CRITICAL EXPONENTS AND RANDOMNESS IN SRTIO3CA

Since its discovery, the SrTiO3:Ca system is known to exhibit a number of features which were thought to arise from an impurity induced disorder. Non-linear dielectric measurements are used to obtain a more quantitative insight into this disorder. For 0.001 < x(Ca) < 0.05, it is found that the non-linear susceptibility diverges at low temperatures. This is similar to what was previously reported in the dielectric random system KTaO3:Na. A method is proposed to quantify the contribution of the disorder to the non-linearities. It is found that the deviation from a true ferroelectric behaviour is not enough to call the low-temperature phase of SrTiO3:Ca a glass one. The maximum non-linearity observed for x(Ca) = 0.003 and 0.004 decreases for higher x(Ca) contents. It is shown that a simple model of temperature dependent dipole moment may account for both the linear and non-linear dielectric behaviour of SrTiO3:Ca.