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 de uterated antifluorite (ND4)(2)PdCl6, but not in its protonated analogu e, and shows the largest transition entropy of all the deuterated ammo nium hexachlorides. The structure of the antifluorite (ND4)(2)PdCl6 ha s been studied at room temperature and at 5 K using neutron powder dif fraction. The data taken at room temperature show strong evidence for disorder of the ND4 molecule. The space group of the compound at 5 K i s shown to be P2(1)/n. The major change in the structure during this i sotopically induced transition is a large rotation of the ND4 ions, wi th relatively little change in the orientation of the PdCl6 units. A c omparison to previous calorimetric and inelastic neutron scattering st udies of the compound leads to the conclusion that the ND4 ions domina te the transition entropy and inelastic spectrum of (ND4)(2)PdCl6. (C) 1997 Academic Press.