Crystal structure and properties of perovskite-type rubidium niobate, a high-pressure phase of RbNbO₃

We synthesized a perovskite-type RbNbO₃ at 1173 K and 4 GPa from non-perovskite RbNbO₃ and investigated its crystal structure and properties towards ferroelectric material design. Single-crystal X-ray diffraction analysis revealed an orthorhombic cell in the perovskite-type structure (space group Amm2, no. 38) with a = 3.9937(2) Å, b = 5.8217(3) Å, and c = 5.8647(2) Å. This non-centrosymmetric space group is the same as the ferroelectric BaTiO₃ and KNbO₃ but with enhanced distortion. Structural transition from orthorhombic to two successive tetragonal phases (Tetra1 at 493 K, Tetra2 at 573 K) was observed, maintaining the perovskite framework before reverting to the triclinic ambient phase at 693 K, with no structural changes between 4 and 300 K. The first transition is similar to that of KNbO₃, whereas the second to Tetra2, marked by c-axis elongation and a significant c_p/a_p ratio jump (from 1.07 to 1.43), is unique. This distortion suggests a transition similar to that of PbVO₃, where an octahedron's oxygen separates along the c-axis, forming a pyramid. Ab initio calculations simulating negative pressure like thermal expansion predicted this phase transition (c_p/a_p = 1.47 at -1.2 GPa), aligning with experimental findings. Thermal analysis revealed two endothermic peaks, with the second transition entailing a greater enthalpy change and volume alteration. Strong second harmonic generation signals were observed across Ortho, Tetra1, and Tetra2 phases, similar to BaTiO₃ and KNbO₃. Permittivity increased during the first transition, although the second transition's effects were limited by thermal expansion-induced bulk sample collapse. Perovskite-type RbNbO₃ emerges as a promising ferroelectric material.