NEUTRON-SCATTERING STUDY OF THE REORIENTATIONAL MOTIONS FOR NH3 GROUPS IN (NH3(CH2)3NH3)MNCL4

Incoherent quasi-elastic neutron scattering experiments have been performed on oriented crystals and on polycrystalline samples of (NH//3(CH//2)//3NH//3)MnCl//4 and (NH//3(CD//2)//3NH//3)MnCl//4 compounds in the 4-300 K temperature range. Backscattering and time of flight spectrometers were used: spectra were obtained using different resolutions (1-170 mu eV) and a large range of momentum transfers (0. 17-2. 57 A** minus **1). It is concluded that NH//3 groups reorientate below 300 K; the experimental spectra have been satisfactorily fitted with a model of 2 pi /3 jumps of the NH//3 groups, associated with a correlation time of 1 multiplied by 10** minus **1**1 s at 300 K and an activation energy roughly equal to 27 kJ/mol. Finally a comparison with literature data, mainly from NMR and Raman results, is presented.