Dramatic softening of the negative thermal expansion material HfW ₂O₈ upon heating through its WO₄ orientational order-disorder phase transition

HfW₂O₈ undergoes a dramatic softening where the average bulk modulus (P = 52-414 MPa) drops from 69 GPa at 298 K to 48 GPa at 430 K as the temperature of the WO₄ orientation order-disorder transition is approached. This is accompanied by increasingly negative thermal expansivity (-10 ppm·K-1 to -15 ppm·K-1) and reversible WO ₄ orientational disordering upon compression in α-HfW ₂O₈. Additionally, α-HfW₂O₈ becomes elastically softer upon compression at constant temperature. The α→β phase transition temperature decreases by ∼30 K between 52 and 414 MPa. Above this phase transition, no further temperature-dependent softening or pressure-dependent changes in the coefficient of thermal expansion occurred.