Phase transition of Zr1-xHfxv2O 7 solid solutions having negative thermal expansion

Abstract

The phase transition of Zr1-xHfxV2O 7 (x = 0-1)solid solutions substituted for Zr(IV) sites was investigated using X-ray diffraction, TMA and DSC methods. X-ray diffraction experiments revealed that all compositions of Zr1-xHf xV2O7 (x = 0-1) were of a single phase having a cubic structure. Lattice parameters of Zr1-xHfxV 2O7 (x = 0-1) at room temperature decreased linearly with increasing Hf content, due to the smaller ionic radius of the Hf (IV) ion than that of the Zr(IV) ion. Abrupt positive thermal expansions in TMA were observed for all samples at two phase transition temperatures in the vicinity of T 1 = 350 K and T2 = 375 K, and negative thermal expansions were seen above about 380 K. DSC measurements clearly showed that all of the Zr1-xHfxV2O7 (x = 0-1) samples also underwent two successive phase transitions between 330 and 390 K. Two transition temperatures and phase transition enthalpies of Zr1-xHf xV2O7 (x = 0-1) samples decreased with the increase of x content, reflecting the decrease of superlattice structure. The sum of transition entropy for successive two phase transitions increased with increasing the normalized relative intensity of the superlattice line at room temperature.