First-Principles Calculation of Lattice Dynamics of Skutterudite Compounds

Lattice dynamics of La-filled skutterudite compounds, LaT 4 X 12 (T = Fe, Ru, and Os, and X = P, As, and Sb), has been investigated from first-principles calculations. The calculations show that distortions of the cage comprised of 12 pnictogens are important for La vibrations. The distortions decrease the energies of the La vibrations, and become large from the phosphorous compounds to the antimonide compounds. The La vibrations in the antimonide compounds can affect electronic states through the accompanying distortions. Although the La vibrations display no sign of Kohn anomaly, three pnictogen vibrations indicate Kohn anomaly, i.e., they show anomalous decrease of their energies around the wavevector q =(1, 0, 0) in a few compounds. One of the modes is non-degenerate mode, and the other two are degenerate. If the non-degenerate one becomes unstable, the resulting structure has the space group Pm 3 . The structure is the same as the low-temperature structure in PrRu 4 P 12 .