Dramatic softening of the negative thermal expansion material HfW2O8 upon heating through its WO4 orientational order-disorder phase transition

HfW2O8 undergoes a dramatic softening where the average bulk modulus (P = 52-414MPa) drops from 69GPa at 298K to 48GPa at 430K as the temperature of the WO4 orientation order-disorder transition is approached. This is accompanied by increasingly negative thermal expansivity (-10ppm . K-1 to -15ppm . K-1) and reversible WO4 orientational disordering upon compression in alpha-HfW2O8. Additionally, alpha-HfW2O8 becomes elastically softer upon compression at constant temperature. The alpha -> beta phase transition temperature decreases by similar to 30K between 52 and 414MPa. Above this phase transition, no further temperature-dependent softening or pressure-dependent changes in the coefficient of thermal expansion occurred. (C) 2014 AIP Publishing LLC.