Observation of the ferroelectric phase transition in Pb5Ge3O11 by the chemical potential changes

Abstract

We observed the transition from the ferroelectric (FE) to paraelectric (PE) phase in the semiconducting, ferroelectric Pb5Ge3O11 single crystal with the use of the contact electrode method. To this purpose a thin, metallic layer was placed onto the Pb5Ge3O11 crystal surface, forming the contact electrode. At opposite ends of the contact electrode, silver wires were glued and a voltage was applied to the contact electrode in such a way that the electric current could flow only through the attached electrode. The electric resistance R(T) of the electrode was measured as a function of temperature. Two series of measurements were performed. In one of them the ferroelectric c-axis of the investigated crystal was perpendicular to the contact electrode. In the second one the c-axis was parallel to the attached electrode. We used gold as the contact electrode material. The anomaly in the R(T) in a form of a kink at T kink = 452 K was found for both c-axis orientations. The measured value of T kink, appearing in the temperature dependence of contact electrode resistance, corresponds exactly to the phase transition temperature T C from the FE to PE phase of the investigated Pb5Ge3O11 material. This result demonstrates that the contact electrode method, primarily proposed exclusively to find critical temperatures of metallic samples, also works well in the case of ferroelectric and semiconducting materials like Pb5Ge3O11. We ascribe the effect of the resistance kink in the temperature dependence of the contact electrode R(T) to thermal excitations of the electrons with different rates below and above T C due to different electronic activation energies in the FE and PE phases of the investigated Pb5Ge3O11 crystal. It, however, means that the phase transition in the electronic subsystem of the Pb5Ge3O11 transfers into the electron gas of the contact electrode via the chemical potential relation µ sample = µ electrode due to the contact between the sample and the electrode. The magnitude of the kink, observed in the R(T) dependence, was higher on heating than on cooling. The additional measurement of the thermally stimulated current (TSC) was carried out on the non-polarised Pb5Ge3O11 sample. In this series of measurements, the sample was covered with gold layers sputtered on the two opposite surfaces of the crystal. The TSC anomaly occurred, related to the residual pyroelectric effect, several degrees below the Curie temperature, T C, and does not disturb the detection of the critical point with the use of the contact electrode method.