Suppression of Ferroelectric Transitions by Symmetry Trapping in Dion–Jacobson-Layered Perovskites Cs(La,Nd)Nb₂O₇

CsNdNb₂O₇ is a hybrid improper ferroelectric (HIF) where oxygen octahedral rotation modes induce spontaneous polarization. While CsLaNb₂O₇ is also believed to be an HIF, its polar structure and ferroelectric properties remain poorly understood. This study investigates their solid solution, CsLa_1-<i>x</i>Nd_<i>x</i>Nb₂O₇, by constructing a temperature-composition phase diagram using synchrotron X-ray diffractometry, first-principles calculations, and dielectric measurements. Upon cooling, the Nd-rich compositions (<i>x</i> = 0.5, 0.75, and 1) undergo phase transitions from a <i>P</i>4/<i>mmm</i> to a <i>C</i>2/<i>m</i> and then to a polar <i>P</i>2₁<i>am</i> phase. Both transition temperatures decrease as x decreases. The polar <i>P</i>2₁<i>am</i> phase, accompanied by ferroelectric polarization switching, is stabilized at room temperature only when <i>x</i> = 0.75 and 1. For La-rich compositions, the nonpolar <i>C</i>2/<i>m</i> phase remains down to room temperature without exhibiting any indication of transformation into the polar <i>P</i>2₁<i>am</i> phase. Theoretical calculations suggest that a symmetry trapping effect hinders the nonpolar (<i>C</i>2/<i>m</i>)-to-polar (<i>P</i>2₁<i>am</i>) phase transition.