Anisotropic glass freezing in KDP-type dipole glass and dielectric properties of low-dimensional hydrogen-bonded crystals

Abstract

Anisotropic glass freezing has been studied by dielectric constant measurement and remanent polarization measurement in the dipole glass systems of rubidium/ammonium phosphate RADP(x) and its deuterated analogue DRADP(x), where x denotes the ammonium concentration. The glass freezing temperatures for a-cut and c-cut crystals as measured from the temperatures of the dielectric loss peaks show clear anisotropy which depends on the degree of deuteration and ammonium mole concentration x. While the protonated sample of RADP(0.43) showed negligible anisotropy between the two axes, the deuterated samples DRADP(0.40) showed a considerable anisotropy at low frequency as large as 7.2K. A higher glass freezing temperature was observed along the c-axis as compared with the a-axis freezing temperature in DRADP(0.40). Among the three deuterated DRADP crystals, anisotropy in glass freezing was observed to be large in the deuterated dipole glass samples DRADP(0.40) whereas very small in the same system but a larger ammonium molar ratio, DRADP(0.59). DRADP(0.47) showed an intermediate strength of anisotropy between theses two crystals. This result conforms with the prediction of the cluster model theory. The origin of the anisotropy in the cluster model theory can be traced to the difference between the two effective order parameters, i.e., the local polarization autocorrelation functions along c- and a-axes. Possible differences between these two effective order parameters may originate from the more frequent local cluster formations of ferroelectric-type Slater units for x ＜ 0.5 or antiferroelectric-type Slater units for x ＞ 0.5. Pyroelectric current measurement also shows anisotropy along the two crystal axes which is very similar to the result of the dielecric constant measurement. A large anisotropy, that is, a large difference between the two peak positions of $dP_R/dT$ for the two a and c axes was observed in the ferroelectric side(x ＜ 0.5) DRADP-x (x=0.40) dipole glas...