Deuteron spin-lattice relaxation in partly and fully deuterated [formula omitted

Abstract

Deuteron spin-lattice relaxation and spectra were studied in partially and fully deuterated ( NH 4 ) 2 PdCl 6 in the temperature range 5–300 K. The relaxation rate maximum was observed at 45 K in ( ND 4 ) 2 PdCl 6 . 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 ND 4 + 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 ( ND 4 ) 2 PdCl 6 . The increase is related to (1) the absence of the order–disorder phase transition and (2) to the enhanced mobility of NH 3 D + 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 ( ND 4 ) 2 PdCl 6 , but approaching them with increasing deuteration. This variation can be explained with different mobilities and varying relative numbers of the various isotopomers NH 4 - n D n + , n = 1 – 4 .