Molecularly Engineered Nitrate Double Perovskites with Elevated Ferroelectric Phase Transition Temperature and Large Piezoelectric Response
Le Ye, Guang‐Yuan Hu, Yarou Duan, Yutong Lin, Ding‐Chong Han, Zi‐Luo Fang, Zhe Dong, Wei‐Xiong Zhang, Liang‐Han Shen, Na Wang, Le‐Ping Miao, Heng‐Yun Ye, Lingling MaoABSTRACT
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.