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

Publisher Website
Google Scholar
Abstract
Above 30% deuteration, ammonium hexachloropalladate undergoes the phase transition Fm3m P21/n, with Tc increasing to 30.2 K for pure (ND4)2PdCl6. Rotational tunnelling of NH4 and NH3D was used to study this phase transition. The transition energies are related to the tunnelling matrix elements hi.NH3D can be 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 respective 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.