166 Er Mössbauer and X-ray diffraction study of ErMn 2 hydrides

Abstract

The pressure-composition isotherm at 22 °C for ErMn 2H x between 1 and 10 4 Torr shows the formation of two hydride phases with compositions x = 4.6 and x = 4.0 and with equilibrium hydrogen pressures of 15 Torr and less than 1 Torr respectively. Both hydrides are expanded versions of the C14 hexagonal Laves structure of ErMn 2 with volume expansions of 23% and 37% for the lower and higher order phases respectively. A slight preferential expansion parallel to the c 0 axis is observed. The 166 Er Mössbauer results show that the magnetic moment of erbium in both hydride phases is below the free-ion value measured for ErMn 2. The ferromagnetic ordering present in ErMn 2 and ErMn 2H 4 is absent in ErMn 2H 4.6 down to 1.5 K. Two erbium sites are detected in ErMn 2H 4 instead of the one expected for the C14 structure. The origin of these sites is discussed in terms of possible configurations of hydrogen site occupancy.