The phases of the antifluorite deuterated-ammonium palladium hexachloride, (ND4)(2)PdCl6

A structural phase transition occurs at T-c = 30.2 +/- 0.1 K in the deuterated antifluorite (ND4)(2)PdCl6, but not in its protonated analogue, and shows the largest transition entropy of all the deuterated ammonium hexachlorides. The structure of the antifluorite (ND4)(2)PdCl6 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 ND4 molecule. The space group of the compound at 5 K is shown to be P2(1)/n. The major change in the structure during this isotopically induced transition is a large rotation of the ND4 ions, with relatively little change in the orientation of the PdCl6 units. A comparison to previous calorimetric and inelastic neutron scattering studies of the compound leads to the conclusion that the ND4 ions dominate the transition entropy and inelastic spectrum of (ND4)(2)PdCl6. (C) 1997 Academic Press.