Study of phonon dispersion relations in forsterite, Mg2SiO4 by inelastic neutron scattering

Abstract

Abstract Phonon dispersion relations in forsterite, Mg2SiO4 (orthorhombic, a = 10.190, b = 5.978, c = 5.753 A , space group PnmaZ = 4) have been determined under ambient conditions using inelastic neutron scattering (INS). Nine acoustic and seventeen optic branches have been measured along the three principal symmetry directions in the Brillouin zone. Seven zone center phonons have been observed with frequencies 104, 144, 184, 192, 200, 258, 315, and 325 cm−1, which are in good agreement with Raman and infra red measurements, except the lowest frequency phonon at 104 cm−1, which is Raman and IR inactive and has been observed for the first time in INS. In order to select suitable wave vector transfers for phonon measurements, the one-phonon neutron structure factors predicted from a lattice-dynamical rigid ion molecular model were used as guides for the INS experiment. The measured dispersion relations agree very well with those predicted from a rigid ‘molecular’ ion model, where the tetrahedral [SiO4] unit is assumed to be a rigid group. The elastic constants determined from the slopes of the acoustic branches show good agreement with those measured by ultrasonic techniques.