Suppression\nof Ferroelectric Transitions by Symmetry\nTrapping in Dion–Jacobson-Layered Perovskites Cs(La,Nd)Nb<sub>2</sub>O<sub>7</sub>

CsNdNb<sub>2</sub>O<sub>7</sub> is\na hybrid improper\nferroelectric\n(HIF) where oxygen octahedral rotation modes induce spontaneous polarization.\nWhile CsLaNb<sub>2</sub>O<sub>7</sub> is also believed to be an HIF,\nits polar structure and ferroelectric properties remain poorly understood.\nThis study investigates their solid solution, CsLa<sub>1–<i>x</i></sub>Nd<sub><i>x</i></sub>Nb<sub>2</sub>O<sub>7</sub>, by constructing a temperature–composition phase diagram\nusing synchrotron X-ray diffractometry, first-principles calculations,\nand dielectric measurements. Upon cooling, the Nd-rich compositions\n(<i>x</i> = 0.5, 0.75, and 1) undergo phase transitions\nfrom 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>2<sub>1</sub><i>am</i> phase. Both transition temperatures decrease\nas x decreases. The polar <i>P</i>2<sub>1</sub><i>am</i> phase, accompanied by ferroelectric polarization switching, is stabilized\nat room temperature only when <i>x</i> = 0.75 and 1. For\nLa-rich compositions, the nonpolar <i>C</i>2/<i>m</i> phase remains down to room temperature without exhibiting any indication\nof transformation into the polar <i>P</i>2<sub>1</sub><i>am</i> phase. Theoretical calculations suggest that a symmetry\ntrapping effect hinders the nonpolar (<i>C</i>2/<i>m</i>)-to-polar (<i>P</i>2<sub>1</sub><i>am</i>) phase transition.