Dielectric behaviour and thermal expansions of ZnTiF6.6H2O/6D2O and MnTiF6.6H2O/6D2O crystals

Abstract

Measurements made using a capacitance bridge of the dielectric constants epsilon b and epsilon c along the b and c directions of the hydrate and deuterate single crystals, and of the linear thermal expansion ( Delta l/l) of the hydrate crystals in the a, b and c directions, in the temperature range 150-300K, are reported and the results shown graphically. During cooling epsilon b and epsilon c for the Zn hydrate crystals undergo a sudden fall of 10% and 5% respectively at about 194K, and the deuterate crystals similarly at about 205K. For the Mn hydrate crystals epsilon undergoes an abrupt fall at about 255K, but for the deuterate only a kink occurs at about 235K. During cooling 103 ( Delta l/l) changes abruptly by 13.4, 1.2 and -6.2 at 182K for the Zn crystals and by 4.9, 9.4 and -8.2 for the Mn crystals at 245K, in the a, b and c directions respectively. It is shown that the sudden decrease occurring in epsilon may be explained theoretically by assuming a coupling between the order parameter for the crystals and the square of the applied field.