Figure 2.

Typical examples of the phase transition for FeH_x in the heating and the cooling paths. Colored symbols indicate the characteristic peaks of each observed phase (red squares: fcc FeH_x, green circles: dhcp FeH_x, blue diamonds: bcc Fe, black asterisks: NaCl). The left column shows a typical example of the phase transition in the heating path from a single bcc Fe phase through the two phases of bcc Fe and fcc FeH_x, and to single fcc FeH_x. The single bcc Fe phase was observed up to 4.0 GPa, 975 K. The fcc FeH_x phase appeared at 4.0 GPa, 1000 K with bcc Fe phase, and the bcc Fe disappeared at 3.8 GPa, 1075 K. The right column shows a typical example of phase transition in the cooling path from the single fcc FeH_x phase to the two phases of fcc FeH_x and dhcp FeH_x. The single fcc FeH_x phase at 8.8 GPa, 1100 K was gradually cooled to 750 K, as shown in Fig. 1. The single fcc FeH_x phase was observed up to 8.0 GPa, 850 K and dhcp FeH_x clearly appeared at 7.8 GPa, 750 K. The neutron diffraction patterns for phase observation were obtained in each P-T condition by 5–10 min keeping time and 5–10 min exposure time after increasing or decreasing the temperature.