Variable-range-hopping transport in ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ (0<x<0.35)

Resistivity measurements on semiconducting single crystals and thin films of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ (x<0.35) grown by molecular-beam epitaxy follow the Mott variable-range-hopping law ρ=${\mathrm{\rho }}_0$exp{(${\mathit{T}}_0$/T${)}^{1/4}$}. A single crystal with x=0.13 was measured over 8 orders of magnitude in resistivity at temperatures between 50 K and room temperature. The values of ${\mathit{T}}_0$ are (3–6)×${10}^8$ K, implying strong localization. Near the insulator-metal transition, the ${\mathit{T}}_0$’s depend on the stoichiometry and the agreement with the Mott law breaks down. The transport is discussed in the context of the electronic and structural phase diagram of the barium potassium bismuthate system. In particular, we find no evidence of the nonlinear transport which might be expected if incommensurate charge-density waves were the origin of the nonmetallicity of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ (0<x<0.35).