Anomalous variations in the volume ofFe69Ni31Invar alloys under high pressure and temperature

We have investigated the variations in the crystal structure of ${\mathrm{Fe}}_{69}{\mathrm{Ni}}_{31}$ Invar alloys under high pressures below $6.5\mathrm{GPa}$ and high temperatures below $1300\mathrm{K}$ 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 ${\mathrm{Fe}}_{69}{\mathrm{Ni}}_{31}$ shows an abrupt decrease at $1300\mathrm{K}$ above $2\mathrm{GPa}$ and at $1100\mathrm{K}$ above $3\mathrm{GPa}$ 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.