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

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