Abstract
Heat capacity and electric conductivity of NbO 2 doped with zirconium and molybdenum were measured simultaneously by a direct-heating pulse method between 450 and 1150 K. A λ-type peak in the heat capacity and rapid increase of the electric conductivity due to phase transition were observed above 1000 K. The transition temperature of Nb 1− x Zr x O 2 decreased rapidly with increasing x, while that of Nb 1− x Mo x O 2 decreased slightly. The change of the transition temperature with mole fraction of dopant supports the assumption that this transition is caused by the change of cation-cation bonding. The excess entropy due to the phase transition decreased rapidly in Nb 1− x Zr x O 2 with increasing x, while it increased slightly in Nb 1− x Mo x O 2. The excess entropy is closely related to the magnitude of the jump in electric conductivity at the transition temperature. The dependence of the excess entropy on x was discussed in terms of the electronic band structure.
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