Structures, lattice tungsten bronze

Molybdenum and Tungsten Bronzes.—The results of the majority of studies of studies of new bronze phases are summarized in Table 9. Sno.2W03 and Sno.3 WO3 have been prepared and their crystal structures determined. Whether they are bronzes is debatable since they do not involve a host lattice of WO ... 

The highest transition temperature for the "tungsten bronze" family was 7.7 K for an acid-etched (71) sample of composition Rb o g3W03. Certain researchers (62), after completing studies on cubic and tetragonai-II (semiconducting) bronzes, made the statement "It appears as though the (cubic) perovskite lattice is not favorable for superconductivity." This statement was made in 1965, prior to the major advances in copper oxides that are considered to have a related-perovskite structure. 

M = V. Cr. Fe. Ga. Ti. Nb. or W X = F or F + O x = 0.35—0.55 0 < y < 1) have been obtained by heating mixtures of the corresponding fluorides and oxides. These materials all crystallize with the hexagonal tungsten bronze structure and their lattice constants have been determined.98... 

The conductivity of H—NiF3 is roughly 1 x 10 6(f).cm) 1 in contrast ofR—NiF3 which is an insulator [47], It is assumed that the conductivity associated with H—NiF3 is due to mobility of the K+ ions in the lattice of the hexagonal tungsten bronze 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 sodium tungsten bronzes already described are examples of incomplete lattice defect structures. Iron(II) oxide is rather similar it has a rock-salt structure but is always deficient in iron. Some Fe ions are always present to maintain electrical neutrality. Fe304 has the spinel structure which has the same arrangement of ions as FeO. Fe203. has also the same arrangement and oxidation of FeO to FcgOg consists of the replacement of Fe2+ ions by two thirds of their number of Fe ions. 

Tungsten bronzes are inert materials M WOy (0 < x < 1) with defect perovskite structures Figure 5.23). Their colour depends on x golden for x a 0.9, red for x 0.6, violet for x a 0.3. Bronzes with x > 0.25 exhibit metallic conductivity owing to a band-like structure associated with W(V) and W(VI) centres in the lattice those with x < 0.25 are semiconductors (see Section 5.8). Similar compounds are formed by Mo, Ti and V. ... 

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