Study on the structure and surface acoustic wave properties of SNGS and STGS piezoelectric crystals

The geometry structure, band structure and density of states of Sr3NbGa3Si2O14 (SNGS) and Sr3TaGa3Si2O14 (STGS) single crystals were investigated by using density functional theory (DFT) method. Parameters including the phase velocity, electromechanical coupling factor and power flow angle (PFA) were calculated for SNGS and STGS at X, Y, Z cuts, respectively. Our calculated data are in good agreement with the experimental results. It is found that the peaks of partial density of states of Sr, Ga, Si and O atoms of STGS have a tendency of shifting to the higher energy levels relative to those of SNGS. Compared with quartz, SNGS and STGS crystals are of lower phase velocities and higher electromechanical coupling factor. The propagation direction of 167.5° of X-cut SNGS, 0° of Y-cut SNGS and 170° of X-cut STGS are found to be optimum direction for SAW device applications.

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► The optimized geometry structure of SNGS and STGS crystals were studied by using the first principles pseudo-potential method. The calculated lattice constants and interatomic distances are in good agreement with the experimental results.
► The band structure and density of states were investigated by using first principles method. Both SNGS and STGS crystals are indirect-gap materials and the indirect band gap are 0.62117 eV, 3.55223 eV, respectively.
► The cuts types are indicated by the Euler angles (φ, θ, χ), and the effect of propagation direction χ on the SAW properties is investigated for the χ varying from 0 to 180°.
► From the theoretical calculation, it was forecasted that the propagation direction of 167.5° of X-cut SNGS, 0° of Y-cut SNGS and 170° of X-cut STGS possess superior SAW performance.