Dopant substitution and oxygen migration in the complex perovskite oxide Ba3CaNb2O9: A computational study

Atomistic simulation methods have been used to study the defect chemistry of the complex perovskite oxide Ba₃CaNb₂O₉. Calculations were carried out for the hexagonal (P-3m1) phase and the cubic (Fm-3m) phase. The hexagonal structure is predicted to be energetically more stable at room temperature. In both structures the most favourable dopant for Nb⁵⁺ is found to be Ca²⁺ rather than Mg²⁺, in contrast to the generally accepted rule that size similarities govern such processes. The diffusion of oxygen vacancies in the hexagonal and cubic phases occurs within different networks of corner-sharing NbO₆ and CaO₆ octahedra. Irrespective of the arrangement of octahedra, however, migration of oxygen vacancies around NbO₆ octahedra takes place with lower activation energies than around the CaO₆ octahedra.     

SrSn3 – eine supraleitende Legierung mit freien Elektronenpaaren

SrSn₃ – a Superconducting Alloy with Non‐bonding Electron Pairs SrSn₃ was synthesized from the elements in a welded niobium ampoule. The crystal structure was determined from X‐ray single crystal data. Space group R3m, a = 6,940(2) Å, c = 33,01(1) Å, Z = 12, Pearson symbol hR48. SrSn₃ shows an ordered atomic distribution on four crystallographic sites. The structure is build up from two closed packed atom layers (Sn1/Sr1 and Sn2/Sr2) each with the composition Sr : Sn = 1 : 3 and with hexagonal symmetry of the Sr atoms. The Sn atoms are shifted with respect to the ideal positions of a closed packed layer in a way that Sn triangles, which are separated by Sr atoms, result. Translational symmetry along the c axis arises from a 12‐layer stacking sequence with hexagonal and cubic closest packing motives. Due to the layer sequence ABABCACABCBC… units of three face‐sharing Sn octahedra result (condensation through Sn2 atoms) which form the Sn partial structure. The octahedra chains run parallel to the c axis and are connected by exclusively vertex sharing Sn octahedra (Sn1 atoms). Temperature dependent susceptibility measurements reveal superconducting properties. LMTO band structure calculations verify the metallic behavior. An analysis of the density of states with the help of the electron localization function (ELF) shows, that two kinds of lone pairs occur in this intermetallic phase: non‐bonding electron pairs with the shape of a sp² orbital hybrid are located at the Sn2 atoms and lone pairs with p orbital character are located at Sn1 atoms. The role of lone pairs with respect to the superconducting property is discussed.     

Structure and phase transitions of the lanthanide metals

The structures and phase transitions of the lanthanide metals can be related to f orbital contributions to the bonding. With increasing availability of the f orbitals the structure sequence hexagonal closest packed, double hexagonal closest packed, δ-samarium, cubic closest packed, and body-centered cubic is observed. Increases in temperature and/or pressure result in an increased availability of the f orbitals resulting in predictable phase transitions.     

Ba0.15WO3: A bronze with an original pentagonal tunnel structure

The structure of a new barium tungsten bronze, Ba_0.15WO₃, has been established by X-ray diffraction and high-resolution microscopy studies. This bronze is orthorhombic, space group Pbm2 or Pbmm, with a = 8.859(3) Å, b = 10.039(8) Å, and c = 3.808(2)Å. The “WO₃” framework is built up from corner-sharing WO₆ octahedra forming pentagonal tunnels where the barium ions are located. Structural relationships with hexagonal tungsten bronze and tetragonal tungsten bronze structures are discussed.     

Über hexagonale Perowskite mit Kationenfehlstellen. XVII Strukturbestimmung an Ba9Nb6W□2O27 - der ersten Stapel-variante eines rhomboedrischen 27 L-Typs

On Hexagonal Perovskites with Cationic Vacancies. XVII. Structure Determination on Ba₉Nb₆W□₂O₂₇ – the First Stacking Polytype of a Rhombohedral 27 L-Type The hexagonal stacking polytype of rhombohedral 27 L -type, Ba₉Nb₆W□₂O₂₇, crystallizes in the space group R3 m with the sequence (4)1(3)1 ? (hhccchhcc)₃ and three formula units for the trigonal setting. The refined, intensity related, R'-value is 9.7percnt;. The octahedral net consists of blocks of three face connected octahedra which are linked to each other alternately through one or two octahedra connected exclusively by common vertices. The cationic vacancies are located in the centers of the groups of three octahedra. With this distribution direct contact between occupied face-sharing octahedra is avoided. The niobium and tungsten atoms are distributed statistically between the remaining octahedral holes. In the blocks of three octahedra they are displaced by ≈ 0.29 Å from their ideal positions in the direction of the central void. The Ba atoms neighbouring a vacancy (all in hexagonal packed BaO₃ sheets) are dislocated in the direction of the void, while the cubic packed BaO₃ sheets maintain nearly regular form.     

Hexagonal Double Perovskite Cs2AgCrCl6

The quaternary halide Cs₂AgCrCl₆ was prepared in the form of dark purple crystals by reaction of CsCl, AgCl, and CrCl₃, at 700 °C. It crystallizes in the trigonal Ba₂NiTeO₆‐type structure [space group R3 m, Z = 6, a = 7.2692(4) Å, c = 36.443(2) Å] belonging to the family of perovskite polytypes containing sequences of hexagonal close‐packed layers. Groups of three face‐sharing octahedra, which are occupied in the sequence Ag–Cr–Ag, are connected through corner‐sharing by Cr‐centered octahedra. The UV/Vis/NIR diffuse reflectance spectrum shows absorptions arising from d–d transitions typical of octahedral Cr³⁺ complexes, as confirmed by electronic structure calculations. The compound melts at 506 °C. Magnetic measurements revealed simple paramagnetic behavior consistent with the presence of isolated Cr³⁺ ions.     

Crystal Structure of BaNb6.3(1)Ti3.6(1)O16 containing Nb6O12, fused Ti3O13 Clusters and Ti3O10 Units

A new phase, BaNb_6.3(1)Ti_3.6(1)O₁₆, has been synthesised. Electron diffraction studies indicate an hexagonal substructure with unit cell parameters a ≈ 8.9 Å and c ≈ 9.5 Å. In some of the ED patterns superstructure reflections are present, indicating a supercell with a = √3 · a_sub and c = c_sub. However, X‐ray single‐crystal diffraction studies of a crystallite yielding reflections corresponding to the supercell revealed it to be monoclinic, with the unit cell parameters a = 26.811(2) Å, b = 15.4798(2) Å, c = 9.414(2) Å, β = γ = 90° and α = 90.0(3)°. The average crystal structure was refined, using the subcell with a = 8.937(2) Å, b = 15.479(2) Å, c = 9.414(2) Å, β = γ = 90° and α = 90.0(3)°, space group Cm11, and Z = 4, to R_I = 3.24% and R_wI = 3.44%. The structure can be described as an hexagonal close packing layers of Nb₆ octahedra, Ba, and O atoms (A1, A2) and layers of O atoms (B1, B2), appearing in the packing sequence: A1B1A2B2. The Nb₆ octahedra are found in isolated Nb₆O₁₂O₆ clusters, and the Ti atoms in Ti₃O₁₃ and Ti₃O₁₀ units in octahedral and tetrahedral voids formed by O atoms, respectively. The Ti positions were found to be only partly occupied. Microanalysis indicates that some Nb atoms are located in the Ti₃ triangles. A model is presented that interprets these not fully occupied Ti₃ triangles as a result of a superimposing of three different structures. Two of these consist of two fused Ti₃O₁₃ units, forming an Ti₆O₁₉ unit, and a Ti₃O₁₀ unit, while the third consists of alternating Ti₃O₁₃ units.     

Crystal structures and magnetic properties of BaTiF5 and CaTiF5

Single crystals of BaTiF₅ and CaTiF₅ were obtained by the Czochralski and Bridgman techniques, respectively. The crystal structures were determined by X-ray diffraction; BaTiF₅: $\text{14}\text{m}\text{, a = 15.091(5)}$Å, c = 7.670(3)Å; CaTiF₅: $\text{I2}\text{c}\text{, a = 9.080(4)}$Å, b = 6.614Å, c = 7.696(3)Å, β = 115.16(3)°. Both structures are characterized by the presence of either branched or straight chains of TiF₆ octahedra. BaTiF₅ contains the unusual dimeric unit (Ti₂F₁₀)^4?. Magnetic susceptibility measurements were performed on both compounds in the temperature range 4.2 to 300 K, however, no evidence for magnetic interactions between the Ti³⁺ moments were observed.     

Closest packing in dense oxides: The structure of a polymorph of Co3(AsO4)2

Crystals of Co₃(AsO₄)₂ were grown from the melt of a mixture of Co₂As₂O₇ and As₂O₅. The crystals are isostructural with Mg₃(AsO₄)₂ and are tetragonal with a = 6.858(2), c = 18.872(5) Å, Z = 6, and space group $\text{I}\text{4}\text{2d}$. A total of 1048 independent reflections were measured by diffractometer and used in the full-matrix refinement to a final R value of 0.069. The structure contains two distinct AsO₄ groups. Two of the cobalt ions are octahedrally coordinated and a third occupies a $\text{4}$ site with four short and four long CoO distances. The crystal structure of Co₃ (AsO₄)₂ is not based on the continuous three-dimensional closest packing of oxygen atoms. Nevertheless the number of oxygen atoms per cubic centimeter is 5.4 × 10²², which falls in the range of values for hexagonal and cubic closest packed structures. A better measure of the degree to which closest packing is achieved by a structure is suggested. It is based on an analysis of the polyhedra of oxygen atoms which surround each of the oxygen atoms in a structure and their relation to the polyhedra in ideally closest packed structures. In order to facilitate the analysis, polytopes of 11- and 12-vertex polyhedra were studied. A new decahexahedral 11-vertex polyhedron was found.     

Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON₂, which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scanning transmission electron microscopy indicate that it crystalizes in the non-centrosymmetric space group P6₃mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX₃ layers. A synergetic second-order Jahn–Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W⁶⁺ cations, induce large distortions at two inequivalent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity.     

Die KristallStruktur von Ti7Cl16 und Ti7Br16: Verbindungen mit trigonalen Ti3-Clustern

Crystal Structure of Ti₇Cl₁₆ and Ti₇Br₁₆: Compounds with Trigonal Ti₃ Clusters The mixed-valence titanium halides Ti₇Cl₁₆ and Ti₇Br₁₆ are isotypic and have orthorhombic unit cells (space group Pnnm) with a = 14.421(4), b = 9.987(3), c = 6.890(2) Å and a = 5.228(4), b = 10.577(3), c = 7.276(2) Å, Z = 2. The crystal structures were determined from single-crystal X-ray diffraction data (R = 0.029 and 0.063). The structures consist of trimeric Ti₃Cl₁₃ and Ti₃Br₁₃ cluster units which are linked three-dimensionally to each other and to isolated TiCl₆ (TiBr₆) octahedra. The Ti? Ti bond lengths in the equilateral Ti₃ triangles of the clusters are strongly dependent from the halogen, being 2.953—2.955(2) Å for Ti₇Cl₁₆ and 3.073—3.097(6) Å for Ti₇Br₁₆. By the Ti? Ti bonds the Ti atoms of the Ti₃Cl₁₃ (Ti₃Br₁₃) groups are displaced from the centres of their octahedral coordination towards the Ti₃ centre. This leads to the Ti? Clⁱ (Ti? Brⁱ) bond lengths of 2.359—2.424(2) Å (2.509—2.574(4) Å) being much shorter than the rest of the Ti? Cl (Ti? Br) bonds of 2.508—2.642(2) Å (2.659—2.826(7) Å).     

Crystallographic analysis of structures associated with a number of Tl- and Ag-containing sulfides: the composition–symmetry correlation

A crystallographic analysis is conducted of the structures of orthorhombic mineral sicherite TlAg₂(As,Sb)₃S₆, monoclinic synthetic sulfide Tl₃Ag₃Sb₂S₆, and triclinic mineral raberite Tl₅Ag₄As₆SbS₁₅. In the first two structures, the large and heavy Tl⁺ cation forms, together with the other cations, ordered “skeletal” frameworks with F and I cation sublattices that are close to cubic ones. In the structure of raberite, the Tl and Ag cations undergo, together with the sulfur anions, two-dimensional ordering by a zone of closely packed crystallographic planes, which generate a pseudohexagonal symmetry. The deviations from the 1 cation/anion stoichiometry are compensated: in the second structure, by a local consolidation of cations (to a distance Tl–Ag = 2.96 Å) and, in the third structure, through the formation of a dumbbell pair As–Ag (2.68 Å), which occupies one position in the sublattice.     

Rb3Ti3Te11

Trirubidium trititanium undecatelluride, Rb₃Ti₃Te₁₁, has been synthesized from the reaction of titanium, tellurium, and Rb₂Te₃ at 773 K. Its structure has been determined from single‐crystal X‐ray data. It is composed of one‐dimensional [Ti₃Te₁₁^3?] chains built by face‐sharing pentagonal TiTe₇ bipyramids and distorted TiTe₆ octahedra. These chains adopt hexagonal closest packing along the [101] direction. Rb atoms are located among these chains. The wide range of Te—Te interactions makes the assignment of formal oxidation states impossible. The compound is isostructural with Cs₃Ti₃Te₁₁.     

New cesium titanate layer structures

A phase study of the Cs₂OTiO₂ system in the composition range 75–100 mole% TiO₂ and the temperature range 850–1200°C revealed the existence of two new cesium titanates, with compositions Cs₂Ti₅O₁₁ and Cs₂Ti₆O₁₃. The former compound undergoes a reversible hydration reaction below 200°C to form Cs₂Ti₅O₁₁ · (1 + x)H₂O, 0.5 < x < 1. The structures of the three phases have been determined. They are based on corrugated layers of edge-shared octahedra, with cesium ions (and H₂O) packing between the layers. In Cs₂Ti₆O₁₃, the layers are continuous in two dimensions, whereas in Cs₂Ti₅O₁₁ and Cs₂Ti₅O₁₁ · (1 + x)H₂O, the layers are periodically stepped to give 5-octahedra wide, corner-linked ribbons.     

Crystal structure of K6CrNb15O42

K_6CrNb_(15)O_(42)crystallizes in the hexagonal system with a=9.126(3)A,c=12.068(3)A,V=870.4(5)A~3,and space group P6_2/mcm,Z=1.The structure was solved using direct method andFourier Techniques.Of the 829 unique reflections measured by counter techniques,448 with I≥3σ(I)were used in the least-squares refinement of the model to R=0.034(R_w=0.044).The structureof KoCrNb_(15)O_(42)may be described as consisting of corner-shared and edge-shared octahedra,the ringunits composed of six octahedra of Nb(1)are corner-shared one another along the c-axis to formhexagonal column octahedra chains which are connected by K~+ and octahedra of Nb(2).     

Synthesis and crystal structure of new complex oxides K10M2+Mo7O27 (M2+ = Mg, Mn, Co)

New compounds, K₁₀M²⁺ Mo₇O₂₇ (M²⁺ = Mg, Mn, Co), have been synthesized and characterized. The compounds have an original structure, as established for the first two phases (orthorhombic, space group Pnm2₁, Z = 2, a = 18353 and 18.402, b = 7.889 and 7.931, c = 10.566 and 10.604 å, R = 0.0345 and 0.0609, respectively). A specific feature of the structure is isolated clusters consisting of face-sharing MoO₆ and M²⁺O₆ octahedra, each of the latter having six MoO4 tetrahedra attached to it by vertices. The general pseudohexagonal motif of the structure of the phases is similar to that of glaserites.     

Contrasting oxide crystal chemistry of Nb and Ta:: The structures of the hexagonal bronzes BaTa2O6 and Ba0.93Nb2.03O6

The high-temperature hexagonal forms of BaTa₂O₆ and Ba_0.93Nb_2.03O₆ have P6/mmm symmetry with unit-cell parameters a=21.116(1) Å, c=3.9157(2) Å and a=21.0174(3) Å, c=3.9732(1) Å, respectively. Single crystal X-ray structure refinements for both phases are generally consistent with a previously proposed model, except for displacements of some Ba atoms from high-symmetry positions. The structures are based on a framework of corner- and edge-connected Nb/Ta-centred octahedra, with barium atoms occupying sites in four different types of [0 0 1] channels with hexagonal, triangular, rectangular and pentagonal cross-sections. The refinements showed that the non-stoichiometry in the niobate phase is due to barium atom vacancies in the pentagonal channels and to extra niobium atoms occupying interstitial sites with tri-capped trigonal prismatic coordination. The origin of the non-stoichiometry is attributed to minimisation of non-bonded Ba-Ba repulsions. The hexagonal structure is related to the structures of the low-temperature forms of BaNb₂O₆ and BaTa₂O₆, through a 30° rotation of the hexagonal rings of octahedra centred at the origin.     

The Crystal Structure of InGaO3(ZnO)4: A Single Crystal X‐ray and Electron Diffraction Study

The crystal structure of the mixed oxide InGaO₃(ZnO)₄ has been determined from electron diffraction and single‐crystal X‐ray diffraction data. The compound crystallises in a hexagonal space group (P6₃/mmc; No. 194), deduced from convergent beam electron diffraction (CBED). Single crystals of InGaO₃(ZnO)₄ were grown from a K₂MoO₄ flux in sealed platinum tubes. Single crystal structure refinement from XRD data [a = 3.2850(2) Å; c = 32.906(3) Å; Z = 2; 4232 data, R₁ = 0.0685] reveals a compound with oxygen anions forming a closest‐packed arrangement. Within this packing In³⁺ cations occupy octahedral interstices, forming layers of edge sharing octahedra. In between these layers are regions with composition [Zn₄GaO₅]⁺ forming a wurtzite type of structure. Inversions of the ZnO₄ tetrahedra occurs (i) at the InO₆ octahedral layer and (ii) halfway in the wurtzite type region, where the inversion boundary is built by Ga³⁺ in trigonal bipyramidal coordination with a long Ga–O_apical distance of 2.19(1) Å. The site occupation of Zn²⁺ and Ga³⁺, respectively, was confirmed by bond valence sum calculations. The compounds described here have the same structural charactistics as other known members with general formula ARO₃(ZnO)_m with m = integer.     

Periodic arrays of identical ions packed with 10, 11, and 12 nearest neighbors

The relationships between 10, 11, and 12 coordination identical-sphere packing arrangements are discussed. Primitive tetragonal packing, which is adopted with minor distortions by rutile, has 11 coordination and may be regarded as an intimate intergrowth of close packing (12 coordination) and body-centered tetragonal (10 coordination) packing. Several other known and postulated structures are shown to represent different arrangements of the 50-50 intergrowth of c-axis square channel and c-axis strings of edge-shared octahedra typical of body-centered tetragonal packing and close packings, respectively. Partial cation occupancy of the square channels in such anion arrays would provide conduction pathways for small cations such as Li⁺. It is also shown that body-centered tetragonal packing is an intermediate of cubic and hexagonal close packing.     

Crystal chemistry in the system MSbO3

Cubic, disordered phases of the compounds MSbO₃ (M = Li, Na, K, Rb, Tl, and Ag) have been investigated. KSbO₃ is readily synthesized in the disordered, cubic structure at high pressure, and the other isomorphic compounds were obtained by ion exchange. The structures of NaSbO₃ and AgSbO₃, which have space group Im3, were solved by X-ray single-crystal analysis. The structures contain an essentially rigid SbO₃ subarray consisting of pairs of edge-shared octahedra sharing common corners. Within this subarray, face-shared octahedra form 〈111〉 tunnels that intersect at the origin and body center of the unit cell, and the M⁺ ions are randomly distributed over two positions within these tunnels. Ordered, cubic phases have the primitive-cubic space group Pn3. The two M positions are different for Na⁺ and for Ag⁺ ions. At one of the Ag⁺-ion positions, the AgO bond length is only 2.26 Å, consistent with the gray-black color of AgSbO₃. Deformation of the 4d¹⁰ Ag⁺-ion core by 4d-5s hybridization appears to be induced by AgO covalent bonding. This conclusion is compatible with the observation that ion exchange is reversible for all compounds but AgSbO₃. Several properties of these compounds are compared with the super ionic conductors M₂O·11Al₂O₃ β-alumina.