Lattice dynamics and structural phase transitions in perovskite-type layer compounds. III. Quasi-elastic neutron scattering study of the cation reorientations in (CD₃NH₃)₂MnCl₄

The three disordered phases THT (tetragonal high temperature), ORT (orthorhombic room temperature) and TLT (tetragonal low temperature) of (CD 3 NH 3 ) 2 MnCl 4 have been studied by quasi-elastic neutron scattering in order to follow the reorientational dynamics of the cation CD 3 NH 3 + . Structural information has been obtained on the geometry of the motions from the incoherent structure factor measured up to Q=2.5 AA -1 for two orientations of the crystal. In particular, it has been confirmed that the cation keeps an orthorhombic configuration in the TLT phase. Furthermore, the more probable reorientational process in all three phases occurs by 90 degrees jumps about the c axis. The corresponding rate constant decreases from about 7.1*10 11 jumps s -1 at 399K (THT) to 3*10 11 jumps s -1 at room temperature (ORT) and to 1.8*10 11 jumps s -1 at 245K (TLT). The other kinds of reorientational motions are slower by a factor of the order of ten. From these data a direct evaluation of the probabilities of the fourth orthorhombic configurations of the cation is possible.