Specific Heat, Optical, and Transport Properties of Hexagonal Tungsten Bronzes
The tungsten bronzes are a series of nonstoichimetric compounds of the general formula M x WO3, where M is usually an alkali metal atom. In this paper the cubic and hexagonal crystal structures are compared. Each structure is composed of linked WO6 octahedra, with M atoms filling the available cavities. In the cubic structure the WO6 matrix has rectangular channels in which the M atom is located. The cubic structure has all M sites full when x = 1. The hexagonal structure is composed of three- and six-member rings of WO6 octahedra. The M-atom sites are located in the hexagonal channels, with x = 1/3 corresponding to all sites full. The radii of the inscribed spheres of the cavities available for occupation by the M atoms are 0.96 Å and 2 Å in the cubic and hexagonal structures, respectively. The smaller M ions such as Na+ (0.95 Å) adopt the cubic structure for 0.5 ≲ x ≲ 1, whereas K+ (1.33 Å) and Rb+ (1.47 Å) form hexagonal bronzes.
KeywordsHeat Capacity Tungsten Bronze Plasma Edge Alkali Metal Atom Hexagonal Crystal Structure
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