Molecularly Engineered Nitrate Double Perovskites with Elevated Ferroelectric Phase Transition Temperature and Large...

Conventional ABX₃ perovskites place organic species at A-sites or as X-site linkers, where the B-site is reserved for metals. Molecular double perovskites with a general formula A₂B^IB^IIIX₆ possess higher tunability and distinct B^I-sites that can accommodate small cations, further regulating lattice connectivity and physical properties. Here, we introduce a new family of B^I-site-organic nitrate double perovskites, (RM3HQ)₂B^IEu(NO₃)₆ (RM3HQ = (R)-N-methyl-3-hydroxyquinuclidinium), forming compounds 1-4 with B^I = ammonium, methylammonium, formamidinium, and dimethylammonium, respectively. Compounds 1 and 2 adopt mixed corner-/face-sharing non-perovskite structures, whereas 3 and 4 form double perovskites. This structural variation raises the ferroelectric phase-transition temperature to 349 K in 4 and 414 K in 3, much higher than that of the inorganic analogue (T_c = 285 K). This further widens the temperature margin for stable room-temperature piezoelectric responses, as demonstrated in 3 by ferroelectric hysteresis loops, ferroelastic domains, and a large piezoelectric coefficient (|d₃₃| ≈ 56 pC N^-1). During these phase transitions, the Eu³⁺ photoluminescence intensity ratio (616 nm/ 592 nm) increases from 0.32 to 2.92 in 3 and from 0.37 to 4.59 in 4, reflecting transition-related changes in the ligand field. This work highlights the B^I-site as an effective handle for tuning ferroelectric phase-transition temperatures and tunable Eu³⁺ photoluminescence.