AB-INITIO ELASTICITY AND LATTICE-DYNAMICS OF AGGASE2

The structural, elastic and vibrational properties of the ternary semi conductor AgGaSe2 chalcopyrite are studied using the pseudopotential m ethod within the local density approximation. The lattice constants an d internal parameter are in good agreement with experiments. The elast ic moduli derived from the stresses generated by small strains compare favourably with experiments and the material is elastically anisotrop ic. All zone-centre phonons are determined by diagonalization of the d ynamical matrix derived from the forces generated by small ionic displ acements. The calculated phonon frequencies are in excellent agreement with experiment and can be grouped into three bands. Two silent Gamma (2) modes are also obtained. The eigenvectors are used to discuss the three-band phonon spectrum and the spectroscopic activity of the modes .