Deuteron spin-lattice relaxation in partly and fully deuterated (NH4)2PdCl6

Deuteron spin-lattice relaxation and spectra were studied in partially and fully deuterated (NH4)2PdCl6 in the temperature range 5-300 K. The relaxation rate maximum was observed at 45 K in (ND4)2PdCl6. Its value is reduced due to limited jumps by about 33% relative to the theoretical value expected for threefold reorientations. Limited jumps correspond to an N-D vector jumping between six directions on a cone around a Pd-N vector, the angle between the N-D and Pd-N vectors being denoted Δ. This motion makes a part of the quadrupole interaction ineffective in relaxation thus reducing the maximum rate at 45 K. The observed reduction leads to the value Δ=21∘. Limited jumps are quenched to a large extent at the order-disorder phase transition and consequently a decrease is observed in the rate. Below the transition ND4+ ions reorient between the tetrahedral orientations of the ordered phase, therefore the quadrupole interaction has the full relaxing efficiency. In the 10% deuterated sample the temperature of the rate maximum is shifted to 35 K and below 20 K the rate itself is one order of magnitude larger than in (ND4)2PdCl6. The increase is related to (1) the absence of the order-disorder phase transition and (2) to the enhanced mobility of NH3D+ because of its electric dipole moment. Limited jumps are claimed to be the dominant relaxation mechanism below 20 K. The relaxation in the disordered 30% deuterated sample is quite similar to that in 10% sample. The 50% and 70% deuterated samples undergo a transition to the ordered phase. The relaxation is biexponential with the characteristic rates somewhat smaller than those in (ND4)2PdCl6, but approaching them with increasing deuteration. This variation can be explained with different mobilities and varying relative numbers of the various isotopomers NH4-nDn+, n=1-4.