Reaction of hydrogen with the high temperature (C14) form of TiCr 2

Abstract

At −78 °C the high temperature (C14) form of the intermetallic compound TiCr 2 will react directly and reversibly with hydrogen to form two non-stoichiometric hydride phases having the nominal compositions of TiCr 1.9H 2.5 and TiCr 1.9H 3.5. The lower hydride phase is more stable than the higher. Standard free energies of formation of these phases at −78 °C are − 1.29 ± 0.04 and + 2.76 ± 0.02 kJ (g atom H 2 2) −1 with corresponding plateau pressures of approximately 0.2 and 30 atm. Relative partial molal thermodynamic quantities are also given for several other hydrogen compositions. At room temperature pressure-composition isotherms and X-ray diffraction data indicate the absence of any hydride phases: the starting (C14) intermetallic simply undergoes a lattice expansion with increasing hydrogen content without phase or structural changes. The critical temperature for the first miscibility gap region is approximately +15−20 ° C. Hysteresis is also absent in this intermetallic-hydrogen system.