Structural phase transitions in deuterated ammonium hexachloroplumbate(IV), (ND4)(2)PbCl6, studied by Cl-35 nuclear quadrupole resonance

The Cl-35 NQR frequency and the spin-lattice and spin-spin relaxation times were measured as functions of temperature, in order to study the structural phase transitions in deuterated ammonium hexachloroplumbate(IV) (ND4)(2)PbCl6. It was shown that the phase transition at 81 K is of a second-order type with an overdamped soft mode, while the deuteration-induced transition at 38 K is of first order. Three NQR frequencies (17.99, 17.86 and 16.35 MHz at about 15 K) were observed below 38 K suggesting an orthorhombic symmetry in the deuteration-induced low-temperature phase. The averaged value of these frequencies almost coincides with the frequency extrapolated from that (17.348 MHz at 39 K) above 38 K. The splitting (about 1.6 MHz) of the frequency below 38 K is quite large compared with that (0.45 MHz) of (ND4)(2)PtCl6, which shows a similar phase transition induced by deuteration at 27.5 K. This indicates a large distortion of the crystal lattice in the present compound.