Tetrahedral rotations in the alkaline-earth metal orthovanadates

The alkaline-earth metal orthovanadate Sr3(VO4)2 with the palmierite structure is reported to host a dielectric anomaly as well as a structural phase transition above the room temperature. With V5+ ions and tetrahedral oxygen coordination, the crystal structure of this compound is not studied in detail from first principles yet. In this work, we perform a detailed analysis of the crystal structure and instabilities of M3(VO4)2 (M = Ca, Sr, Ba) orthovanadates with the palmierite structure using first-principles density functional theory. We find that as the M2+ cation size decreases, a significant structural distortion that changes the symmetry from R3 over bar m to C2/c emerges. This change is accompanied with a rotation of the oxygen tetrahedra. Our calculations also indicate that the polar instability in these compounds is suppressed by these tetrahedral rotations.