Anisotropic electronic properties of the diphosphate tungsten bronzes K₂P₈W₂₄O₈₈, K₂P₈W₂₈O₁₀₀ and their substituted compounds

Electronic properties of single crystals of K₂P₈W₂₄O₈₈ and K₂P₈W₂₈O₁₀₀, the sixth and seventh members of the potassium diphosphate tungsten bronze series K_x(P₂O₄)₄(WO₃)_4m, were studied. Temperature-dependent resistivity measurements on oriented single crystals reveal quasi-two-dimensional metallic behavior with room-temperature resistivities of ρ{variant}_a* = 5.4 × 10^-3, ρ{variant}_b = 1.2 × 10^-4, and ρ{variant}_c* = 3.9 × 10^-4 Ω cm in K₂P₈W₂₄O₈₈ and ρ{variant}_a* = 8.9 × 10^-3, ρ{variant}_b = 1.5 × 10^-4, and ρ{variant}_c* = 3.4 × 10^-4 Ω cm in K₂P₈W₂₈O₁₀₀. Magnetic susceptibility measurements indicate Pauli paramagnetic behavior. In agreement with the above results band electronic calculations on a W₂₄O₈₄ slab of Rb_1.8P₈W₂₄O₈₈ (isostructural with K₂P₈W₂₄O₈₈) and a W₂₈O₉₆ slab found in Rb_1.74P₈W₂₈O₁₀₀ show that the Fermi level cuts both one- and two-dimensional bands, and the conductivity is greater along the b axis. Solid-state insertion of Co²⁺, Fe²⁺, and Sn²⁺ ions into K₂P₈W₂₄O₈₈ and partial substitution of K⁺ by alkali metal ions Li⁺, Na⁺, and Rb⁺, respectively, in K₂P₈W₂₄O₈₈ lead to an enhancement of the conductivity and an increase in the extent of the anisotropy. For K_xA_yP₈W₂₄O₈₈ with A = Li and Na, single crystals form with the monophosphate tungsten bronze structure with pentagonal tunnels rather than the diphosphate tungsten bronze structure with hexagonal cavities as observed for the host compound. Partial substitution of W by Mo in K₂P₈W₂₄O₈₈ results in the intergrowth of K₂P₈W_24-xMo_xO₈₈ and K₂P₈W_28-xMo_xO₁₀₀. The electronic properties of the Mo-substituted phases change gradually from metal to semiconductor with increasing Mo content.