Investigation of the Rb–W–O system in connexion with the superconducting properties of the hexagonal tungsten bronzes

Resolving old problems with layered polytungstates related to hexagonal tungsten bronze: phase formation, structures, crystal chemistry and some properties

A 60-year-old problem with the atomic arrangements and exact compositions of alkali polytungstates related to hexagonal tungsten bronze (HTB) was solved. The systems A₂WO₄-WO₃ (A = K, Rb) were restudied and the average monoclinic layered structures of stoichiometric polytungstates A₄W₁₁O₃₅ (A = K, Rb, Cs, Tl) and A₂W₇O₂₂ (A = K, Rb, Cs) were first successfully determined. The structures resemble those of "MoW₁₁O₃₆" and "MoW₁₄O₄₅" (J. Graham and A. D. Wadsley, Acta Crystallogr., 1961, 14, 379-383) and are derived from HTB by breaking into slabs parallel to (100) due to the ordered omission of some [WO]_∞ chains along the hexagonal tunnels. The slabs in A₄W₁₁O₃₅ (A = Cs, Tl) and A₂W₇O₂₂ (A = Rb, Cs) are mutually shifted by the a/2 HTB unit cell axis. These data mainly confirmed our preliminary structural models of HTB-like alkali polytungstates (S. F. Solodovnikov, N. V. Ivannikova, Z. A. Solodovnikova and E. S. Zolotova, Inorg. Mater., 1998, 34, 845-853) and revealed a new similar thallium polytungstate. The structures of the HTB-like polytungstates and related compounds form a homologous series of layered complex oxides or fluorides A_n+2-xM_3n+2X_9n+8 where n = 2, 3 and 4 are equal to the numbers of HTB hexagonal tunnels across the polytungstate slab width for Tl₂W₄O₁₃, A₄W₁₁O₃₅ and A₂W₇O₂₂ (A = K, Rb, Cs or Tl), respectively. The structures of the HTB-like polytungstates seem to intergrow with HTB-type A_xWO₃ to form, in particular, higher homologues of the series. Our group-supergroup analysis, measurements of nonlinear optical activity and electrical conductivity, and calculations of the bond-valence site energy barriers indicate possible ferroelectric/ferroelastic properties and moderate 2D oxide-ion mobility within the HTB-type slabs of the studied polytungstates.

Tantalum and vanadium substitution in hexagonal K0.3WO3 bronze: synthesis and characterization

Polycrystalline samples of tantalum and vanadium single and double substituted hexagonal potassium tungsten bronzes (K-HTB’s) with nominal compositions of K0.3(W6+ 0.7W5+ 0.3–yTa5+ y)O3 (0≤y≤0.3), K0.3(W6+ 0.7W5+ 0.3–y V5+ y)O3 (0≤y≤0.18) and K0.3(W6+ 0.7W5+ 0.3–yTa5+ y/2V5+ y/2)O3 (0≤y≤0.3) were synthesized by solid state reactions in quartz tubes at 10–7 MPa and 1073 K. The applied synthesis condition allowed K0.3WO3 to crystallize in space group P6322, confirmed by X-ray powder diffration and Raman spectroscopic analyses. In this K-HTB composition, W5+ could fully be replaced by Ta5+, whereas V5+ could only be substituted up to y=0.16. The degree of W5+ substitution was explained in terms of second-order Jahn–Teller (SOJT) distortion of the d0 cations W6+, Ta5+ and V5+. The applied distortion index also demonstrates why a complete substitution of W5+ in K0.3(W6+W5+)O3 was allowed by a concomitant sharing of Ta5+ and V5+, which are statistically distributed on the W5+/W6+ sites. As W5+(d1) is not SOJT susceptible, it is also shown that the concentration of W5+ in tungsten bronzes plays an important role in the local WO6 octahedral symmetry as well as in its coordination.

Octahedral tilting in the tungsten bronzes

Possibilities for `simple' octahedral tilting in the hexagonal and tetragonal tungsten bronzes (HTB and TTB) have been examined, making use of group theory as implemented in the computer programISOTROPY. For HTB, there is one obvious tilting pattern, leading to a structure in space groupP63/mmc. This differs from the space groupP63/mcmfrequently quoted from X-ray studies – these studies have in effect detected only displacements of the W cations from the centres of the WO6octahedra. The correct space group, taking account of both W ion displacement and the octahedral tilting, isP6322 – structures in this space group and matching this description have been reported. A second acceptable tilting pattern has been found, leading to a structure inP6/mmmbut on a larger `2 × 2 × 2' unit cell – however, no observations of this structure have been reported. For TTB, a search at the special points of the Brillouin zones revealed only one comparable tilting pattern, in a structure with space-group symmetryI4/mon a `21/2 × 21/2by 2' unit cell. Given several literature reports of larger unit cells for TTB, we conducted a limited search along the lines of symmetry and found structures with acceptable tilt patterns inBbmmon a `21/22 × 21/2 × 2' unit cell. A non-centrosymmetric version has been reported in niobates, inBbm2 on the same unit cell.

Tuning of the crystal engineering and photoelectrochemical properties of crystalline tungsten oxide for optoelectronic device applications

WO₃crystals with {002} or {111} facets primarily exposed, WO₃films with dominant orientations, doping and heterostructuring are highlighted.