Heat capacity, structural and thermodynamic properties of synthetic klockmannite CuSe at temperatures from 5 K to 652.7 K. Enthalpy of decomposition

The heat capacity of CuSe has been determined by adiabatic shield calorimetry from T approximate to 5 K to T = (652.7 +/- 0.1) K. At the upper temperature, CuSe disproportionates into Cu2-xSe and a liquid rich in Se. The low temperature hexagonal superstructure, which is based on the covellite-type structure, changes to an orthorhombic structure at T = (326.8 +/- 0.1) K. A transition back to hexagonal structure of the covellite-type occurs at T approximate to 410 K. The positions of the atoms in the two high-temperature modifications are determined by powder neutron and X-ray diffraction, and the temperature-induced changes in the atomic arrangements are discussed. The thermodynamic properties have been evaluated and the values of molar heat capacity, entropy, and enthalpy at T = 298.15 K and T = 652.7 K are: [GRAPHICS] The molar enthalpy of transition of CuSe with hexagonal superstructure to the orthorhombic structure is Delta(trs)H(m)(o) (T = 326.8 K) = (855.3 +/- 2.0) J . mol(-1), and the enthalpy of decomposition is Delta(dec)H(m)(o) (T = 652.7 K) = (1 1800 +/- 30) J . mol(-1). (C) 1996 Academic Press Limited.