Heat capacity and order–disorder phase transition in negative thermal expansion compound ZrW 2O 8

Abstract

Heat capacity of ZrW 2O 8 was measured in the (1.8 to 483) K temperature range by using two adiabatic and a commercial relaxation calorimeters. Standard thermodynamic functions of ZrW 2O 8 were estimated from the measured molar heat capacity. A large λ-type anomaly of heat capacity due to an order–disorder phase transition was observed at T=(440±0.5) K. The enthalpy and entropy of phase transition were determined to be (1.56 ± 0.01) kJ · mol −1 and (4.09 ± 0.03) J · K −1 · mol −1, respectively. The magnitude of the entropy of transition supports the order–disorder mechanism previously suggested by the present authors.