Surface and pseudo surface acoustic waves in langatate

Langatate (LGT, La3Ga5.5Ta0.5O14) is a recent addition to new materials of the trigonal crystal class 32, which also include langasite, langanite, and gallium phosphate. These materials belong to the same crystal class as quartz and all have promising quartz-like characteristics with respect to temperature. They have higher electromechanical coupling coefficients that will lead to higher bandwidth devices for surface and bulk acoustic wave (SAW and BAW) applications. These materials present higher densities and permittivities than quartz, with resulting SAW phase velocities values which are 13 to 25% smaller than that of ST-X quartz. While such a reduction in the phase velocity may pose a problem for higher frequency applications, due to the increased difficulty in the actual device fabrication, repeatability, yield, and ultimately cost, it would extend the use of SAWs to lower frequencies. Pseudo surface acoustic waves (PSAWs) are a possible solution to the high frequency problem, since both the PSAW and the high velocity or HVPSAW have phase velocities which are 40 to 100% higher than that of the SAW. In the present work SAW contour plots of the phase velocity, the electromechanical coupling coefficient, and the power flow angle, are given showing the orientations in space where high coupling are obtained, with the corresponding power flow angles and phase velocity characteristics for these orientations. For the PSAW and HVPSAW modes, calculations are presented for these types of waves for some selected rotated cuts. The present study shows that propagation losses for the PSAWs of about 0.01 dB/wavelength, and phase velocities approximately 20% higher than the SAW, are observed along specific orientations planes for the PSAW, thus providing for higher frequency surface acoustic wave device applications.