Bond-bending modes and stability of tetrahedral semiconductors under high pressure: a puzzle of AlN

Lattice vibrations of the wurtzite-type AlN have been studied by Raman spectroscopy under high pressure up to the structural phase transition at 20 GPa. We have shown that the widely debated bond-bending E_2^1 mode of w-AlN has an abnormal positive pressure shift up to the threshold of the phase transition, whereas in many tetrahedral semiconductors the bond-bending modes soften on compression. This finding disagrees with the results of ab initio calculations, which give a "normal" negative pressure shift. Combination of high dynamical and low thermodynamical stability of AlN breaks the correlation between the mode Gruneisen parameters for the bond-bending modes and the transition pressure, which holds for CdS, InP, ZnO, ZnTe, ZnSe, ZnS, Ge, Si, GaP, GaN, SiC and BeO.