  The effect of high pressures to 40 GPa on the crystal structure and lattice
dynamics of AlB2 was studied by synchrotron x-ray powder diffraction, Raman
spectroscopy, and first-principles calculations. There are no indications for a
pressure-induced structural phase transition. The Raman spectra of the metallic
sample exhibit a well-defined peak near 980 cm^{-1} at 0 GPa which can be
attributed to the Raman-active E_{2g} zone-center phonon. Al deficiency of ~11%
in AlB2, as indicated by the x-ray data, changes qualitatively the electronic
structure, and there are indications that it may have a sizable effect on the
pressure dependence of the E_{2g} phonon frequency. Similar changes of the
pressure dependence of phonon frequencies, caused by non-stoichiometry, are
proposed as an explanation for the unusually large variation of the pressure
dependence of T_c for different samples of MgB2.
