High pressure studies of the FCC chromium hydride

The equilibrium pressure of hydrogen gas above solid face centered cubic (fcc) phase of chromium hydride has been determined at 150 degrees C. As compared to the hex agonal close packed (hcp), chromium hydride plateau for the alpha + beta phase region has been found to be lower by about 500 bars and equal to 0.25 +/- 0.01 GPa. As a consequence, lower value of standard free energy of formation of the fee chromium hydride has been calculated and a lower equilibrium pressure at normal temperature as well. One can also assume a lower hydrogen pressure necessary for a direct synthesis of the fcc hydride, as compared to the hcp phase. The search for possible pressure induced phase transition from the fcc to the hcp structure has been unsuccessful up to 37 GPa. The compressibility of the fcc hydride is similar to that of the hcp phase, showing pressure independence of the hydrogen volume in the metal lattice, and confirming the general rule observed in transition metal hydrides.