Improper Inversion Symmetry Breaking and Piezoelectricity through Oxygen Octahedral Rotations in Layered Perovskite Family, LiRTiO₄ (R = Rare Earths)

An improper mechanism for breaking inversion symmetry is revealed and thus inducing piezoelectricity in the family of layered perovskites, LiRTiO₄ (R = rare earths), which are previously reported as centrosymmetric. Noncentrosymmetry in this family of compounds arises from TiO₆ octahedral rotation represented by a⁻b^oc^o/b^oa⁻c^o in the perovskite blocks between RO rock salt and LiO antifluorite layers. X-ray diffraction and optical second harmonic generation complemented by density functional theory predictions are crucial in determining the new structures. High transition temperature (T_ac) of up to 1200 K from noncentrosymmetric to centrosymmetric phase is observed. Piezoelectric coefficients (d₃₆) of up to −15 pC/N are predicted, and piezoelectric force microscopy experiments confirm a piezoelectric response. The demonstrated improper mechanism in this and other layered oxide families, with a wide range of available topologies and chemistries, could aid in the search for high temperature piezoelectrics.