Calorimetric and x-ray diffraction studies of α-to-β structural phase transitions inHfW2O8andZrW2O8

Abstract

A powder x-ray diffraction experiment was performed on cubic ${\mathrm{HfW}}_{2}{\mathrm{O}}_{8}$ and ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ from 90 to 560 K. The lattice parameter and absolute negative thermal expansion coefficient of ${\mathrm{HfW}}_{2}{\mathrm{O}}_{8}$ are smaller than those of ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}.$ The heat capacity of ${\mathrm{HfW}}_{2}{\mathrm{O}}_{8}$ was measured from 340 to 520 K using an adiabatic scanning calorimeter. An anomaly in heat capacity of ${\mathrm{HfW}}_{2}{\mathrm{O}}_{8}$ corresponding to the \ensuremath{\alpha}-to-\ensuremath{\beta} structural phase transition was clearly detected around 463 K. The phase transition temperature of ${\mathrm{HfW}}_{2}{\mathrm{O}}_{8}$ by the calorimeter is 26 K higher than that of ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}.$ The entropy of transition of ${\mathrm{HfW}}_{2}{\mathrm{O}}_{8}$ is the same magnitude as that of ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8},$ indicating that the mechanism of the phase transition of ${\mathrm{HfW}}_{2}{\mathrm{O}}_{8}$ and ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is the same.