The Phases of the Antifluorite Deuterated-Ammonium Palladium Hexachloride, (ND4)2PdCl6

Abstract A structural phase transition occurs at T c =30.2±0.1 K in the deuterated antifluorite (ND 4 ) 2 PdCl 6 , but not in its protonated analogue, and shows the largest transition entropy of all the deuterated ammonium hexachlorides. The structure of the antifluorite (ND 4 ) 2 PdCl 6 has been studied at room temperature and at 5 K using neutron powder diffraction. The data taken at room temperature show strong evidence for disorder of the ND 4 molecule. The space group of the compound at 5 K is shown to be P 2 1 / n . The major change in the structure during this isotopically induced transition is a large rotation of the ND 4 ions, with relatively little change in the orientation of the PdCl 6 units. A comparison to previous calorimetric and inelastic neutron scattering studies of the compound leads to the conclusion that the ND 4 ions dominate the transition entropy and inelastic spectrum of (ND 4 ) 2 PdCl 6 .