The generalised phonon density of states of (CH3NH3)2MnCl4: experiments and model calculations

Abstract
The generalised phonon density of states has been studied by neutron inelastic scattering from powder samples of (CH3NH3)2MnCl4 at 4.2, 77, 195, 300 and 405K for the frequency range of the external modes of these molecular crystals. The experimental results are compared with lattice dynamical calculations employing a rigid-ion model. The effects of anharmonicity are partly taken into account by including a finite phonon linewidth increasing with frequency and with temperature in the calculations. The results show that the main peak in the generalised phonon density states is due to modes with a high portion of rotational motion of the molecules which exhibit a rather drastic frequency shift with increasing temperature. At 300K, a completely deuterated sample was also studied for comparison.