Determination and experimental verification of high-temperature SAW orientations on langatate

Langatate (LGT) is a member of the langasite family of crystals appropriate for high-temperature frequency control and sensing applications. This paper identifies multiple LGT SAW orientations for use at high temperature, specifically in the 400 to 900°C range. Orientations with low sensitivity to temperature are desired for frequency control devices and many sensors, conversely large temperature sensitivity is a benefit for temperature sensors. The LGT SAW temperature behavior has been calculated along orientation searches, sweeping the Euler angles (0°, Θ, ψ), (90°, Θ, ψ), and (Φ, 90°, ψ). The temperature coefficient of delay (TCD) and total frequency change over the temperature range were analyzed from 400 to 900°C. Multiple SAW orientations were found with zero-TCD between 400 and 500°C, but no orientations were identified with turn-over temperatures above 500°C. Other orientations have a low variation, -0.8%, in frequency over the range from 400 to 800°C. Additionally, temperature-sensitive orientations were identified with TCD up to 75 ppm/°C at 900°C, for use as temperature sensors at high temperature. The predictions are shown to agree with measured behavior of LGT SAW delay lines fabricated along 6 orientations in the (90°, 23°, "F) plane.