Specific Heat, Optical, and Transport Properties of Hexagonal Tungsten Bronzes

Abstract

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.

Research supported by U.S. Air Force Office of Scientific Research, Office of Aerospace Research, under Grant No. AFOSR 68-1510C.