  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.
