Anomalous variations in the volume of Fe69Ni31 Invar alloys under high pressure and temperature

We have investigated the variations in the crystal structure of Fe(69)Ni(31) Invar alloys under high pressures below 6.5 GPa and high temperatures below 1300 K using energy dispersive x-ray diffraction. The crystal structure is stabilized as a face-centered-cubic (fcc) lattice structure over the entire investigated range. However, the volume of the fcc in Fe(69)Ni(31) shows an abrupt decrease at 1300 K above 2 GPa and at 1100 K above 3 GPa with increasing pressure. These results show that a pressure-induced phase transition to a low-volume state occurs under high-temperature and high-pressure conditions. We consider this transition would be that from the high-spin state to the low-spin state, which has been speculated in some previous theoretical studies.