Two New Non-centrosymmetric n = 3 Layered Dion–Jacobson Perovskites: Polar RbBi₂Ti₂NbO₁₀ and Nonpolar CsBi₂Ti₂TaO₁₀

Two new non-centrosymmetric (NCS) n = 3 layered perovskites, RbBi₂Ti₂NbO₁₀ and CsBi₂Ti₂TaO₁₀, have been synthesized through high temperature solid state reactions. X-ray diffraction analyses suggest that RbBi₂Ti₂NbO₁₀ crystallizes in the orthorhombic polar space group, Ima2 (No. 46), whereas CsBi₂Ti₂TaO₁₀ crystallizes in the tetragonal nonpolar space group, P4̅ (No. 81). Interestingly, CsBi₂Ti₂TaO₁₀ is the first Dion–Jacobson (DJ)-type layered perovskite with a NCS nonpolar space group. Powder second-harmonic generation (SHG) measurements with 1064 nm radiation reveal that both of the reported materials have SHG efficiencies of approximately 100 times that of α-SiO₂ and are phase-matchable (type I). Piezoelectricity measurements suggest that the estimated d₃₃ values for polar RbBi₂Ti₂NbO₁₀ and nonpolar CsBi₂Ti₂TaO₁₀ are 170 and 67 pm V^–1, respectively. The net polarization direction arising from the constituent distorted polyhedra and the key component causing microscopic deformations in NCS structure are introduced. Polarization measurements are also presented along with other characterizations such as the UV–vis diffuse reflectance, and infrared spectra, elemental analyses, and electronic structure calculations for the reported materials.