Perovskite tungsten bronzes Phase

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

Introduction of sodium into empty 12-coordinate sites of the WO3 lattice results in a series of oxide bronzes. These have general formula NaxWOa, where 0.0sodium tungsten bronzes retain the monoclinic structure of the parent WO3 and they are n-type semiconductors [276,277]. With increasing Na content the structure evolves through two distinct tetragonal phases and for x>0.43 the bronzes adopt an essentially cubic perovskite structure [278] closely related to that of ReOs (fig. 18). The Na 3s levels lie about 10 eV above the bottom of the W 5d bands and each added Na atom is therefore ionised to Na, with donation of one electron into the W 5d band of local t2g symmetry [279]. For x values of less than 0.26, the 5d electrons are localised, probably by an interplay between polaronic effects, disorder... 

The stability of the perovskite structure depends on Goldschmidt s tolerance factor (rA +ro)/> (rB +ro), with 0.7 < t < 1.2. For tolerance factors outside this range nonperovskite structure are stable, including pyrochlore-type structures A2B2X6Z, AuriviUius phases (AuriviUius, 1949), or tetragonal tungsten bronze (TTB)-type structures Ba2MTi2X30i5 (M = Ln,Bi X = Nb,Ta) (Stennett et al., 2005). 

Geusic J.E., Kurts S.K., Van Uitert L.G., Wemple S.H. Electrooptic properties of some ABO3 perovskite in the paraelectric phase. Appl. Phys. Lett. 1964 4 141-143 Giess E.A., Scott B.A., Burns G., O kane D.F., Segmuller A. Alkali strontiirm-barium-lead niobate systems with a tungsten bronze structure Crystallographic properties and Curie points. J. Am. Ceram. Soc. 1969 52 276-281... 

---