Thermal expansion and ionic conductivity of K₅Pb_0,5Zr_1,5 (MoO₄)₆

The present study is aimed at the directed synthesis of a new ternary molybdate K₅Pb_0.5Zr_1.5(MoO₄)₆ isostructural to K₅Pb_0.5Hf_1.5(MoO₄)₆ by solid-phase reaction within the temperature range of 350–550 °С (for 100 h). The compound crystallizes in the rhombohedral system with the space group R3̅  and unit cell parameters of a = 10.6604(2) Å, c = 37.9769(9) Å, and V = 3737.6(2) ų. The structure was refined using the Rietveld method. The atomic positions were refined in the isotropic approximation, with soft constraints on Mo–O distances and O–Mo–O bond angles. The crystal structure constitutes a 3D scaffold comprising PbO₆ and ZrO₆ octahedrons and MoO₄ tetrahedrons sharing oxygen vertices. The thermal expansion of K₅Pb_0.5Zr_1.5(MoO₄)₆ was studied via high-temperature X-ray powder diffraction. The calculated thermal expansion coefficients along both crystallographic axes remain positive over the entire temperature range. In this case, the value of α_a remains constant, while that of α_c increases with rising temperature. The obtained ternary molybdate belongs to materials with high thermal expansion (α_V = 60×10^-6 °С^-1). The significant anisotropy in the crystallographic direction c can be attributed to the soft K–O and Pb-O bonds. The electrical conductivity of K₅Pb_0.5Zr_1.5(MoO₄)₆ was studied via impedance spectroscopy within the temperature range of 30–500 °С; at 500 °С, the conductivity amounted to 0.7×10^-4 S/cm, with E_a = 0.59 eV.

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