Lattice Dynamical Properties of the Rare-Earth Dihydrides XH2 (X=Sc, Y, and La)

The rare-earth hydrides have received tremendous attention due to their potential applications for hydrogen storage technology, they exhibit intriguing electronic, optical, and mechanical properties. In particular, XH2 series (X= Sc, Y, and La), the prototype for rare-earth dihydrides, are very attractive due to their excellent properties: high pressure stability, mechanical strength, favourable kinetics feature, small neutron absorption cross-section, and dramatic optical properties. At ambient conditions, XH2 (X= Sc, Y, and La) crystallize in the cubic fluorite structure with a Fm3m space group, typically having a fcc lattice where the tetrahedral sites are filled with hydrogen atoms. The theoretical investigations of phonon properties of XH2 are scarce as these properties are very helpful to explore the nature of XH2. Besides the structural aspects, the study of their vibrational properties is also important as they are closely related to several fundamental properties such as inter atomic interaction, phonon spectra, specific heat etc. Therefore, in this work a de launey angular force constant model has been used to study the phonons in XH2 (X= Sc, Y, and La) by considering the interatomic interactions up to third nearest neighbor. It is found that the ranges of phonon frequencies for acoustic and optical modes decrease gradually from ScH2 to LaH2. The calculated results are compared and analyzed with available experimental results.