Structural, electronic, elastic, and piezoelectric properties of α -quartz and MX O4 ( M=Al, Ga, Fe; X=P, As) isomorph compounds: A DFT study

We report the structural, electronic, elastic, and piezoelectric properties of some α -quartz SiO2 isotypes, namely, AlPO4, GaPO4, GaAsO4, and FePO4. This family of crystals is well known for its elastic and piezoelectric properties related to their M O4 and X O4 tetrahedral units, especially the M-O-X bridging angle θ and the tilt angle δ, the most distorted structures being expected to exhibit the highest piezoelectric coupling constant. We have then computed the optimized structure of each MX O4 isomorph compound in order to study the variation in the elastic and piezoelectric tensors with respect to θ and δ. A comparison between our results at the density-functional theory level and the available data (theoretical and experimental ones) has been made. The differences observed for the whole class of systems has been discussed and a comparison with the SiO2 α -quartz behavior is made.