Structural relations between weberite and zirconolite polytypes—refinements of doped 3T and 4M Ca2Ta2O7 and 3T CaZrTi2O7

New weberite-type Ca₂Ta₂O₇ and zirconolite-type CaZrTi₂O₇ polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca_0.8Ti_1.35Zr_1.3Th_0.15Al_0.4O₇, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca_1.92Ta_1.92Nd_0.08Zr_0.08O₇, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3₁21, Z=6. The 4M Ca₂Ta₂O₇ polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, space group C2, Z=16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM₄ tetrahedra is not maintained in the Ca₂Ta₂O₇ weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta₃Ca interstice to an adjacent Ta₂Ca₄ octahedral interstice in the weberite polytypes.     

The crystal chemistry of L-Ta2O5 and related structures

Single crystals of a new form of L-Ta₂O₅ with a 19×b superstructure have been synthesised by flux growth. The phase is most likely stabilised by the incorporation of a small amount of lithium (0.14 wt% Li) from the flux. The phase has C-centred monoclinic symmetry with , (), , γ=90.00(1)°. The structure was refined in space group C112/m to R₁=0.044 for 814 unique reflections with F>4σ(F). The structure can be described as comprising chains of edge-shared TaO₇ pentagonal bipyramids that are regularly folded at (010) planes to give sinusoidal chains along [010]. These chains are interconnected along [100] and [001] by corner sharing, creating inter-chain regions that are occupied by isolated TaO₆ octahedra and pairs of corner-shared octahedra. A comparison with published data for high-quality refinements of related structures has led to the development of a general model that can explain the structural chemistry variations in the known L-Ta₂O₅-related structures. A shorthand notation is presented for representing the structures, based on the sequence along [010] of the interchain octahedra.     

Chemical twinning of the pyrochlore structure in the system Bi2O3-Fe2O3-Nb2O5

New ternary bismuth iron niobates having structures based on chemical twinning of pyrochlore are described. Bi_5.67Nb₁₀FeO₃₅ has hexagonal symmetry, P6₃/mmc, , , Z=2 and Bi_9.3Nb_16.9Fe_1.1O_57.8 has rhombohedral symmetry, R-3m, , , Z=3. The structures of both phases were determined and refined to R₁=0.04 using single-crystal X-ray data. They can be described as being derived from the pyrochlore structure by chemical twinning on (111)_py oxygen planes. The chemical twin operation produces pairs of corner-connected hexagonal tungsten bronze (HTB) layers as in the HTB structure, so the structures may alternatively be described as pyrochlore:HTB unit-cell intergrowth structures. In the hexagonal phase the pyrochlore blocks have a width of 12 Å, whereas the rhombohedral phase has pyrochlore blocks of two widths, 6 and 12 Å, alternating with HTB blocks. It is proposed that the previously reported binary 4Bi₂O₃:9Nb₂O₅ phase has a related structure containing pyrochlore blocks all of width 6 Å. A feature of the structures is partial occupancy (∼65%) of the Bi sites and displacement of the Bi atoms from the ideal pyrochlore A sites towards the surrounding oxygen atoms, as observed in Bi-containing pyrochlores.     

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.     

Pyrochlore formation, phase relations, and properties in the CaO-TiO2-(Nb,Ta)2O5 systems

Phase equilibria studies of the CaO:TiO₂:Nb₂O₅ system confirmed the formation of six ternary phases: pyrochlore (A₂B₂O₆O′), and five members of the (110) perovskite-slab series Ca_n(Ti,Nb)_nO_3n+2, with n=4.5, 5, 6, 7, and 8. Relations in the quasibinary Ca₂Nb₂O₇−CaTiO₃ system, which contains the Ca_n(Ti,Nb)_nO_3n+2 phases, were determined in detail. CaTiO₃ forms solid solutions with Ca₂Nb₂O₇ as well as CaNb₂O₆, resulting in a triangular single-phase perovskite region with corners CaTiO₃-70Ca₂Ti₂O₆:30Ca₂Nb₂O₇-80CaTiO₃:20CaNb₂O₆. A pyrochlore solid solution forms approximately along a line from 42.7:42.7:14.6 to 42.2:40.8:17.0 CaO:TiO₂:Nb₂O₅, suggesting formulas ranging from Ca_1.48Ti_1.48Nb_1.02O₇ to Ca_1.41Ti_1.37Nb_1.14O₇ (assuming filled oxygen sites), respectively. Several compositions in the CaO:TiO₂:Ta₂O₅ system were equilibrated to check its similarity to the niobia system in the pyrochlore region, which was confirmed. Structural refinements of the pyrochlores Ca_1.46Ti_1.38Nb_1.11O₇ and Ca_1.51Ti_1.32V_0.04Ta_1.10O₇ using single-crystal X-ray diffraction data are reported (Fd3m (#227), a=10.2301(2) Å (Nb), a=10.2383(2) Å (Ta)), with Ti mixing on the A-type Ca sites as well as the octahedral B-type sites. Identical displacive disorder was found for the niobate and tantalate pyrochlores: Ca occupies the ideal 16d position, but Ti is displaced 0.7 Å to partially occupy a ring of six 96g sites, thereby reducing its coordination number from eight to five (distorted trigonal bipyramidal). The O′ oxygens in both pyrochlores were displaced 0.48 Å from the ideal 8b position to a tetrahedral cluster of 32e sites. The refinement results also suggested that some of the Ti in the A-type positions may occupy distorted tetrahedra, as observed in some zirconolite-type phases. The Ca-Ti-(Nb,Ta)-O pyrochlores both exhibited dielectric relaxation similar to that observed for some Bi-containing pyrochlores, which also exhibit displacively disordered crystal structures. Observation of dielectric relaxation in the Ca-Ti-(Nb,Ta)-O pyrochlores suggests that it arises from the displacive disorder and not from the presence of polarizable lone-pair cations such as Bi³⁺.