The Zintl-Klemm concept applied to cations in oxides. I. The structures of ternary aluminates

High-pressure phase transition and amorphization of BaV2O6

The structural evolution of metavanadate compounds under high pressure offers valuable insights into phase transitions and changes in material properties. This study explores the structural behavior of BaV2O6 under pressures up to 12 GPa using powder X-ray diffraction and density-functional theory (DFT) simulations. The results indicate a phase transition from the ambient pressure orthorhombic phase (space group C222) to a monoclinic phase (space group C2) at 4 GPa, likely driven by the distortion of the vanadium oxide polyhedron. Above 10 GPa, the C2 phase undergoes amorphization, attributed to the breakdown of the infinite [VO4] chains into [VO3]- units. Additionally, BaV2O6 exhibits anisotropic lattice contraction and a relatively low bulk modulus (B0 ≈ 50 GPa). DFT calculations further explore the pressure dependence of enthalpy differences, Raman modes, and band structures, providing insights into the structural and electronic transformations of BaV2O6 under high pressure. This work deepens the understanding of the structural and band structure development of the metavanadate family under high pressure, contributing to advancements in materials science under extreme conditions.

Navigation through high-dimensional chemical space: discovery of Ba5Y13[SiO4]8O8.5 and Ba3Y2[Si2O7]2

Two compounds were discovered in the well-studied BaO-Y2O3-SiO2 phase field. Two different experimental routines were used for the exploration of this system due to the differences of synthetic conditions and competition with a glass field. The first phase Ba5Y13[SiO4]8O8.5 was isolated through a combination of energy dispersive X-ray spectroscopy analysis and diffraction techniques which guided the exploration. The second phase Ba3Y2[Si2O7]2 was located using iterative algorithmic identification of target compositions. The structure solution of the new compounds was aided by continuous rotation electron diffraction, and the structures were refined against combined synchrotron and neutron time-of-flight powder diffraction. Ba5Y13[SiO4]8O8.5 crystallizes in I4̄2m, a = 18.92732(1), c = 5.357307(6) Å and represents its own structure type which combines elements of structures of known silicates embedded in columns of interconnected yttrium-centred polyhedra characteristic of high-pressure phases. Ba3Y2[Si2O7]2 has P21 symmetry with a pseudo-tetragonal cell (a = 16.47640(4), b = 9.04150(5), c = 9.04114(7) Å, β = 90.0122(9)°) and is a direct superstructure of the Ca3BaBi[P2O7]2 structure. Despite the lower symmetry, the structure of Ba3Y2[Si2O7]2 retains disorder in both Ba/Y sites and disilicate network, thus presenting a superposition of possible locally-ordered fragments. Ba5Y13[SiO4]8O8.5 has low thermal conductivity of 1.04(5) W m-1 K-1 at room temperature. The two discovered phases provide a rich structural platform for further functional material design. The interplay of automated unknown phase composition identification with multiple diffraction methods offers acceleration of the time-consuming exploration of high-dimensional chemical spaces for new structures.

A re-interpretation of the structure of the silver borate, Ag16B4O10, in the light of the extended Zintl-Klemm concept

The borate Ag₁₆B₄O₁₀ was synthesized at high temperature and at elevated oxygen pressures [Kovalevskiy et al. (2020). Chem. Sci. 11, 962-969]. Its structure consists of [B₄O₁₀]^8- polyanions (isostructural to P₄O₁₀) embedded in an Ag matrix. According to the standard valences Ag⁺, B³⁺ and O^2-, the formula has an excess of eight e^- which the above authors proposed were located, pairwise, in four Ag₄ tetrahedra within the silver substructure. That conclusion was based on the semiconducting and diamagnetic properties, as well as the very small `attractors' of the Electron Localization Function (ELF) found at the centre of these Ag₄ tetrahedra. However, a large overlap of the projected density of states (DOS) of silver and oxygen indicated possible dispersion interactions between both atomic species. In this article, an alternative description is proposed based on the extended Zintl-Klemm concept. The anion [B₄O₁₀]^8- can be formulated as Ψ-[N₄O₁₀] P₄O₁₀, if it is assumed that the eight e^- are transferred to the four B atoms, so converting them into Ψ-N, this then makes sense of its similarity with P₄O₁₀, [N₄(CH₂)₆], adamantane and tetraisopropyladamantane. When the Ag atoms connect to the O atoms, they arrange as the H atoms do in hexamethylenetetramine (HMTA). If the two lone pairs of each of the bridging O atoms in Ψ-[N₄O₁₀] are equated to the C-H bonds in HMTA, then, this same equivalence exists between the C-H bonds and the O-Ag bonds in the compound Ag₁₆B₄O₁₀. The 24 Ag atoms surrounding each [B₄O₁₀]^8- group prolong the sphalerite structure of the borate anion by means of Ag-O bonds which also fit the sphalerite structure formed of AgO. The eight excess electrons might then be distributed between the Ag and the O atoms, so making sense of the mixing of the Ag and O states. The Ag atoms bonded to the O atoms of the [B₄O₁₀]^8- groups form a coat that interconnects the borate anions through Ag-O bonds. To establish the validity of this new proposal, the study needs to be extended to the compound Ag₃B₅O₉.

Rationalization of the crystal structure of eudidymite Na2Be2[Si[4]6O15]·H2O in light of the extended Zintl-Klemm concept

The structure of eudidymite is described in light of the extended Zintl-Klemm concept which considers that Na and Be atoms transfer their six valence electrons to the six Si atoms, converting them into Ψ-P which forms a skeleton characteristic of pentels (Group 15 elements) and is similar to that described in the compound (NH₄)₂Ge^[6][Ge^[4]₆O₁₅] when analysed in the same manner. The Si^[4] skeleton is formed of bilayers that are connected through Be₂O₆ groups which are in fact fragments of the β-BeO structure which bridge the two contiguous Si-bilayers by sharing O atoms. In this context, the Be atoms play a dual role, i.e. on the one hand converting the Si atoms into Ψ-P, on the other hand replicating fragments of its own β-BeO structure. The Be atoms partially reproduce their own structure despite it being enclosed in a more complex network such as in Na₂Be₂[Si^[4]₆O₁₅]·H₂O. Calculations of the ionic strength I considering Si as Ψ-P is energetically more favourable than when I is calculated on the basis of tetravalent Si in the silicate, justifying this new approach of developing the theory of pseudo-structure generation. This approach offers a major new development in the study of crystal structures.

High pressure phase transitions of paracelsian BaAl2Si2O8

Three new polymorphs of aluminosilicate paracelsian, BaAl₂Si₂O₈, have been discovered using synchrotron-based in situ high-pressure single crystal X-ray diffraction. The first isosymmetric phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The phase transition is associated with the formation of pentacoordinated Al³⁺ and Si⁴⁺ ions, which occurs in a stepwise fashion by sequential formation of Al-O and Si-O bonds additional to those in AlO₄ and SiO₄ tetrahedra, respectively. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic (P2₁/c) to orthorhombic (Pna2₁). The structure of paracelsian-III consists of SiO₆ octahedra, AlO₆ octahedra and distorted AlO₄ tetrahedra, i.e. the transition is reconstructive and associated with the changes of Si⁴⁺ and Al³⁺ coordination, which show rather complex behaviour with the general tendency towards increasing coordination numbers. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna2₁ to Pn. The transition has a displacive character. In the course of the phase transformation pathway up to 32 GPa, the structure of polymorphs becomes denser: paracelsian-II is based upon elements of cubic and hexagonal close-packing arrangements of large O²⁻ and Ba²⁺ ions, whereas, in the crystal structure of paracelsian-III and IV, this arrangement corresponds to 9-layer closest-packing with the layer sequence ABACACBCB.

Pressure Impact on the Stability and Distortion of the Crystal Structure of CeScO3

The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite-type cerium scandate were studied in situ under high pressure by means of synchrotron X-ray powder diffraction, using a diamond-anvil cell. We found that the perovskite-type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell parameters with pressure indicated an anisotropic compression. The room-temperature pressure-volume equation of state (EOS) obtained from the experiments indicated the EOS parameters V₀ = 262.5(3) ų, B₀ = 165(7) GPa, and B₀' = 6.3(5). From the evolution of microscopic structural parameters like bond distances and coordination polyhedra of cerium and scandium, the macroscopic behavior of CeScO₃ under compression was explained and reasoned for its large pressure stability. The reported results are discussed in comparison with high-pressure results from other perovskites.

Crystal behavior of potassium bromate under compression

We report on high-pressure angle-dispersive X-ray diffraction data up to 15 GPa and ab initio total-energy calculations up to 242 GPa for KBrO3. No phase transition was found below 15 Pa in contrast to previously reported data. Its experimental bulk modulus in the quasi-hydrostatic regime is B0 = 18.8 (9) GPa with a bulk modulus pressure derivative B'0 = 8.2 (4). However, according to our ab initio calculations, KBrO3 significantly reduces its rhombohedral distortion via small cooperative movements of the atoms and the structure progressively approaches the cubic symmetry, where the KBr subarray would adopt a topology similar to that of the corresponding B2-type bromide. This rearrangement of atoms is directly related to the Buerger's mechanism of the B1-B2 phase transition for halides, confirming that cations (second neighbors) do not arrange in an arbitrary way. The O atoms forming the [BrO3] pyramidal units move smoothly with pressure to the center of the [K8] cube faces, where electron localization function calculations present their maxima in other B2-type compounds, eventually adopting the perovskite-type structure at P ≃ 152 GPa. Our data on KBrO3 has been compared with chemically substituted isostructural halates, providing new insights on the compressibility of this family of compounds.

In-situ laser synthesis of Nd-Al-O coatings: the role of sublattice cations in eutectic formation

Neodymium aluminate coatings have been prepared in-situ by the laser zone melting (LZM) method, using a CO2 SLAB-type laser emitting at 10.6 µm. Polycrystalline Al2O3 commercial plates have been used as substrates, and coatings were prepared from the corresponding mixtures of powdered neodymium and aluminium oxides as starting materials. Microstructure, studied by SEM and phase composition, studied by XRD, proved the in-situ formation of a NdAlO3/NdAl11O18 eutectic. As a result, a well integrated composite coating was formed. Nanoindentation tests are consistent with excellent integration between coating and substrate. Structural similarities between the eutectic components within the coating, as well as between these and the substrate, are consistent with the crystallographic concepts proposed by Vegas (Ramos-Gallardo & Vegas, 1997), where cation sub-arrays play an important role governing metal oxide structures. These structure sublattices are suggested as the driving force behind eutectic oxide formation.

Borohydrides: from sheet to framework topologies

The five novel compounds ALiM(BH4)4 (A = K or Rb; M = Mg or Mn) and K3Li2Mg2(BH4)9 crystallizing in the space groups Aba2 and P2/c, respectively, represent the first two-dimensional topologies amongst homoleptic borohydrides. The crystal structures have been solved, refined and characterized by synchrotron X-ray powder diffraction, neutron powder diffraction and solid-state DFT calculations. Minimal energies of ordered models corroborate crystal symmetries retrieved from diffraction data. The layered Li-Mg substructure forms negatively charged uninodal 4-connected networks. It is shown that this connectivity cannot generate the long sought-after, bimetallic Li-Mg borohydrides without countercations when assuming preferred coordination polyhedra as found in Mg(BH4)2 and LiBH4. The general properties of the trimetallic compound series are analogous with the anhydrous aluminosilicates. Additionally, a relationship with zeolites is suggested, which are built from three-dimensional Al-Si-O networks with a negative charge on them. The ternary metal borohydride systems are of interest due to their potential as novel hydridic frameworks and will allow exploration of the structural chemistry of light-metal systems otherwise subject to eutectic melting.

An alternative approach to rationalizing the structures of the cyclotrisilicates: Rb10[Si6O17], Cs8[Si6O16] and Na3Y[Si6O15] by viewing them in light of the extended Zintl-Klemm concept

The structures of the three dimeric cyclotrisilicate anions [Si6O17](10-), [Si6O16](8-) and [Si6O15](6-) forming part of the compounds Rb10[Si6O17] or Rb14[Si4][Si6O17], Cs8[Si6O16] and Na3Y[Si6O15], respectively, have been described as different ways of condensing the more elemental cyclotrisilicate anions [Si3O9](6-) [Hlukhyy & Fässler (2013). Z. Anorg. Allg. Chem. 639, 231-233]. Such a description prevents a rational explanation of the connectivity between the Si atoms. So, in this article the Si skeletons of the above silicate anions are put on a common basis in light of the extended Zintl-Klemm concept. Their connectivity is directly related to the ratio Ψ-S/Ψ-P resulting from the number of electrons transferred from the electropositive Na, Y, Rb and Cs atoms, offering a new example of the fact that there exists a univocal link between chemical composition (in terms of pseudo-atoms) and structure: the generalized Zintl-Klemm concept. The three skeletons fit the connectivity of hypothetical Zintl polyanions of composition [Ψ-S4P2](10-), [Ψ-S2P4](8-) and [Ψ-P6](6-) respectively.

Structural phase transitions on AgCuS stromeyerite mineral under compression

The structural behavior of mineral Stromeyerite, AgCuS, has been studied by means of angle-dispersive X-ray diffraction measurements up to 13 GPa and ab initio total-energy calculations. Two high-pressure phase transitions are found at 1.4 and 5.7 GPa, from the initial distorted Ni(2)In-type phase (AuRbS-type, RP, space group Cmc2(1)) through an anti-PbClF-type phase (HP1, space group P4/nmm) to a monoclinic distortion of this latter phase (HP2, space group P2(1)/m). The collapse of the metal-metal interatomic distances at the RP-HP1 transition suggests a stronger metallic behavior of the high-pressure phase. The compressibility of the lattice parameters and the equation of state of the first pressure-induced phase have been experimentally determined. First-principles calculations present an overall agreement with the experimental results in terms of the high-pressure sequence and provide chemical insight into the AgCuS behavior under hydrostatic pressure.

Unique thermodynamic relationships for Δf H o and Δf G o for crystalline inorganic salts. I. Predicting the possible existence and synthesis of Na2SO2 and Na2SeO2

The concept that equates oxidation and pressure has been successfully utilized in explaining the structural changes observed in the M 2S subnets of M 2SO x (x = 3, 4) compounds (M = Na, K) when compared with the structures (room- and high-pressure phases) of their parent M 2S `alloy' [Martínez-Cruz et al. (1994), J. Solid State Chem. 110, 397–398; Vegas (2000), Crystallogr. Rev. 7, 189–286; Vegas et al. (2002), Solid State Sci. 4, 1077–1081]. These structural changes suggest that if M 2SO2 would exist, its cation array might well have an anti-CaF2 structure. On the other hand, in an analysis of the existing thermodynamic data for M 2S, M 2SO3 and M 2SO4 we have identified, and report, a series of unique linear relationships between the known Δf H o and Δf G o values of the alkali metal (M) sulfide (x = 0) and their oxyanion salts M 2SO x (x = 3 and 4), and the similarly between M 2S2 disulfide (x = 0) and disulfur oxyanion salts M 2S2O x (x = 3, 4, 5, 6 and 7) and the number of O atoms in their anions x. These linear relationships appear to be unique to sulfur compounds and their inherent simplicity permits us to interpolate thermochemical data (Δf H o) for as yet unprepared compounds, M 2SO (x = 1) and M 2SO2 (x = 2). The excellent linearity indicates the reliability of the interpolated data. Making use of the volume-based thermodynamics, VBT [Jenkins et al. (1999), Inorg. Chem. 38, 3609–3620], the values of the absolute entropies were estimated and from them, the standard Δf S o values, and then the Δf G o values of the salts. A tentative proposal is made for the synthesis of Na2SO2 which involves bubbling SO2 through a solution of sodium in liquid ammonia. For this attractive thermodynamic route, we estimate ΔG o to be approximately −500 kJ mol−1. However, examination of the stability of Na2SO2 raises doubts and Na2SeO2 emerges as a more attractive target material. Its synthesis is likely to be easier and it is stable to disproportionation into Na2S and Na2SeO4. Like Na2SO2, this compound is predicted to have an anti-CaF2 Na2Se subnet.

On the charge transfer between conventional cations: the structures of ternary oxides and chalcogenides of alkali metals

The structures of ternary oxides and chalcogenides of alkali metals are dissected in light of the extended Zintl-Klemm concept. This model, which has been successfully extended to other compounds different to the Zintl phases, assumes that crystal structures can be better understood if the cation substructures are contemplated as Zintl polyanions. This implies the occurrence of charge transfer between cations, even if they are of the same kind. In this article, the charge transfer between cations is even more illustrative because the two alkali atoms have different electronegativity, so that the less electropositive alkali metal and the O/S atom always form skeletons characteristic of the group 14 elements. Thus, partial structures of the zincblende-, wurtzite-, PbO- and SrAl(2)-type are found in the oxides/sulfides. In this work, such an interpretation of the structures remains at a topological level. The analysis also shows that this interpretation is complementary to the model developed by Andersson and Hyde which contemplates the structures as the intergrowth of structural slabs of more simple compounds.

PbCa2[Al8O15] with a novel three-dimensional aluminate anion

The asymmetric unit of the title compound, lead(II) dicalcium octaaluminate, contains one Pb, one Ca, four Al and eight O atoms, with the Pb atom and one O atom situated on mirror planes. Three Al atoms exhibit slightly distorted tetrahedral coordinations with a mean Al—O bond length of 1.76 Å. The fourth Al atom is in a considerably distorted trigonal–bipyramidal coordination with a mean Al—O bond length of 1.89 Å. One AlO4tetrahedron forms infinite chains parallel to [100]viacorner-sharing. These chains are linked by parallel chains of edge-sharing AlO5trigonal bipyramids into layersAof six-membered double rings extending parallel to (010). The second layerBis made up of the remaining two AlO4tetrahedra. These tetrahedra share corners, resulting in likewise six-membered double rings. Finally, the parallel layersAandBare linked into a three-dimensional framework by common corners. Charge compensation is achieved by the Pb2+and Ca2+cations, which are situated in the cavities of the anionic framework, and which are surrounded by seven and six O atoms, respectively, both within highly irregular coordination polyhedra.

Towards a generalized vision of oxides: disclosing the role of cations and anions in determining unit-cell dimensions

Theoretical calculations of the electron-localization function show that, at the volumes of the two CaO phases (rocksalt and CsCl type), the parent Ca structures (fcc: face-centred cubic and sc: simple cubic, respectively) exhibit charge concentration zones which coincide with the positions occupied by the O atoms in their oxides. Similar features, also observed for the pairs Ca/CaF(2) and BaSn/BaSnO(3), are supported by recent high-pressure experiments as well as electron-localization function (ELF) calculations, carried out on elemental K. At very high pressures, the elemental K adopts the hP4 structure, topologically identical to that of the K atoms in high-pressure K(2)S and high-temperature alpha-K(2)SO(4). Moreover, the ELF for the hP4 structure shows charge concentration (approximately 2 electrons) at the sites occupied by the S atoms in the high-pressure K(2)S phase. All these features confirm the oxidation/high-pressure equivalence as well as the prediction of how cation arrays should be metastable phases of the parent metals. For the first time to our knowledge, the structure type, dimension and topology of several oxides and fluorides (CaO, CaF(2) and BaSnO(3)) are explained in univocal physical terms.

Compounds with a 'stuffed' anti-bixbyite-type structure, analysed in terms of the Zintl-Klemm and coordination-defect concepts

Compounds with a ‘stuffed anti-bixbyite’ structure, such as Li₃AlN₂, were analysed in terms of both the extended Zintl–Klemm concept and the coordination-defect concept. For the first time, inorganic crystal structures are seen as a set of ‘multiple resonance structures’ (Klemm pseudo-structures) which co-exist as the result of unexpected electron transfers between any species pair comprising either like or unlike atoms, cations or anions. If this is the driving force controlling crystal structures, the conventional oxidation states assigned to cations and anions lose some of their usefulness.

The bixbyite structure (Mn₂O₃) () is often described as a distorted face-centered cubic (f.c.c.) array of Mn atoms, with O atoms occupying 3/4 of the tetrahedral holes. The empty M₄ tetrahedra are centred at 16c. In anti-bixbyite structures (Mg₃N₂), cation vacancies are centred in empty N₄ tetrahedra. If 16 hypothetical atoms were located at this site they would form the structure of γ-Si. This means that anti-bixbyite structures are ideally prepared to accommodate Si(Ge) atoms at these holes. Several compounds (Li₃AlN₂ and Li₃ScN₂) fully satisfy this expectation. They are really anti-bixbyites ‘stuffed’ with Al(Sc). The presence of these atoms in 16c is illuminated in the light of the extended Zintl–Klemm concept (EZKC) [Vegas & García-Baonza (2007 ▶). Acta Cryst. B63, 339–345], from which a compound would be the result of ‘multiple resonance’ pseudo-structures, emerging from electron transfers between any species pair (like or unlike atoms, cations or anions). The coordination-defect (CD) concept [Bevan & Martin (2008) ▶. J. Solid State Chem.181, 2250–2259] is also consistent with the EZKC description of the pseudo-structures. A more profound insight into crystal structures is gained if one is not restricted to the contemplation of classical anions and cations in their conventional oxidation states.