Dielectric Relaxation of Rochelle Salt

Abstract

The complex dielectric constant of Rochelle salt has been measured at frequencies from 2.5 to 13 GHz, and at 155 MHz. The measurements were made in a temperature range from -50 to 45°C which includes the ferroelectric region and significant parts of both paraelectric regions. Measurements were made both with and without dc biases. The dielectric constant is found to undergo a relaxation of a strictly Debye character. Over the entire temperature range, the relaxation time is found to be proportional to $T^{1.25}$ times the difference between the low-frequency, clamped, differential dielectric constant and its limiting high-frequency value. This result is shown to be consistent with a model of the high-frequency dielectric behavior based on the Kubo susceptibility formalism. Following Mitsui, it is assumed that the ferroelectric properties of Rochelle salt arise from the rotation of hydroxyl groups within the tartrate molecules. The basic relaxation process probably involves phonon-induced transitions between the lowest states of these hydroxyl ions. However, the cooperative dipolar interaction between ions reduces the over-all relaxation rate of the coupled system. An approximate model Hamiltonian of the coupled hydroxyl system is constructed in terms of the Pauli matrices and is used to calculate an explicit expression for the relaxation rate.