AB INITIO STUDY OF THE ELECTRONIC STRUCTURE AND PHONONDISPERSIONS FOR TiH2 AND ZrH2

摘要

Pseudo-potential band-structure and phonon-structure calculations arecarried out for the fluorite-type and tetragonal phases of TiH2 and ZrH2. Band structures,densities of states, total energy and phonon dispersion curves are computed. The total-energy curves as functions of the c/a ratio show two minima for c/a < 1 and c/a > 1.The results show that the Jahn-Teller splitting is responsible for the tetragonal distortion ofthe dihydrides in agreement with previous interpretations. Soft acoustic phonon modesin the long-wave range are observed in the calculated phonon dispersion curves of thecubic phases, pointing to the structural instability with respect to tetragonal distortion.Although the experimental findings show that the cubic-to-tetragonal phase transitionin TiH2 at 310 K is of the second order, the calculated characteristics clearly indicatethe first-order transition that is close to the second-order one.