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Report No.

Tetragonal distortion and successive disproportionation reaction of fcc lanthanum dihydride under high pressure

Machida, Akihiko; Hattori, Takanori   ; Honda, Mitsunori*; Sano, Asami   ; Watanuki, Tetsu; Katayama, Yoshinori; Aoki, Katsutoshi; Komatsu, Kazuki*; Arima, Hiroshi*; Oshita, Hidetoshi*; Ikeda, Kazutaka*; Suzuya, Kentaro; Otomo, Toshiya*; Tsubota, Masami*; Ichikawa, Takayuki*; Kojima, Yoshitsugu*; Fujihisa, Hiroshi*; Kim, D. Y.*

We have investigated the structural properties of rare-earth metal hydrides under high pressure. LaH$$_2$$ has the CaF$$_2$$ type structure in which the H atoms locate at the tetrahedral interstitial sites (T-sites) of the fcc metal lattice. Synchrotron radiation X-ray diffraction and infrared reflection experiments revealed disproportionation reaction of LaH$$_2$$ into the H-poor and H-rich phases around 11 GPa. Before the disproportionation reaction, we have found that the fcc metal lattice transformed into tetragonal lattice. The ordering of the H atoms in the octahedral-sites (O-sites) causes the tetragonal distortion of LaH$$_{2+delta}$$. The tetragonal transformation and successive disproportionation reaction of LaH$$_2$$ would closely relate to the inter-site transfer of the H atoms between the T- and O-sites. We have performed the neutron diffraction experiments of LaD$$_2$$ to investigate the change of the positions and occupancies of the hydrogen atoms under high pressure.



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