INELASTIC NEUTRON-SCATTERING INVESTIGATION OF EXTERNAL-MODES IN INCOMMENSURATE AND COMMENSURATE A2BX4 MATERIALS

Inelastic neutron scattering experiments in K2SeO4, Cs2SeO4 and K2CrO4 are reported. The lowest SIGMA3-SIGMA2 phonon branches along the a*-axis in their Pnam phase have been characterized for the three compounds. The results are compared with recent lattice dynamical simulations using rigid-ion models. Potassium chromate, that simulations suggested to be incommensurate at low temperatures, evidences no structural phase transition down to 12 K, but its lowest SIGMA2 phonon branch exhibits a significative softening and decreases more than 20% as the temperature is lowered from 300 K to 36 K. The softening mechanism in this compound and in potassium selenate is related to an increase of the effective interaction between an acoustic and an optical branch.