A CRYSTALLOGRAPHIC STUDY OF ORTHORHOMBIC TO CUBIC PHASE TRANSITION IN YTTERBIUM DIHYDRIDE

The crystal structures of ytterbium and calcium dihydrides were investigated at temperatures from 25 ° to 600 °C by X-ray diffraction. The gradual transition of ytterbium dihydride from orthorhombic to face-centered cubic, beginning at 125 °C, was found. The kinetics of the transition were studied at 200 ° and 250 °C. The two cubic phases of Warf and Hardcastle, of lattice constants 5.192 ± 0.004 A ̊ and 5·253 ± 0.015 A ̊, were confirmed. The latter was definitely metastable; the former was stable at room temperature for the duration of the investigation. Surface oxidation at and above 400 °C prevented investigation of a similar expected transition in calcium hydride. Coefficients of expansion and volume changes on transition were calculated. The role of polarization in the transition is discussed. © 1968.