Heat capacity and other thermodynamic properties of hcp-CrH and hcp-CrD

Polycrystalline samples of chromium hydride CrH and deuteride CrD with a hexagonal close-packed ( hcp ) metal lattice were synthesized at a hydrogen (deuterium) pressure of 7.4 GPa and a temperature of 873 K, rapidly cooled to 100 K, recovered to ambient pressure and studied by differential scanning calorimetry at temperatures from 120 to 220–240 K. The calorimetric study and density functional theory calculations helped to determine the isobaric heat capacity C P of hcp -CrH at temperatures up to 1000 K and to clarify the shape and position of its acoustic phonon band and the coefficient of electronic heat capacity. The investigation of hcp -CrD demonstrated the approximately harmonic change in the energy of its optical vibrations compared with hcp -CrH. Using the obtained C P ( T ) dependence of hcp -CrH for the calculation of its Gibbs energy gave the line of the Cr + (1/2)H 2 CrH equilibrium in the T-P diagram of the Cr–H system. The position of this line corroborated the debatable opinion that the pressure of thermodynamic equilibrium is much closer to the pressure of hydride decomposition than to the pressure of its formation. • CrH and CrD are synthesized under high hydrogen (deuterium) pressure. • Isobaric heat capacities of CrH and CrD are measured with differential scanning calorimetry. • Standard Gibbs free energies of CrH and CrD are calculated. • Pressure-temperature phase diagram of the Cr–H system is constructed at T = 0–1000 K.