ANISOTROPIC GLASS FREEZING IN RUBIDIUM AMMONIUM DIHYDROGEN PHOSPHATE MIXED-CRYSTAL AND ITS DEUTERATED ANALOG/

Anisotropic glass freezing has been studied via dielectric measurement s in the dipole glass systems of rubidium/ammonium dihydrogen phosphat e RADP(x) and its perdeuterated analogue D-RADP(x), where x denotes th e ammonium concentration. The glass freezing temperatures for a-cut an d c-cut crystals are measured, and found to reveal very small anisotro py in the protonated sample of RADP(0.43) but a considerable anisotrop y (similar to 7.2 K) at low frequency in the deuterated sample of D-RA DP(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 theo ry 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.