Vibrational spectroscopy studies of a pressure-induced disproportionation reaction of LaH2

The pressure-induced disproportionation reaction of LaH2 was investigated by infrared reflection and Raman measurements at ambient temperature. The relative reflection intensity in the 4000–6000 cm−1 region began to decrease significantly at a pressure of about 12 GPa and fell to 10% of the initial value at 20 GPa. Absorption peaks, which appeared around 1200 and 700 cm−1 at pressures above 14 GPa, were assigned to the hydrogen vibrations at the tetrahedral and octahedral sites of the fcc metal lattice, respectively. The peak frequencies measured in the 14–30 GPa range were similar to those observed in LaH3. These infrared results indicated insulating LaH3 precipitated from metallic LaH2. Above 20 GPa, a Raman peak related to the hydrogen vibration in the octahedral sites appeared around 700 cm−1, and was likely due to lattice distortion. The disproportionation reaction of LaH2 into LaH 2 + δ and solid solution LaHx ( x < 1 ) was confirmed.