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 A 2 WO 4 -WO 3 ( A = K, Rb) were restudied and the average monoclinic layered structures of stoichiometric polytungstates A 4 W 11 O 35 ( A = K, Rb, Cs, Tl) and A 2 W 7 O 22 ( A = K, Rb, Cs) were first successfully determined. The structures resemble those of "MoW 11 O 36 " and "MoW 14 O 45 " (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 A 4 W 11 O 35 ( A = Cs, Tl) and A 2 W 7 O 22 ( 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- x M 3 n +2 X 9 n +8 where n = 2, 3 and 4 are equal to the numbers of HTB hexagonal tunnels across the polytungstate slab width for Tl 2 W 4 O 13 , A 4 W 11 O 35 and A 2 W 7 O 22 ( A = K, Rb, Cs or Tl), respectively. The structures of the HTB-like polytungstates seem to intergrow with HTB-type A x WO 3 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.