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

ABSTRACT Conventional ABX 3 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 A 2 B I B III X 6 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) 2 B I Eu(NO 3 ) 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 (| d 33 | ≈ 56 pC N −1 ). During these phase transitions, the Eu 3+ 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 Eu 3+ photoluminescence.