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First‐principles study of phonon‐mode softening under pressure: the case of GaN and AlN

The origin of the mode softening under pressure of the low‐frequency E2 phonon of GaN and AlN is investigated by first‐principles density‐functional calculations. Using the eigenvectors of these modes as obtained from diagonalizing a dynamical matrix which is calculated using density‐functional perturbation theory, a frozen‐phonon calculation shows that the softening is mainly caused by an increased destabilization due to the Ewald energy, which is nearly counterbalanced by the other contributions to the total energy in the case of AlN, but only partially so for GaN.