First-principles calculations of lattice vibrations on LaT2Zn20 (T Ru and Ir)

Lattice vibrations of LaRu2Zn20, and LaIr2Zn20 have been investigated by the local density approximation. Although these compounds have a cage structure in which a La atom is surrounded by Zn, the calculated results show that La vibrations have large dispersion and relatively high frequencies. In both compounds, it is found that the vibrations of Zn at 16c site in the plane perpendicular to the three-fold axis have very low frequencies and can be unstable. This result suggests that the transitions in LaRu2Zn20 at 150 K and in LaIr2Zn20 at 200 K are caused by these unstable modes.