Conductivity and complex permittivity of langatate at high temperature up to 900°C

There are a large number of high-temperature sensing and frequency control applications that can be addressed using acoustic wave devices capable of operation at high-temperatures. For those applications, it is important to characterize the acoustic properties of the piezoelectric crystal used as substrate at elevated temperatures. Langatate (LGT) is one of the crystals which allow the fabrication of SAW devices at elevated temperatures. In a previous work, the authors measured and discussed the LGT elastic constants up to 900°C. This paper reports the langatate complex dielectric permittivity and conductivity from 25 to 900°C. The constants were extracted from impedance measurements of parallel-plate capacitors fabricated with Pt/Rh/ZrO₂ electrodes on LGT wafers aligned along the X and Z crystalline axes. The real permittivities, ε\´₁₁ and ε"₃₃, were found to change significantly in the range from 25 to 900°C with a 38% increase and a 49% decrease of their room temperature values, respectively. Thus, it is important to include the extracted high temperature permittivities when designing LGT acoustic wave devices and not simply to use extrapolated low temperature data. Both LGT conductivity and imaginary permittivity are necessary to quantify the electrical losses of sensors, signal-processing, and frequency-control devices operating with this substrate at high-temperatures.