T-P phase diagrams and isotope effects in the Mo-H/D systems

Lines of formation and decomposition of the hcp(epsilon) molybdenum deuteride in the T-P phase diagram of the Mo-D system are constructed via the measurement of isotherms of the electrical resistance of molybdenum in a deuterium atmosphere at temperatures up to 550degreesC and pressures up to 6 GPa. The diagram is compared to that of the Mo-H system studied earlier. The pressure of phase equilibrium in both systems is shown to be much closer to the decomposition pressure than to the formation pressure. A new explanation is suggested for this asymmetry in the hysteresis loop that seems to be characteristic of many metal-hydrogen systems. From the position of the decomposition lines, the standard entropy, enthalpy and Gibbs energy of formation are estimated for molybdenum deuteride and hydride.