Structural frustration effects by mixed alkali ions in ferroelectric Dion–Jacobson layered perovskites (Cs,Rb)NdNb2O7

Abstract

Two Dion–Jacobson (DJ) double-layered perovskites CsNdNb₂O₇ and RbNdNb₂O₇ exhibit hybrid improper ferroelectricity, in which spontaneous polarization is induced by coupling of two types of nonpolar oxygen octahedral rotations (OORs). Their crystal structures adopt common polar distortion modes, although they exhibit distinct OOR patterns. In this study, complete solid-solution Cs_1−xRb_xNdNb₂O₇ ceramics (x = 0, 0.25, 0.5, 0.75, and 1) have been synthesized by conventional solid-state reaction methods, and the temperature-composition phase diagram has been established by various methods including variable-temperature synchrotron X-ray diffractometry, differential scanning calorimetry, optical second harmonic generation, and dielectric permittivity measurements. The powder X-ray diffractometry reveals that Cs-rich compounds (x ≤ 0.25) crystalize into polar P2₁am structures with a⁻a⁻c⁺/a⁻a⁻c⁺ OORs at room temperature, while Rb-rich compounds (x ≥ 0.5) adopt polar I2cm structures with a⁻a⁻c⁺/−(a⁻a⁻c⁺) OORs. All the solid solutions exhibit ferroelectricity at room temperature. Upon heating, successive phase transitions involving the loss of polar distortions and OORs have been observed. The phase transition temperatures of the solid solutions with intermediate compositions are lower than those predicted based on Vegard's law, indicating structural frustration effects due to the random arrangement of two alkali metal ions. This work provides insight into the influence of alkali metal ions in the intervening layers on the structural and ferroelectric properties of DJ layered perovskites.