Alkali-Metal-Mediated Phase Transition Behavior and Photoluminescence Property in Tb ³⁺ -Based Hybrid Double Perovskite-Type Ferroelastics

The rational control of the phase transition temperature (Ttr) remains a critical challenge in the development of stimuli-responsive hybrid materials. Herein, we demonstrate that alkali-metal ion engineering in two-dimensional hybrid rare-earth double perovskites, (HQ)4[MTb(NO3)8] (HQ = quinuclidine cation, M = Rb+, 1; Cs+, 2), enables a finely tuned in Ttr. Notably, the Ttr changes accordingly from 345 K for 1 to 381 K for 2. This dramatic shift is quantitatively rationalized through Hirshfeld surface analysis, which reveals that the larger Cs+ enhances the intermolecular interaction network, significantly increasing the proportion of strong H···O contacts from 49.0% to 51.6%. The resulting reinforcement of the hydrogen-bonding network elevates the energy barrier for the concerted order–disorder transition of the HQs and the inorganic framework. This mechanism is corroborated by the direct observation of reversible ferroelastic domain switching via polarized light microscopy. Furthermore, the local coordination geometry of the Tb3+ centers is preserved upon alkali-metal ion substitution, not only maintaining the characteristic green emission and long fluorescence lifetime but also improving the quantum yield. This work establishes alkali-metal ions engineering as an effective strategy for tailoring phase transition behavior and optical performance in hybrid multifunctional materials.