B'-site NH₄⁺ induces ferroelasticity in 2D rare-earth hybrid double perovskites

In recent years, organic-inorganic hybrid perovskites have been structured with a dual B-site (A₄B'B″X₈) configuration, offering enhanced compositional flexibility and thus attracting interest. A rare-earth-based hybrid double perovskite, namely (HQ)₄NH₄Tb(NO₃)₆·(NO₃)₂ (HQ = quinuclidine cation) (1), was successfully synthesized in this work. Variable-temperature single-crystal X-ray diffraction shows that 1 undergoes a reversible monoclinic (P2/m) to orthorhombic (Pnma) phase transition at ∼309 K, which is a typical ferroelastic phase transition in the Aizu classification. This phase transition is attributed to the weakening of N-H⋯O hydrogen bonds during heating, which reduces the constraints on structural units within the region and thereby induces a transition from a low-symmetry phase to a high-symmetry phase. Furthermore, a step-like dielectric anomaly observed near T_t in variable-temperature dielectric measurements further confirms the reversible phase transition. Owing to the intrinsic properties of Tb³⁺, 1 shows the characteristic green emission of Tb³⁺ and a fluorescence lifetime of 7.133 ms. This work demonstrates that the incorporation of NH₄⁺ into the B' sites of hybrid perovskites provides a feasible experimental strategy for the development of stimulus-responsive and multifunctional rare-earth hybrid perovskite materials.