Single crystals of rare earth oxides from reducing halide melts

Wet to Dry Controls Lanthanide Scandate Synthesis

The choice of temperature and gas conditions used in a water pressure-controlled reactor is guided by density functional theory (DFT) to synthesize nearly phase-pure lanthanide scandate nanoparticles (LnScO₃, Ln = La, Nd, Sm, Gd). In this synthetic method, low water-vapor partial pressures, well below water's gas liquidus, inhibit particle growth, while an excess of water vapor results in undesired rare-earth hydroxide and oxyhydroxide secondary phases. The optimal humidity for high-purity LnScO₃ particle synthesis is shown to vary with the lanthanide; DFT is used to calculate the thermodynamics of secondary phase formation for each lanthanide tested such that the role of water vapor may be quantified and used to maintain phase purity (greater than 96 mol %) across the series. The combination of thermodynamic calculation and experimental confirmation with this pressure-controlled reactor provides an opportunity to explore analogous syntheses of other inorganic perovskite nanoparticles.

Synthesis and crystal structure of two scandium oxotellurates(IV): Sc2Te3O9 and Sc2Te4O11

The two new scandium oxotellurates(IV) Sc2Te3O9 and Sc2Te4O11 were synthesized through firing appropriate mixtures of Sc2O3, TeO2 and CsBr (as flux) in evacuated glassy silica ampoules at 850 °C for 10 days. Both of them crystallize in the monoclinic space group P21/c with Z = 4 (Sc2Te3O9: a = 523.36(3), b = 2438.23(14), c = 731.98(4) pm, β = 116.221(3)°; Sc2Te4O11: a = 949.51(6), b = 779.12(5), c = 1341.93(9) pm, β = 90.829(3)°). Both crystal structures contain two crystallographically unique Sc3+ cations. In the case of Sc2Te3O9, they reside in six- and sevenfold oxygen coordination arranged as distorted uncapped or capped octahedra, while for Sc2Te4O11, they only exhibit six oxygen atoms in the coordination polyhedra, but one of them has also a certain tendency to thrive for a higher coordination number (C.N. = 6 + 1). The [(Sc1)O6)]9− and [(Sc2)O6+1)]11− polyhedra in Sc2Te3O9 are condensed via common edges to form serrated   ∞ 1 { [ Sc 2 O 6 / 1 t O 1 / 2 v O 4 / 2 e ] 11 − } ${\text{ }}_{\infty }^{1}\left\{{\left[{\text{Sc}}_{2}{\text{O}}_{6\text{/}1}^{\text{t}}{\text{O}}_{1\text{/}2}^{\text{v}}{\text{O}}_{4\text{/}2}^{\text{e}}\right]}^{11-}\right\}$ chains running along [100], whereas the two [ScO6]9− octahedra in Sc2Te4O11 only share common vertices, generating   ∞ 1 { [ Sc 2 O 6 / 1 t O 3 / 2 v ] 9 − } ${\text{ }}_{\infty }^{1}\left\{{\left[{\text{Sc}}_{2}{\text{O}}_{6\text{/}1}^{\text{t}}{\text{O}}_{3\text{/}2}^{\text{v}}\right]}^{9-}\right\}$ double strands along [010]. In both compounds, the three-dimensional framework and the charge balance are accomplished by the discrete ψ1-tetrahedral [TeO3]2− anions with non-bonding lone-pair electrons located at their central Te4+ cations. Moreover, strong secondary Te4+···O2− interactions, which are generally quite common for rare earth metal(III) oxotellurates(IV), occur in both crystal structures, but much more pronounced in Sc2Te4O11, where three quarters of the Te4+ cations reside in the centers of ψ eq 1 ${{\psi}}_{\text{eq}}^{1}$ -trigonal bipyramids [TeO4]4− as compared to Sc2Te3O9, which can well be written as Sc2[TeO3]3.

High-Pressure Phases and Properties of the Mg3Sb2 Compound

Pressure always plays an important role in influencing the structure configuration and electronic properties of materials. Here, combining density function theory and structure prediction algorithm, we systematically studied the Mg3Sb2 system from its phase transition to thermodynamic and electronic properties under high pressure. We find that two novel phases, namely Cm and C2/m, are stable under high pressure. Calculation results of phonon dispersions showed that both novel phases have no imaginary frequency, which indicates that the novel phases are thermodynamically stable. Due to the larger ionic radius of Sb compared to N, P, and As elements, the Mg3Sb2 compound shows a different electronic property at high pressure. The electronic calculations show that the novel phases of Cm and C2/m of Mg3Sb2 possess metallic behavior under high pressure. These results provide new insights for understanding the Mg3Sb2 compound.

In situ high-temperature X-ray diffraction study of Sc-doped titanium oxide nanocrystallites

Titanium dioxide is an inexpensive wide-gap highly ionic semiconductor with striking photocatalytic capabilities in several heterogeneous photoredox reactions. A small crystal size is desirable to maximize the surface area, since photocatalytic reactions occur at the surface of a photocatalyst. Presented here are the synthesis and microstructural characterization of 4 at.% Sc-doped TiO2 (4SDT) prepared by water-based co-precipitation. The crystal structure of 4SDT was examined via in situ high-temperature powder X-ray diffraction experiments from 25 to 1200°C. Rietveld analysis revealed single-phase anatase up to 875°C, while at 900°C the anatase-to-rutile phase transformation occurred and at higher temperatures additional reflections of Sc-rich phases (Sc2TiO5 from 975°C and Ti3Sc4O12 or Sc2O3 at 1200°C) were observed. Debye function analysis (DFA) was applied to model the total scattering pattern directly in reciprocal space, allowing the reconstruction of Ti vacancies. Both Rietveld and DFA methods were applied to estimate the nanocrystallite size and shape with consistent growth in crystallite size with temperature: an ellipsoid shape with equatorial ∼4.7 nm / axial (001) ∼6.9 nm at 25°C to equatorial ∼27.9 nm / axial (001) ∼39.6 nm at 900°C refined by Rietveld analysis, versus a cylinder shape with D a,b = 4.3 nm and size dispersion σ ab = 1.5 nm, L c = 4.9 nm and σ c = 2.3 nm at 25°C to D a,b = 21.4 nm, σ ab = 8.3 nm, L c = 23.9 and σ c = 10.9 nm at 900°C estimated by DFA. The microstructural changes obtained by Rietveld and DFA methods were supported by high-resolution transmission electron microscopy image analysis, as well as by the less direct nitrogen sorption techniques that provide information on the size of non-agglomerated and dense particles. The Ti site-occupancy factor showed a linear increase from 0.6–0.8 at 25°C to unity at 900°C for anatase, and from ∼0.7 at 900°C to unity at 1200°C for rutile, via Rietveld analysis and DFA.

Das Ytterbium(III)-Oxidbromid-Oxidotellurat(IV) Yb3O2Br[TeO3]2

The ytterbium(III) oxide bromide oxidotellu-rate(IV) Yb3O2Br[TeO3]2 was obtained from a mixture of Yb2O3, YbBr3 and TeO2 in a molar ratio of 2:1:2 along with an excess of KBr as fluxing agent in evacuated fused silica ampoules after 10 days at T = 800 °C and subsequent slow cooling to room temperatures as colorless, plate-shaped single crystals. Its triclinic crystal structure (a = 663.97(5), b = 697.46(5), c = 1080.15(8) pm, α = 105.102(3), β = 90.931(3), γ = 100.034(3)°; Z = 2, space group: P 1 ‾ $‾{1}$ ) displays three crystallographically different Yb3+ cations with coordination numbers of six, seven and eight. Six out of eight distinct oxygen atoms belong to two independent ψ1-tetrahedral [TeO3]2−anions, whereas the other two represent O2− anions in tetrahedral coordination of four Yb3+ cations each, not having any contact to tellurium. Condensed via common vertices and edges, these [OYb4]10+ tetrahedra form cationic layers ∞ 2 ${}_{\infty }{}^{2}$ {[O2Yb3]5+}, which spread out parallel to the (001) plane. Two discrete [TeO3]2− groups and one Br− anion per formula unit take care of their three-dimensional interconnection along [001] and the overall charge balance of Yb3O2Br[TeO3]2. Remarkable interactions between the lone pair of electrons at the Te4+ cations of the ψ1-tetrahedral [TeO3]2− anions and those at the Br− anions are discussed.

On the phosphors Na5M(WO4)4 (M = Y, La–Nd, Sm–Lu, Bi) – crystal structures, thermal decomposition, and optical and magnetic properties

An exploration of the crystal chemistry of exciting luminescent title compounds – possible phosphors – is discussed also employing magnetic measurements.

Prediction of pressure-induced phase transformations in Mg3As2

We have performed an extensive structural study on cubic Mg₃As₂ in a pressure range of 0–100 GPa by using a combination of structure predictions and first-principle calculations.

The lanthanide hydride oxides SmHO and HoHO

Metal hydride oxides are an interesting class of compounds with potential for hydride ion conduction and as host materials for luminescence. SmHO and HoHO were prepared from mixtures of the sesquioxides Ln 2O3 and the hydrides LnH2+ x at 1173 K as gray powders (Ln=Sm, Ho). They crystallize in a fluorite type crystal structure with disordered anion distribution (Fm3̅m; SmHO: a=5.46953(6) Å, V=163.625(5) Å3; HoHO: a=5.27782(3) Å, V=147.016(2) Å3, based on powder X-ray diffraction) and show stability towards air. Lanthanide-oxygen and -hydrogen distances are 2.36838(3) Å in SmHO and 2.28536(1) Å in HoHO and comparable to those in binary lanthanide oxides and hydrides. Elemental analyses confirm the composition LnHO. Quantum-mechanical calculations show a negative enthalpy for the reaction RE 2O3+REH3→3 REHO for all lanthanides and Y, with increasing values for decreasing ionic radii.

The influence of cation ordering, oxygen vacancy distribution and proton siting on observed properties in ceramic electrolytes: the case of scandium substituted barium titanate

The origin of the 2-order of magnitude difference in the proton conductivity of the hydrated forms of hexagonal and cubic oxygen deficient BaSc_xTi_1-xO_3-δ (x = 0.2 and x = 0.7) was probed using a combination of neutron diffraction and density functional theory techniques to support published X-ray diffraction, conductivity, thermogravimetric and differential scanning calorimetry studies. Cation ordering is found in the 6H structure type (space group P6₃/mmc) adopted by BaSc_0.2Ti_0.8O_3-δ with scandium preferentially substituting in the vertex sharing octahedra (2a crystallographic site) and avoiding the face-sharing octahedra (4f site). This is coupled with oxygen vacancy ordering in the central plane of the face-sharing octahedra (O1 site). In BaSc_0.7Ti_0.3O_3-δ a simple cubic perovskite (space group Pm3[combining macron]m) best represents the average structure from Rietveld analysis with no evidence of either cation ordering or oxygen vacancy ordering. Significant diffuse scattering is observed, indicative of local order. Hydration in both cases leads to complete filling of the available oxygen vacancies and permits definition of the proton sites. We suggest that the more localised nature of the proton sites in the 6H structure is responsible for the significantly lower proton conduction observed in the literature. Within the 6H structure type final model, proton diffusion requires a 3-step process via higher energy proton sites that are unoccupied at room temperature and is also likely to be anisotropic whereas the highly disordered cubic perovskite proton position allows 3-dimensional diffusion by well-described modes. Finally, we propose how this knowledge can be used to further materials design for ceramic electrolytes for proton conducting fuel cells.

Crystal structure of monoclinic samarium and cubic europium sesquioxides and bound coherent neutron scattering lengths of the isotopes 154Sm and 153Eu

The crystal structures of monoclinic samarium and cubic europium sesquioxide, Sm2O3 and Eu2O3, were reinvestigated by powder diffraction methods (laboratory X-ray, synchrotron, neutron). Rietveld analysis yields more precise structural parameters than previously known, especially for oxygen atoms. Interatomic distances d(Sm–O) in Sm2O3 range from 226.3(4) to 275.9(2) pm [average 241.6(3) pm] for the monoclinic B type Sm2O3 [space group C2/m, a = 1418.04(3) pm, b = 362.660(7) pm, c = 885.48(2) pm, β = 100.028(1)°], d(Eu–O) in Eu2O3 from 229.9(2) to 238.8(2) pm for the cubic bixbyite (C) type [space group Ia3̅, a = 1086.87(1) pm]. Neutron diffraction at 50 K and 2 K did not show any sign for magnetic ordering in Sm2O3. Isotopically enriched 154Sm2O3 and 153Eu2O3 were used for the neutron diffraction work because of the enormous absorption cross section of the natural isotopic mixtures for thermal neutrons. The isotopic purity was determined by inductively coupled plasma – mass spectrometry to be 98.9% for 154Sm and 99.8% for 153Eu. Advanced analysis of the neutron diffraction data suggest that the bound coherent scattering lengths of 154Sm and 153Eu need to be revised. We tentatively propose b c(154Sm) = 8.97(6) fm and b c(153Eu) = 8.85(3) fm for a neutron wavelength of 186.6 pm to be better values for these isotopes, showing up to 8% deviation from accepted literature values. It is shown that inaccurate scattering lengths may result in severe problems in crystal structure refinements causing erroneous structural details such as occupation parameters, which might be critically linked to physical properties like superconductivity in multinary oxides.

Charge distribution as a tool to investigate structural details. III. Extension to description in terms of anion-centred polyhedra

The charge distribution (CHARDI) method is a self-consistent generalization of Pauling's concept of bond strength which does not make use of empirical parameters but exploits the experimental geometry of the coordination polyhedra building a crystal structure. In the two previous articles of this series [Nespoloet al.(1999).Acta Cryst.B55, 902–916; Nespoloet al.(2001).Acta Cryst.B57, 652–664], we have presented the features and advantages of this approach and its extension to distorted and heterovalent polyhedra and to hydrogen bonds. In this third article we generalize CHARDI to structures based on anion-centred polyhedra, which have drawn attention in recent years, and we show that computations based on both descriptions can be useful to obtain a deeper insight into the structural details, in particular for mixed-valence compounds where CHARDI is able to give precise indications on the statistical distribution of atoms with different oxidation number. A graph-theoretical description of the structures rationalizes and gives further support to the conclusions obtainedviathe CHARDI approach.

Structural phase transformations of Mg(3)N(2) at high pressure: experimental and theoretical studies

The structural behavior of Mg(3)N(2) has been investigated up to 40.7 GPa at room temperature by means of angle-dispersive X-ray diffraction. A reversible, first-order structural phase transition from the ambient cubic phase (Ia3) to a high-pressure monoclinic phase (C2/m) is found to start at approximately 20.6 GPa and complete at approximately 32.5 GPa for the first time. The equation of state determined from our experiments yields bulk moduli of 110.7(2) and 171.5(1) GPa for the cubic and monoclinic phases, respectively, indicating higher incompressibility of the high-pressure phase of Mg(3)N(2). First-principles calculations reproduced the phase stability and transition pressure determined in our experiment. In addition, a second phase transition from the monoclinic phase to a hexagonal phase (P3m(1)) was predicted around 67 GPa for Mg(3)N(2). The electronic band structures of three phases of Mg(3)N(2) are also calculated and discussed.

M4,667Ch[SiO4]3 (M = Nd, Sm; Ch = O, S): structural comparison of two apatite-type lanthanoid chalcogenide ortho-oxosilicates

The lanthanoid chalcogenide ortho-oxosilicates M4.667Ch[SiO4]3 (M = Nd, Sm; Ch = O, S) with apatite-type crystal structure have been synthesized by the reaction of either M2O3 and SiO2 or M2O3, M, S and SiO2 using CsCl as flux in evacuated silica tubes at 850 °C for 7d. The title compounds crystallize hexagonally in the space group P63 /m with two formula units per unit cell. Their crystal structure contains two crystallographically independent lanthanoid cations, of which (M1) is surrounded by nine O ligands in the shape of a tricapped trigonal prism. Additionally, this site is only occupied by 81–84% to assure that the composition is electroneutral. The (M2) cations have an environment of seven O anions in the case of M4.667O[SiO4]3, which form a pentagonal bipyramid. Considering M4.667S[SiO4]3 this polyhedron changes, when one of the apical oxide particles is substituted by two sulfide ligands which then form a “two-legged” pentagonal bipyramid. Both the non-silicon bonded oxygen in the oxide derivative and the sulfur in the sulfide homologue are situated in a channel along [001]. While oxygen prefers a trigonal planar cationic coordination at the Wyckoff position 2a (CN = 3), the sulfur represents the centre of a trigonal antiprism at the Wyckoff position 2b (CN = 6).