The isotope effect and phase transitions in ammonium hexachloropalladate studied by neutron tunnelling spectroscopy

Above 30% deuteration, ammonium hexachloropalladate undergoes the phase tra nsition Fm(3)m --> P2(1)/n, with T-c increasing to 30.2 K for pure (ND4)(2) PdCl6. Rotational tunnelling of NH4 and NH3D was used to study this phase t ransition. The transition energies are related to the tunnelling matrix ele ments h(i). NH3D can De described as a one-dimensional rotor, with the one- dimensional matrix elements being almost exactly the averages of the three- dimensional ones. In the high-temperature phase the intensities of the resp ective tunnelling transitions confirm a statistical occurrence probability of the isomers. In the low-temperature phase the strength of the rotational potential increases by about 30%. A broadening of the NH3D tunnelling line with decreasing temperature above the crystallographic phase transition is interpreted as an intermediate glass phase which evolves due to long-range dipolar coupling combined with positional disorder.