Atomistic configurational effects on piezoelectric properties of La3Ta0.5Ga5.5O14 and a new piezoelectric crystal design

Single crystalline langatate (LTG, La3Ta0.5Ga5.5O14) has been widely used as a sensor material in high temperature applications because of its thermally stable piezoelectric properties. In this research, to elucidate the relationship between piezoelectric tensors and local ionic configurations, first-principles calculations based on density functional perturbation theory (DFPT) were performed on various local ionic structures. The results indicate that two independent relaxed-ion piezoelectric coefficients, e11 and e14, increased with increases in La(3e)-O and Ta(1a)-O distances or decreases in Ga(3f,2d)-O distances. Thus, to obtain high piezoelectric constants in this crystal, ions larger than La3+ should be incorporated at 3e sites to open the distance between 3e ions and oxygen ions, and ions smaller than Ga3+ should be introduced at 2d and 3f sites to reduce the distance between Ga and O ions. Finally, from this design rule, a new crystal, BTAS (Ba3TaAl3Si2O14), which belongs to the same P321 group, is proposed. The calculated relaxed-ion piezoelectric coefficient e11 of BTAS was 17.7% higher than the coefficient of a LTG crystal. This significant increase confirms BTAS as a useful new piezo-material, especially in applications where there is also a need to reduce the use of more expensive elements.