Pressure-dependence of the phase transitions and thermal expansion in zirconium and hafnium pyrovanadate

Abstract Low or negative thermal expansion (NTE) has been previously observed in members of the ZrP 2 O 7 family at temperatures higher than their order-disorder phase transitions. The thermoelastic properties and phase behavior of the low temperature superstructure and high temperature negative thermal expansion phases of ZrV 2 O 7 and HfV 2 O 7 were explored via in situ variable temperature/pressure powder x-ray diffraction measurements. The phase transition temperatures of ZrV 2 O 7 and HfV 2 O 7 exhibited a very strong dependence on pressure (∼700 K GPa), with moderate compression suppressing the formation of their NTE phases below 513 K. Compression also reduced the magnitude of the coefficients of thermal expansion in both the positive and negative thermal expansion phases. Additionally, the high temperature NTE phase of ZrV 2 O 7 was found to be twice as stiff as the low temperature positive thermal expansion superstructure (24 and 12 GPa respectively).