Abstract
A Raman study of single-crystal hexa-ammine nickel (II) chloride between 20 and 300K is presented. A static distortion of the lattice is observed at Tc=76+or-5K but the evidence suggests that proton orientational ordering begins before the lattice distortion occurs and continues down to T0 with Tc-T0 approximately 8K. A critical exponent of (0.48+or-0.03) is obtained for the growth of the light scattering intensity from internal modes associated with ammonia vibration as the proton groups order. The spectra obtained are consistent with a doubling of the primitive cell to a distorted D3d space group below Tc.