Above-Room-Temperature Ferroelastic Phase Transitions in Two Tetrafluoroborate-Based Hexagonal Molecular Perovskites.

ABX 3 -type molecular perovskites provide an important platform to tune phase transitions, via judiciously choosing A-, B-, and X-site components, to approach advanced functional materials for applications. Although tetrafluoroborate can act as X-site component to assemble ten instances of ABX 3 molecular perovskites, only two of them possess hexagonal perovskite structures. Herein, we report two tetrafluoroborate-based hexagonal molecular perovskites, A[Na(BF 4 ) 3 ], by judiciously choosing two different A-site cations: 1-methyl-1,4-diazabicyclo[2.2.2]octane-1,4-diium (Hmdabco 2+ ) for 1 and 1-methylpiperazine-1,4-diium (H 2 mpz 2+ ) for 2 . They have high-temperature phases in the same space group ( P 6 3 / mmc ) revealing highly disordered A-site cations. Upon cooling, 1 undergoes two-step P 6 3 / mmc ↔ P 3̅ c 1 ↔ P 2 1 / n transitions at 344 and 338 K, respectively, including a ferroelastic one (3̅ mF 2/ m ) accompanied by a spontaneous strain of 0.013. In contrast, the smaller H 2 mpz 2+ cation with more adoptable conformations induces a one-step sharp P 6 3 / mmc ↔ P 2 1 / c ferroelastic transition (6/ mmmF 2/ m ( s )) at 418 K in 2 , leading to more significant symmetry breaking and a considerable spontaneous strain of 0.129. This study provides important clues to modulate structural phase transitions by tuning diverse components for the multicomponent dense hybrid crystals.