The open-circuit voltage (OCV) was measured using a high impedance electrometer (Keithley 6514) having input impedance > 200TΩ for voltage measurements and also charge measurements down to 10fC to avoid any current leakage and minimize the meter contribution to the sample resistance. The actual sample temperature was measured using a K-type thermocouple (Omega) and a precision multimeter (Keithley 2000) with 0.001 °C resolution. In each series of experiments, the samples were kept at 1000 °C for 100 hours for possible degradation studies.