High Solubility of Hetero-Valence Ion (Cu²⁺) for Reducing Phase Transition and Thermal Expansion of ZrV_1.6P_0.4O₇

Large thermal expansion at room temperature and high phase transition temperature of ZrV₂O₇ limit its practical applications and are reduced by the high solubility of hetero-valence ion (Cu²⁺) on the basis of an equal-valence substitution of P⁵⁺ for V⁵⁺. The temperature-dependent Raman spectra show that Zr_0.9Cu_0.1V_1.6P_0.4O_6.9 maintains a normal parent cubic structure till 173 K and transforms to a 3×3×3 cubic superstructure below 173 K. Temperature dependent x-ray diffraction patterns of Zr_0.9Cu_0.1V_1.6P_0.4O_6.9 show near zero and negative thermal expansion. High solubility of lower valence Cu²⁺ relates to an equal-valence substitution of smaller P⁵⁺ for V⁵⁺, which extends the bond angle of V(P)–O–V in ZrV_1.6P_0.4O₇ close to 180°. The change of microstructure is considered to be responsible for reduced phase transition temperature and thermal expansion.