainly Ta⁵⁺ or Nb⁵⁺ as cations. This work takes advantage of the outstanding properties of Ti⁴⁺ demonstrated in the context of photocatalytic water splitting. A fully titanium-based oxychloride, La_2.1Bi_2.9Ti₂O₁₁Cl, with a double-layered Sillén-Aurivillius intergrowth structure is fabricated via a facile one-step solid-state synthesis. A detailed crystal structure analysis is performed via powder X-ray diffraction and correlated to density functional theory calculations, providing a detailed understanding of the site occupancies in the unit cell. The chemical composition and the morphology are studied using scanning and transmission electron microscopy together with energy-dispersive X-ray analysis. The capability of the compound to absorb visible light is demonstrated by UV-vis spectroscopy and analyzed by electronic structure calculations. The activity toward the hydrogen and the oxygen evolution reaction is evaluated by measuring anodic and cathodic photocurrent densities, oxygen evolution rates, and incident-current-to-photon efficiencies. Thanks to the incorporation of Ti⁴⁺, this Sillén-Aurivillius-type compound enables best-in-class photoelectrochemical water splitting performance at the oxygen evolution side under visible light irradiation. Thus, this work highlights the potential of Ti-containing Sillén-Aurivillius-type compounds as stable photocatalysts for visible light-driven solar water splitting.

Substrate-Enabled Room-Temperature Electrochemical Deposition of Crystalline ZnMnO3

Mixed transition metal oxides have emerged as promising electrode materials for electrochemical energy storage and conversion. To optimize the functional electrode properties, synthesis approaches allowing for a systematic tailoring of the materials' composition, crystal structure and morphology are urgently needed. Here we report on the room-temperature electrodeposition of a ternary oxide based on earth-abundant metals, specifically, the defective cubic spinel ZnMnO₃ . In this unprecedented approach, ZnO surfaces act as (i) electron source for the interfacial reduction of MnO₄ ^- in aqueous solution, (ii) as substrate for epitaxial growth of the deposit and (iii) as Zn precursor for the formation of ZnMnO₃ . Epitaxial growth of ZnMnO₃ on the lateral facets of ZnO nanowires assures effective electronic communication between the electroactive material and the conducting scaffold and gives rise to a pronounced 2-dimensional morphology of the electrodeposit forming - after partial delamination from the substrate - twisted nanosheets. The synthesis strategy shows promise for the direct growth of different mixed transition metal oxides as electroactive phase onto conductive substrates and thus for the fabrication of binder-free nanocomposite electrodes.

Resolving the structure of V3O7·H2O and Mo-substituted V3O7·H2O

Vanadate compounds, such as V₃O₇·H₂O, are of high interest due to their versatile applications as electrode material for metal-ion batteries. In particular, V₃O₇·H₂O can insert different ions such as Li⁺, Na⁺, K⁺, Mg²⁺ and Zn²⁺. In that case, well resolved crystal structure data, such as crystal unit-cell parameters and atom positions, are needed in order to determine the structural information of the inserted ions in the V₃O₇·H₂O structure. In this work, fundamental crystallographic parameters, i.e. atomic displacement parameters, are determined for the atoms in the V₃O₇·H₂O structure. Furthermore, vanadium ions were substituted by molybdenum in the V₃O₇·H₂O structure [(V_2.85Mo_0.15)O₇·H₂O] and the crystallographic positions of the molybdenum ions and their oxidation state are elucidated.

Nanoscale transformations of amphiboles within human alveolar epithelial cells

Amphibole asbestos is related to lung fibrosis and several types of lung tumors. The disease-triggering mechanisms still challenge our diagnostic capabilities and are still far from being fully understood. The literature focuses primarily on the role and formation of asbestos bodies in lung tissues, but there is a distinct lack of studies on amphibole particles that have been internalized by alveolar epithelial cells (AECs). These internalized particles may directly interact with the cell nucleus and the organelles, exerting a synergistic action with asbestos bodies (AB) from a different location. Here we document the near-atomic- to nano-scale transformations induced by, and taking place within, AECs of three distinct amphiboles (anthophyllite, grunerite, “amosite”) with different Fe-content and morphologic features. We show that: (i) an Fe-rich layer is formed on the internalized particles, (ii) particle grain boundaries are transformed abiotically by the internal chemical environment of AECs and/or by a biologically induced mineralization mechanism, (iii) the Fe-rich material produced on the particle surface does not contain large amounts of P, in stark contrast to extracellular ABs, and (iv) the iron in the Fe-rich layer is derived from the particle itself. Internalized particles and ABs follow two distinct formation mechanisms reaching different physicochemical end-states.

Deep hydration of an Li7-3xLa3Zr2MIIIxO12 solid-state electrolyte material: a case study on Al- and Ga-stabilized LLZO

Single crystals of an Li-stuffed, Al- and Ga-stabilized garnet-type solid-state electrolyte material, Li7La3Zr2O12 (LLZO), have been analysed using single-crystal X-ray diffraction to determine the pristine structural state immediately after synthesis via ceramic sintering techniques. Hydrothermal treatment at 150 °C for 28 d induces a phase transition in the Al-stabilized compound from the commonly observed cubic Ia-3d structure to the acentric I-43d subtype. LiI ions at the interstitial octahedrally (4 + 2-fold) coordinated 48e site are most easily extracted and AlIII ions order onto the tetrahedral 12a site. Deep hydration induces a distinct depletion of LiI at this site, while the second tetrahedral site, 12b, suffers only minor LiI loss. Charge balance is maintained by the incorporation of HI, which is bonded to an O atom. Hydration of Ga-stabilized LLZO induces similar effects, with complete depletion of LiI at the 48e site. The LiI/HI exchange not only leads to a distinct increase in the unit-cell size, but also alters some bonding topology, which is discussed here.

Modified H2 V3 O8 to Enhance the Electrochemical Performance for Li-ion Insertion: The Influence of Prelithiation and Mo-Substitution

Nanostructured H₂ V₃ O₈ is a promising high-capacity cathode material, suitable not only for Li⁺ but also for Na+, Mg²⁺ , and Zn²⁺ insertion. However, the full theoretical capacity for Li⁺ insertion has not been demonstrated experimentally so far. In addition, improvement of cycling stability is desirable. Modifications like substitution or prelithiation are possibilities to enhance the electrochemical performance of electrode materials. Here, for the first time, the substitution of vanadium sites in H₂ V₃ O₈ with molybdenum was achieved while preserving the nanostructure by combining a soft chemical synthesis approach with a hydrothermal process. The obtained Mo-substituted vanadate nanofibers were further modified by prelithiation. While pristine H₂ V₃ O₈ showed an initial capacity of 223 mAh g^-1 and a retention of 79 % over 30 cycles, combining Mo substitution and prelithiation led to a superior initial capacity of 312 mAh g^-1 and a capacity retention of 94 % after 30 cycles.

Multi-scale characterization of glaucophane from Chiavolino (Biella, Italy): implications for international regulations on elongate mineral particles

In this paper, we present the results of a multi-analytical characterization of a glaucophane sample collected in the Piedmont region of northwestern Italy. Investigation methods included optical microscopy, powder X-ray diffraction, Fourier-transform infrared spectroscopy, μ-Raman spectroscopy, Mössbauer spectroscopy, electron probe microanalysis, environmental scanning electron microscopy and energy-dispersive X-ray spectroscopy, and scanning/transmission electron microscopy combined with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. In addition to the crystal-chemical characterization of the sample from the mesoscale to the near-atomic scale, we have also conducted an extended study on the morphology and dimensions of the mineral particles. The main finding is that studying the same particle population at different magnifications yields different results for mineral habit, dimensions, and dimensional distributions. As glaucophane may occur as an elongate mineral particle (e.g., asbestiform glaucophane occurrences in California and Nevada), the observed discrepancies therefore need to be considered when assessing potential breathability of such particles, with implications for future regulations on elongate mineral particles. While the sample preparation and particle counting methods are not directly investigated in this work, our findings suggest that different magnifications should be used when characterizing an elongate mineral particle population, irrespective of whether or not it contains asbestiform material. These results further reveal the need for developing improved regulation for elongate mineral particles. We thus propose a simple methodology to merge the datasets collected at different magnifications to provide a more complete description and a better risk evaluation of the studied particle population.

Single-crystal neutron and X-ray diffraction study of garnet-type solid-state electrolyte Li6La3ZrTaO12: an in situ temperature-dependence investigation (2.5 ≤ T ≤ 873 K)

Single-crystal neutron and X-ray diffraction methods were used to evaluate the exact crystal structure, Li site occupation and diffusion path of nominal Li₆La₃ZrTaO₁₂ at six temperatures between 2.5 K and 873 K.

Large single crystals of garnet-type Li₆La₃ZrTaO₁₂ (LLZTO) were grown by the Czochralski method and analysed using neutron diffraction between 2.5 and 873 K in order to fully characterize the Li atom distribution, and possible Li ion mobility in this class of potential candidates for solid-state electrolyte battery material. LLZTO retains its cubic symmetry (space group Ia3d) over the complete temperature range. When compared to other sites, the octahedral sites behave as the most rigid unit and show the smallest increase in atomic displacement parameters and bond length. The La and Li sites show similar thermal expansion in their bond lengths with temperature, and the anisotropic and equivalent atomic displacement parameters exhibit a distinctly larger increase at temperatures above 400 K. Detailed inspection of nuclear densities at the Li1 site reveal a small but significant displacement from the 24d position to the typical 96h position, which cannot, however, be resolved from the single-crystal X-ray diffraction data. The site occupation of Li^I ions on Li1 and Li2 sites remains constant, so there is no change in site occupation with temperature.

Wet-Environment-Induced Structural Alterations in Single- and Polycrystalline LLZTO Solid Electrolytes Studied by Diffraction Techniques

Li₇La₃Zr₂O₁₂ (LLZO) is one of the potential candidates for Li metal-based solid-state batteries owing to its high Li⁺ conductivity (≈10^-3 S cm^-1) at room temperature and large electrochemical stability window. However, LLZO undergoes protonation under the influence of moisture-forming Li₂CO₃ layers, thereby affecting its structural and transport properties. Therefore, a detailed understanding on the impact of the exchange of H⁺ on Li⁺ sites on structural alteration and kinetics under the influence of wet environments is of great importance. The present study focuses on the Li⁺/H⁺ exchange in single-crystal and polycrystal Li₆La₃ZrTaO₁₂ (LLZTO) garnets prepared using the Czochralski method and solid-state reactions subjected to weathering in air, aqueous solutions at room temperature, and in aqueous solution at 363 K using X-ray diffraction (XRD) and neutron diffraction (ND) techniques. Based on 36 single-crystal diffraction and 88 powder diffraction measurements, we found that LLZTO crystallizes with space group (SG) Ia3̅d with Li located in 96h (Li(2)) and 24d (Li(1)) sites, whereas the latter one is displaced toward the general position 96h forming shorter Li(1)-Li(2) jump distances. The degradation in air, wet air, water, and acetic acid leads to a Li⁺/H⁺ exchange that preferably takes place at the 24d site, which is in contrast to previous reports. Higher Li⁺/H⁺ was observed for LLZTO aged in water at 363 K that reduced the symmetry to SG I4̅3d from SG Ia3̅d. This symmetry reduction was found to be related to the site occupation behavior of Li at the tetrahedral 12a site in SG I4̅3d. Moreover, Li⁺ is exchanged by H⁺ preferably at the 48e site (equivalent to 96h site). We also found that the equilibrium H⁺ concentrations in all media tested remains very similar, which is related to the H⁺ diffusion in the LLZTO-controlled exchange process. Only the increase in temperature led to a significant increase in the exchange capacity as well as in the Li⁺/H⁺ exchange rate. Overall, we found that the exchange rate, exchange capacity, site occupation behavior of Li⁺ and H⁺, as well as the structural stability of LLZTO, strongly depend on the composition. These findings suggest that measurements on a single LLZTO variant sample do not lead to a general conclusion for all garnets to guide the field toward better materials. In contrast, each composition has to be analyzed exclusively to understand the interplay of composition, structure, and exchange kinetic properties.

P21/c Postorthopyroxene γ-LiScGe2O6, a New Dense High-Pressure Polymorph and Its Direct Transformation from the Pbca Structure

Orthorhombic β-LiScGe₂O₆ single crystals were compressed hydrostatically up to 10.35 GPa using a diamond anvil cell and investigated in situ by means of X-ray diffraction and Raman spectroscopy. Crystal-structure investigations at ambient conditions and at high pressure show a structural transition from an orthopyroxene-type Pbca structure (a ≈ 18.43 Å, b ≈ 8.85 Å, and c ≈ 5.34 Å at 8.6 ± 0.1 GPa) to a postorthopyroxene type P2₁/c structure of the new dense γ-LiScGe₂O₆ (a ≈ 18.62 Å, b ≈ 8.85 Å, c ≈ 5.20 Å, and β ≈ 93.1° at 9.5 ± 0.1 GPa). The structure refinements reveal displacive shifts of O atoms associated with a rotation of every other tetrahedral-chain unit from the O- to S-type position similar to the postorthopyroxene-type MgSiO₃. As a consequence of the oxygen displacement, the coordination number of Li atoms is changing from [5 + 1] to a proper 6-fold coordination. The transition around P_c = 9.0 ± 0.1 GPa is associated with a volume discontinuity of ΔV = −1.6%. This orthopyroxene (OEn-Pbca) to postorthopyroxene (pOEn-P2₁/c) transition is the second example of this type of transformation. Precise lattice parameters have been determined during isothermal compression. The fit of the unit-cell volumes of β-LiScGe₂O₆, using a third-order Birch–Murnaghan equation of state, yields V₀ = 943.63 ± 0.11 ų, K₀ = 89.8 ± 0.6 GPa, and dK/dP = 4.75 ± 0.18 as parameters. Evaluation of the data points beyond the critical transition pressure using a second-order Birch–Murnaghan equation suggests V₀ = 940.6 ± 4.4 ų and K₀ = 82.4 ± 4.8 GPa. A series of high-pressure Raman spectra confirm the symmetry-related structural transition, with band positions shifting in a noncontinuous manner, thus confirming the proposed first-order transition.

Synthetic lithium−scandium germanate crystals (space group Pbca at ambient conditions) were the subject of in situ high-pressure solid-state investigations (single-crystal X-ray diffraction, lattice parameter, and Raman spectra) in a diamond anvil cell up to 10 GPa. One phase transition, apparently of first order, is identified at 9.0 ± 0.1 GPa. The Birch−Murnaghan equations of state were fitted to the P−V data, and the obtained parameters are given. The transition is of the type orthoenstatite (Pbca) to postorthoenstatite (P2₁/c).

The crystal structure of KScP2O7

Single crystals of KScP2O7, potassium scandium diphosphate, were grown in a borate flux. The title compound crystallizes isotypically with KAlP2O7 in space-group type P21/c, Z = 4. The main building block is an {ScP2O11}9- unit, forming layers parallel to (001). These layers are stacked along [001] via common corners of octa-hedral and tetra-hedral units to span up large hepta-gonal cavities that host the potassium cations with a coordination number of 10. The P-O-P bridging angle increases with increasing size of the octa-hedrally coordinated M III cation, as do the K-O distances within a series of KM IIIP2O7 compounds (M III = Al to Y with ionic radii r = 0.538 to 0.90 Å).

Structural and Raman spectroscopic characterization of pyroxene-type compounds in the CaCu1-xZnxGe2O6 solid-solution series

Pyroxene-type germanate compounds with the composition CaCuGe₂O₆-CaZnGe₂O₆ have been synthesized via a solid-state ceramic sintering route. Phase-pure polycrystalline and small single-crystal material was obtained all over the series, representing a complete solid-solution series. Differential thermal analysis, single-crystal X-ray diffraction and Raman spectroscopy were used to characterize phase stability, phase changes and structural alterations induced by the substitution of Cu²⁺ with Zn²⁺. Whereas pure CaCuGe₂O₆ exhibits P2₁/c symmetry with a strong distortion of the M1 octahedra and two different Ge sites, one of them with an unusual fivefold coordination, the replacement of Cu²⁺ by Zn²⁺ induces a chemically driven phase change to the C2/c symmetry. The phase change takes place around Zn²⁺ contents of 0.12 formula units and is associated with large changes in the unit-cell parameters. Here, the increase of c by as much as 3.2% is remarkable and it is mainly controlled by an expansion of the tetrahedral chains. Further differences between the P2₁/c and C2/c structures are a more regular chain of edge-sharing M1 octahedra as a consequence of more and more reduced Jahn-Teller distortion and a less kinked, symmetry-equivalent tetrahedral chain. The coordination of the Ca site increases from sevenfold to eightfold with large changes in the Ca-O bond lengths during the phase change. Raman spectroscopy was mainly used to monitor the P2₁/c to C2/c phase change as a function of composition, but also as a function of temperature and to follow changes in specific Raman modes throughout the solid-solution series.

Synthesis, Crystal Structure, and Stability of Cubic Li7-xLa3Zr2-xBixO12

Li oxide garnets are among the most promising candidates for solid-state electrolytes in novel Li ion and Li metal based battery concepts. Cubic Li₇La₃Zr₂O₁₂ stabilized by a partial substitution of Zr⁴⁺ by Bi⁵⁺ has not been the focus of research yet, despite the fact that Bi⁵⁺ would be a cost-effective alternative to other stabilizing cations such as Nb⁵⁺ and Ta⁵⁺. In this study, Li_7–xLa₃Zr_2–xBi_xO₁₂ (x = 0.10, 0.20, ..., 1.00) was prepared by a low-temperature solid-state synthesis route. The samples have been characterized by a rich portfolio of techniques, including scanning electron microscopy, X-ray powder diffraction, neutron powder diffraction, Raman spectroscopy, and ⁷Li NMR spectroscopy. Pure-phase cubic garnet samples were obtained for x ≥ 0.20. The introduction of Bi⁵⁺ leads to an increase in the unit-cell parameters. Samples are sensitive to air, which causes the formation of LiOH and Li₂CO₃ and the protonation of the garnet phase, leading to a further increase in the unit-cell parameters. The incorporation of Bi⁵⁺ on the octahedral 16a site was confirmed by Raman spectroscopy. ⁷Li NMR spectroscopy shows that fast Li ion dynamics are only observed for samples with high Bi⁵⁺ contents.

The cubic modification of Li₇La₃Zr₂O₁₂ can be stabilized by a by a partial substitution of Zr⁴⁺ by Bi⁵⁺. The incorporation of Bi⁵⁺ leads to an increase in the unit-cell parameters. Samples prepared by a low-temperature preparation route are sensitive to CO₂ and H₂O from air, causing a protonation of the garnet phase. ⁷Li NMR spectroscopy shows that fast translational Li ion dynamics are only observed for samples with high Bi⁵⁺ contents.

Crystal structure of spinel-type Li0.64Fe2.15Ge0.21O4

Spinel-type Li0.64Fe2.15Ge0.21O4, lithium diiron(III) germanium tetra-oxide, has been formed as a by-product during flux growth of an Li-Fe-Ge pyroxene-type material. In the title compound, lithium is ordered on the octa-hedral B sites, while Ge(4+) orders onto the tetra-hedral A sites, and iron distributes over both the octa-hedral and tetra-hedral sites, and is in the trivalent state as determined from Mössbauer spectroscopy. The oxygen parameter u is 0.2543; thus, the spinel is close to having an ideal cubic closed packing of the O atoms. The title spinel is compared with other Li- and Ge-containing spinels.

Ca3ZnGeO2[Ge4O12]: a Ca-Zn germanate related to the mineral taikanite

The title compound, Ca3ZnGeO2[Ge4O12] (tricalcium zinc germanium dioxide dodecaoxidotetragermanate), adopts a taikanite-type structure. The tetrahedral [Ge4O12] chain geometry is very similar to that of the silicate chain of taikanite, i.e. BaSr2Mn(3+)2O2[Si4O12], while the major difference is found parallel to the c axis. In taikanite, Mn(3+) octahedra form an infinite zigzag chain, whereas the title compound has a chain of distorted ZnO6 octahedra, which alternates with distorted GeO4 tetrahedra connected to each other via two common edges. Eightfold-coordinated Ca(2+) polyhedra and ZnO6 octahedra form a slab parallel to (001) which alternates with another slab containing the tetrahedrally coordinated Ge sites along the c axis.

Thin water films and magnesium hydroxide fiber growth

On oxide nanostructures thin water films act as reactant and provide a reaction medium for hydroxide fiber growth.

Pyroxene-type compounds NaM3+Ge2O6, with M = Ga, Mn, Sc and In

The four title compounds, namely sodium gallium germanate, NaGaGe2O6, sodium manganese vanadate germanate, NaMnV0.1Ge1.9O6, sodium scandium germanate, NaScGe2O6, and sodium indium germanate, NaInGe2O6, adopt the high-temperature structure of the pyroxene-type chain germanates, with monoclinic symmetry and space group C2/c. The lattice parameters, the individual and average bond lengths involving M1, and the distortion parameters scale well with the ionic radius of the M1 cation. NaGaGe2O6 has more distorted M1 sites and more extended tetrahedral chains than NaInGe2O6, in which a high degree of kinking is required to maintain the connection between the octahedral and tetrahedral building units of the pyroxene structure. An exceptional case is NaMnGe2O6, in which the strong Jahn-Teller effect of Mn(3+) results in more distorted octahedral sites than expected according to linear extrapolation from the other NaM(3+)Ge2O6 pyroxenes. In contrast with the literature, minor incorporations of V(5+) in the tetrahedral site and a corresponding reduction of Mn(3+) to Mn(2+) in the octahedral sites in the present sample lower the Jahn-Teller distortion and stabilize the Mn-bearing pyroxene, even allowing its synthesis at ambient pressure.

The polar phase of Li2Ge4O9at 298, 150 and 90 K

Dilithium tetragermanate is orthorhombic, space groupP21ca, at 298 K, and is thus in a polar and probably a ferroelectric state. The structure contains two independent Li, four Ge and nine O atoms, all on general positions with site symmetry 1. Three tetrahedrally coordinated Ge positions form crumpled crankshaft-like chains, forming sheets within theacplane, and these are interconnected by the fourth, octahedrally coordinated, Ge sites along thebdirection. The GeO4tetrahedra and GeO6octahedra form a three-dimensional framework containing two different cavities, hosting the two 4+1-coordinated Li sites. Cooling to 90 K does not alter the space-group symmetry; the tetrahedral chains behave as a rigid unit and changes occur mainly within the Li coordination spheres.

The tetragermanatesA2Ge4O9(A= Na, K and Rb)

The crystal structures of three alkali tetragermanatesA2Ge4O9(A= Na, K and Rb) [namely disodium tetragermanate, Na2Ge4O9, dipotassium tetragermanate, K2Ge4O9, and dirubidium tetragermanate, Rb2Ge4O9] are trigonal (space groupP\overline{3}c1). The main building units are a three-membered ring of tetrahedra, oriented within the (001) plane and forming tetrahedral sheets. These sheets are connected to each other by two different regular isolated GeO6octahedraviacorner-sharing to build up a tetrahedral–octahedral framework. The alkali cations are located in cavities within this framework and are sevenfold coordinated. The increasing size of theA-site cation is accommodated by twist deformations of the tetrahedral rings and alterations in the Ge—O—Ge angles. With increasing size of theA-site cation, both the tetrahedral and octahedral sites become more regular, with slightly decreasing 〈Ge—O〉 distances from Na2Ge4O9to Rb2Ge4O9. This goes hand-in-hand with a more uniform distribution of bonds around theA-site cation. All these observations make Rb2Ge4O9the most regular member of thisA2Ge4O9octahedral–tetrahedral framework structure series.

Magnetic spin structure of pyroxene-type MnGeO(3)

Polycrystalline MnGeO(3) material was synthesized at 1473 K and ambient pressures using ceramic sintering techniques. Under these conditions the pyroxene-type compound crystallizes in the orthorhombic modification with space group Pbca, as determined from the refinement of the neutron diffraction data. The monoclinic modification, space group C 2/c, was also present at a level of 8.8(4) wt% and the magnetic structures for the two polymorphs at low temperatures have been found simultaneously. The monoclinic form orders magnetically below 34 K; the spin structure can be described using k = (0, 0, 0) in the magnetic space group C 2'/c, having an antiferromagnetic spin arrangement within and between the chains of M1 and M2 sites. The orthorhombic phase of MnGeO(3) transforms to a magnetically ordered state with k = (0, 0, 0) and magnetic space group Pb'c'a below 12 K. Spins on M1 sites are aligned along the crystallographic a-axis with a slight non-collinear antiferromagnetic spin arrangement with and between the M1 chains. Spins on M2 sites are also antiferromagnetically coupled; however, one of the three different M1-M2 superexchange pathways within the band of M1 and M2 sites displays a ferromagnetic interaction, while the other two allow antiferromagnetic interaction.

Structural variation and crystal chemistry of LiMe3+Si2O6 clinopyroxenes Me3+ = Al, Ga, Cr, V, Fe, Sc and In

A total of 32 synthetic end-member and solid-solution compounds of the LiM3+Si2O6 (Li = M2 site, M3+ = M1 site = Al, Ga, V, Fe, Sc and In) clinopyroxene series have been investigated by single-crystal X-ray diffraction. Except LiCrSi2O6, all compounds show C2/c symmetry at 295 K. LiCrSi2O6 has space group P21/c but transforms to the high temperature C2/c structure at 335 K. The variations of structural parameters in the LiMe3+Si2O6 clinopyroxenes are dominated by the Me3+ site. The average M1—O bond length is linearly correlated with the ionic radius of the M1 cation. Octahedra reflect the increasing size of the M1 cations by steadily increasing bond and edge lengths, the variations however are not uniform. With increasing size of the M1 cation, octahedra deviate from ideal octahedral geometry. Octahedral edges, shared with other structural units, are distinctly hampered in expansion with increasing size of the M1 cation. The increasing size of the M1 octahedral chain is compensated by changing the kinking of the tetrahedral chain and by alterations of bond and edge lengths as well as the bond angles within the tetrahedron. Three different mechanisms of adapting of the structural building units with increasing M3+ cationic radius can be identified: (i) expansion of the tetrahedral chain by stretching (ii) transition form “O” to “S” chain conformation after full expansion and (iii) finally a limit of expansion in a direction. We stress that cations larger than In3+ cannot be substituted at the M1 site because of too large geometrical differences between octahedral and tetrahedral chains.

Structural changes upon the temperature dependent C2/c → P21/c phase transition in LiMe3+Si2O6 clinopyroxenes, Me = Cr, Ga, Fe, V, Sc and In

Within the Li-clinopyroxene series LiMe3+Si2O6, a temperature induced C2/c → P21/c phase transition has been observed for Me = Cr, Ga, V, and Sc at temperatures of 330(1), 286(1), 205(3) and 234(1) K, respectively. There is no phase transition for the Al3+ and the pure In3+ compound down to 80 K. Within the LiSc1– x In x Si2O6 solid solution se ries, the transition temperature rapidly decreases with increasing In3+ content and drops below 90 K between x = 0.26 and 0.30. The C → P phase transition is found only in samples with nearly fully extended tetrahedral chains. The phase transition in LiScSi2O6, with its “O-rotated” and distinctly more kinked tetrahedral chains in C2/c (O3—O3—O3 angle = 175.7(1)° at 298 K), exhibits a different character: the decay of the b-type Bragg reflections hkl: h + k ≠ 2n and the structural changes in the vicinity of the phase transition are less rapid. The temperature dependent evolution of the order parameter Q2 (as expressed by the decay of the b-type Bragg reflections) suggests a second order thermodynamic character of the C → P phase transition in the Sc3+ compound, whereas it is close to a tri-critical behaviour in the other ones. The structural changes, taking place at the phase transition and below are similar to those in LiFeSi2O6, i.e. dislinkage and appearance of two differently kinked tetrahedral chains in P21/c, decrease of the coordination number of Li+ from 6 to 5, slight alterations of octahedral bond lengths. The structural changes at the phase transition are of similar magnitude in the Ga, Cr, V and Fe compound. LiScSi2O6 is again exceptional. Different thermal expansion of octahedral and tetrahedral sites and increasing site distortion of the M1 site as a consequence of tetrahedral chain kinking in C2/c are assumed to be factors inducing the C2/c → P21/c phase transition.

Chromium-based clinopyroxene-type germanates NaCrGe(2)O(6) and LiCrGe(2)O(6) at 298 K

The structure analyses of sodium chromium digermanate, NaCrGe(2)O(6), (I), and lithium chromium digermanate, LiCrGe(2)O(6), (II), provide important structural information for the clinopyroxene family, and form part of our ongoing studies on the phase transitions and magnetic properties of clinopyroxenes. (I) shows C2/c symmetry at 298 K, contains one Na, one Cr (both site symmetry 2 on special position 4e), one Ge and three O-atom positions (on general positions 8f) and displays the well known clinopyroxene topology. The basic units of the structure of (I) are infinite zigzag chains of edge-sharing Cr(3+)O(6) octahedra (M1 site), infinite chains of corner-sharing GeO(4) tetrahedra, connected to the M1 chains by common corners, and Na sites occupying interstitial space. (II) was found to have P2(1)/c symmetry at 298 K. The structure contains one Na, one Cr, two distinct Ge and six O-atom positions, all on general positions 4e. The general topology of the structure of (II) is similar to that of (I); however, the loss of the twofold symmetry makes it possible for two distinct tetrahedral chains, having different conformation states, to exist. While sodium is (6+2)-fold coordinated, lithium displays a pure sixfold coordination. Structural details are given and chemical comparison is made between silicate and germanate chromium-based clinopyroxenes.

Raman spectroscopic and visible absorption investigation of LiCrSi2O6 pyroxene under pressure

The first observation of the vibrational spectrum of the synthetic pyroxene Li-kosmochlor (LiCrSi2O6) is reported herein. The Raman and visible spectra are reported as a function of pressure. Though the pyroxene retains its P21/c symmetry, changes in the Raman spectra are observed between 6.8 and 7.7 GPa, possibly due to the formation of an additional bond between Li and O3 or some other transition that retains the mineral's P21/c space group. Splitting of the peak appearing at approximately 700 cm(-1), used to characterize the P21/c phase in other studies, is not observed. Comparison is made with the Raman spectra of LiAlSi2O6 and LiFeSi2O6 in the P21/c phase and the visible spectra of NaCrSi2O6 at high pressures.

On the crystal chemistry of olivine-type germanate compounds, Ca1 + x M 1 − x GeO4 (M 2+ = Ca, Mg, Co, Mn)

Germanate compounds, CaMGeO4 with M 2+ = Ca, Mg, Co and Mn, were synthesized as single crystals by slow cooling from the melt or by flux growth techniques. All the compositions investigated exhibit Pnma symmetry at 298 K and adopt the olivine structure. The M2 site is exclusively occupied by Ca2+, while on M1 both Ca2+ and M 2+ cations are found. The amount of Ca2+ on M1 increases with the size of the M1 cation, with the smallest amount in the Mg compound (0.1 atoms per formula unit) and the largest in the Mn compound (0.20 atoms per formula unit), while in Ca2GeO4, also with olivine structure, both sites are completely filled with Ca2+. When compared with those of Ca silicate olivine, the lattice parameters a and c are distinctly larger in the analogous germanate compounds, while b has essentially the same values, regardless of the tetrahedral cation, meaning that b is independent of the tetrahedral cation. Structural variations on the octahedrally coordinated M1 site are largely determined by the size of the M1 cation, the average M1—O bond lengths being identical in Ca silicate and Ca germanate olivine. Increasing the size of the M1 cation induces an increasing polyhedral distortion, expressed by the parameters bond-length distortion, octahedral angle variance and octahedral quadratic elongation. However, the Ca germanate olivine compounds generally have more regular octahedra than the analogous silicates. The octahedrally coordinated M2 site does not exhibit large variations in structural parameters as a consequence of the constant chemical composition; the same is valid for the tetrahedral site.

Synthetic aenigmatite analog Na2(Mn5.26Na0.74)Ge6O20: structure and crystal chemical considerations

Disodium hexamanganese(II,III) germanate is the first aenigmatite-type compound with significant amounts of manganese. Na(2)(Mn(5.26)Na(0.74))Ge(6)O(20) is triclinic and contains two different Na positions, six Ge positions and 20 O positions (all with site symmetry 1 on general position 2i of space group P1). Five out of the seven M positions are also on general position 2i, while the remaining two have site symmetry 1 (Wyckoff positions 1f and 1c). The structure can be described in terms of two different layers, A and B, stacked along the [011] direction. Layer A contains pyroxene-like chains and isolated octahedra, while layer B is built up by slabs of edge-sharing octahedra connected to one another by bands of Na polyhedra. The GeO(4) tetrahedra show slight polyhedral distortion and are among the most regular found so far in germanate compounds. The M sites of layer A are occupied by highly charged (trivalent) cations, while in layer B a central pyroxene-like zigzag chain can be identified, which contains divalent (or low-charged) cations. This applies to the aenigmatite-type compounds in general and to the title compound in particular.

Ca3GeO4Cl2 with a norbergite-like structure

The title compound, tricalcium monogermanate dichloride, is orthorhombic and consists of one distinct Ge site on special position 4c, site symmetry m, and two different Ca sites, Ca1 and Ca2, one on general position 8d, site symmetry 1, and the other on special position 4c. Two of the O atoms occupy the 4c position (symmetry m); the third O atom is situated on the general 8d position, symmetry 1, as is the one distinct Cl position. By sharing common edges, the distorted Ca1 octahedra form infinite crankshaft-like chains parallel to the b direction. Along a and c, these chains are connected to one another via common corners, thereby forming a three-dimensional framework of edge- and corner-sharing Ca1O(4)Cl(2) octahedra. Triangular prisms of Ca2O(4)Cl(2) polyhedra and GeO(4) tetrahedra fill the interstitial space within the Ca1 polyhedral framework. Relationships between the structures of the title compound and the humite-type materials norbergite (Mg(3)SiO(4)F(2)) and Mn(3)SiO(4)F(2) are discussed.

Dicalcium heptagermanate Ca2Ge7O16revised

The structure of dicalcium heptagermanate, previously described with an orthorhombic space group, has been redetermined in the tetragonal space group P(overline4)b2. It contains three Ge positions (site symmetry 1, ..2 and 2.22, respectively), one Ca position (..2) and four O atoms, all on general 8i positions (site symmetry 1). A sheet of four-membered rings of Ge tetrahedra (with Ge on the 8i position) and isolated Ge tetrahedra (Ge on the 4g position) alternate with a sheet of Ge octahedra (Ge on the 2d position) and eightfold-coordinated Ca sites along the c direction in an ABABA... sequence. The three-dimensional framework of Ge sites displays a channel-like structure, evident in a projection on to the ab plane.

Cu(Cu0.44Cr4.56)Ge2O12: a close-packed oxide with CuO4 tetrahedra

The structure of copper(I,II) pentachromium(III) germanate, Cu(Cu(0.44)Cr(4.56))Ge(2)O(12), contains one Cu position (m2m), one Ge position (m) and three Cr positions (2/m, m and 2). The close-packed structure is described in terms of slabs of edge-sharing Cr(3+)O(6) octahedra and isolated CuO(4) and GeO(4) tetrahedra. These slabs are aligned parallel to the bc plane and are separated from each other by GeO(4) tetrahedra along a. The tetrahedral coordination observed for the Cu(+)/Cu(2+) ions represents an unusual feature of the structure. The Cr-O and Cu-O bond lengths are compared with literature data.

Temperature-dependent crystal structure refinement and 57Fe Mössbauer spectroscopy of Cu2Fe2Ge4O13

The germanate compound Cu2Fe2Ge4O13, dicopper diiron germanate, was synthesized by solid-state reaction at 1403 K and ambient pressure. There is no change of space-group symmetry between 10 and 900 K. Between 40 K and room temperature the a lattice parameter shows a negative thermal expansion which can be connected to a decreasing Cu—Cu interatomic distance. Above room temperature all the lattice parameters are positively correlated with temperature. Among the structural parameters several alterations with temperature occur, which are most prominent for the distorted Fe3+ octahedral site. Besides an increase of the average bond length and of the interatomic Fe—Fe distances, distortional parameters also increase with temperature, while the average Cu—O bond length remains almost constant between 100 and 900 K, as do the average Ge—O distances. 57Fe Mössbauer spectroscopy was used to detect long-range magnetic ordering in Cu2Fe2Ge4O13. While around 100 K, which is the temperature at which a broad maximum is observed in the magnetic susceptibility, no magnetic ordering was detected in the Mössbauer spectrum, below 40 K a narrow split sextet is developed which is indicative of a three-dimensional magnetic ordering of the sample.

Ca7.96Cu0.04Ge5O18: a new calcium germanate with GeO4 and Ge3O10 units

The title compound, octacalcium copper pentagermanium octadecaoxide, represents a new intermediate phase between CaO and GeO2, and has not previously been reported in the literature. The structure consists of three different Ge sites, two of them on general 8d positions, site symmetry 1, one on special position 4d, site symmetry 2. Three of the five Ca sites occur on 8d positions, site symmtery 1, one Ca is on 4b with site symmetry -1 and one Ca is on 4c with site symmetry 2. All nine O atoms have symmetry 1 (8d position). By sharing common edges, the Ca sites form infinite bands parallel to the c axis, and these bands are interconnected by isolated GeO4 and Ge3O10 units. These (100) layers are stacked along a in an ABAB... sequence, with the B layer being inverted and displaced along b/2.

Commensurate monoclinic form of Ca2Zn0.97Co0.03Ge2O7

Cobalt-doped dicalcium zinc germanate, synthesized by slow cooling from the melt, is monoclinic and has a layered structure, which is different from the modulated melilite-type structure of Ca(2)ZnGe(2)O(7). The monoclinic form has two different Ca, one Zn and two Ge sites, and seven independent O-atom positions; all are in general position 4e of the space group P2(1)/n. The topology of the structure is described and compared with that of Ca(2)ZnGe(1.25)Si(0.75)O(7).

Manganoan rockbridgeite Fe4.32Mn0.62Zn0.06(PO4)3(OH)5: structure analysis and 57Fe Mössbauer spectroscopy

The structure of the basic iron phosphate rockbridgeite [iron manganese zinc tris(phosphate) pentahydroxide] was reinvestigated with special emphasis on the cation distribution deduced from new X-ray and 57Fe Mössbauer data. Rockbridgeite is orthorhombic, space group Cmcm, and shows three different Fe sites, one with 2/m symmetry, another with m symmetry and the third in a general position. One phosphate group has the P atom on a site with m symmetry, while the other has the P atom at a site with mm symmetry. Two Fe sites are fully occupied by ferric iron, while Mn3+ and Fe2+ are situated at a third, principally Fe, site. Structural data, bond-valence sums and polyhedral distortion parameters suggest a new interpretation of the rockbridgeite 57Fe Mössbauer spectrum.

Structure of the clinopyroxene-type compound CaCuGe2O6 between 15 and 800 K

CaCuGe2O6 shows a strongly distorted clinopyroxene-type structure with P2(1)/c symmetry at 298 K. The Cu2+ ion at the M1 site is coordinated by six O atoms forming an octahedron, which deviates significantly from ideal geometry. Individual M1 sites are connected via common edges to form an infinite zigzag chain parallel to the crystallographic c axis. The Ca2+ ion at M2 shows a sevenfold coordination. M2 sites are connected to the M1 chain via three common edges, thereby forming a metal layer within the bc plane. Besides the strong Jahn-Teller distortion of the Cu site, the structure of the title compound differs from ;normal' clinopyroxenes by a distortion of alternate layers of Ge sites. While the Ge(A) site is fourfold coordinated by O atoms, forming infinite chains of corner-sharing chains parallel to the c axis, the Ge(B) site exhibits a fivefold coordination, thereby forming a true two-dimensional layer of edge-sharing GeO5 bipyramids. Decreasing the temperature causes a magnetic phase transition at 40 K, as monitored by a broad maximum in the magnetic susceptibility and by discontinuities in the lattice parameters. Increasing the temperature causes variations in bond lengths, edge lengths and bond angles. Most prominent is the increase of one bond length of the Ge(B) site and the increase of the tetrahedral bridging angle of the Ge(A) site. At 660 K a crystallographic phase transition is observed where the symmetry changes from P2(1)/c to C2/c. The transition is accompanied by large changes in the lattice parameters which are indicative of distinct topological changes of several structural building units. The high-temperature C2/c structure is similar to that of the germanate clinopyroxene CaMgGe2O6.

A comparison of the clinopyroxene compounds CaZnSi2O6and CaZnGe2O6

Single crystals of CaZnSi2O6 (calcium zinc silicate) and CaZnGe2O6 (calcium zinc germanate) were synthesized at 1623 K and 2.5 GPa by slow cooling of the melts from 1473 K. Structure solution using Patterson methods revealed the two compounds to be isomorphous and thus isostructural. They adopt the clinopyroxene structure type with space group C2/c. The substitution of Ge4+ for Si4+ increases the distortion of the tetrahedra and octahedra. The increased size of the tetrahedral GeO4 chain is mainly compensated by (i) increasing the kinking of the tetrahedral chain and (ii) lengthening the Zn-O bonds.

The ferriannite KFe\bf{_3^{2+}}(Al0.26Fe\bf{_{0.76}^{3+}Si3)O10(OH)2at 100 and 270 K

Unusually large and good-quality single crystals of the synthetic trioctahedral mica KFe(3)(2+)(Al(0.26)Fe(0.76)(3+)Si(3))O(10)(OH)(2) [potassium triiron(II) aluminasilaferrate(III) decaoxide dihydroxide] have been grown hydrothermally. X-ray diffraction data measured at 270 and 100 K have been used to refine the crystal structure, including the positions of the H atoms. This synthetic mica is similar to annite, KFe(3)AlSi(3)O(10)(OH)(2), and crystallizes with the same monoclinic C2/m symmetry. No phase transition has been observed down to 100 K. At low temperature, the ditrigonal distortion of the mica structure increases markedly, while the octahedral and tetrahedral bond lengths tend to decrease and increase, respectively. A detailed comparison of structural parameters in various Fe-rich micas is presented.

Single-crystal structure refinement of NaTiSi2O6 clinopyroxene at low temperatures (298 < T < 100 K)

The alkali-metal clinopyroxene NaTi(3+)Si2O6, one of the rare compounds with trivalent titanium, was synthesized at high temperature/high pressure and subsequently investigated by single-crystal X-ray diffraction methods between 298 and 100 K. One main difference between the high- and the low-temperature form is the sudden appearance of two different Ti(3+)-Ti3+ interatomic distances within the infinite chain of the TiO6 octahedra just below 197 K. This change can be seen as direct evidence for the formation of Ti-Ti singlet pairs in the low-temperature phase. Mean Ti-O bond lengths smoothly decrease with decreasing temperature and the phase transition is associated with a slight jump in the Ti-O bond length. The break in symmetry, however, causes distinct variations, especially with respect to the two Ti-O(apex) bond lengths, but also with respect to the four Ti-O bonds in the equatorial plane of the octahedron. The TiO6 octahedron appears to be stretched in the chain direction with a slightly larger elongation in the P1; low-temperature phase compared with the C2/c high-temperature phase. Polyhedral distortion parameters such as bond-length distortion and octahedral angle variance suggest the TiO6 octahedron in P1; to be closer to the geometry of an ideal octahedron than in C2/c. Mean Na-O bond lengths decrease with decreasing temperature and the variations in individual Na-O bond lengths are the result of variations in the geometry of the octahedral site. The tetrahedral site acts as a rigid unit, which does not show pronounced changes upon cooling and through the phase transitions. There are neither large changes in bond lengths and angles nor in polyhedral distortion parameters, for the tetrahedral site, when they are plotted. In contrast with the C2/c-->P2(1)/c phase transition, found especially in LiMSi2O6 clinopyroxenes, no very large variations are found for the tetrahedral bridging angle. Thus, it is concluded that the main factor inducing the phase transition and controlling the structural variations is the M1 octahedral site.

LiInSiO4: a new monovalent-trivalent olivine

The structure of the olivine LiInSiO(4) (lithium indium silicate) is isotypic with LiScSiO(4) and MgMgSiO(4) (forsterite). The main differences between the title compound and the divalent-divalent olivines are found for the bond lengths and angles opposite common edges between the tetrahedron and the Li(+) and In(3+) ion sites. The tetrahedron shares one common edge with the Li(+) site and two common edges with the In(3+) site. The tetrahedron is distinctly distorted, as are the Li(+) and In(3+) sites.