Anisotropic glass freezing in rubidium/ammonium dihydrogen phosphate mixed crystal and its deuterated analogue

Abstract

Anisotropic glass freezing has been studied via dielectric measurements in the dipole glass systems of rubidium/ammonium dihydrogen phosphate RADP(x) and its perdeuterated analogue D-RADP(x), where x denotes the ammonium concentration. The glass freezing temperatures for a-cut and c-cut crystals are measured, and found to reveal very small anisotropy in the protonated sample of RADP(0.43) but a considerable anisotropy (similar to 7.2 K) at low frequency in the deuterated sample of D-RADP(0.40). A higher glass freezing temperature was observed along the c-axis as compared with the a-axis freezing temperature for D-RADP(0.40) crystal. This conforms with the prediction of the cluster model theory for RADP(x < 0.5). An Arrhenius plot of the logarithmic relaxation frequency versus the reciprocal temperature for D-RADP(0.40) gave the deuteron activation energies Delta E-a similar or equal to 11.5 kJ mol(-1) along the a-axis and Delta E-c similar or equal to 13.5 kJ mol(-1) along the c-axis.