Abstract
ABX3-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 ABX3 molecular perovskites, only two of them possess hexagonal perovskite structures. Herein, we report two tetrafluoroborate-based hexagonal molecular perovskites, A[Na(BF4)3], by judiciously choosing two different A-site cations: 1-methyl-1,4-diazabicyclo[2.2.2]octane-1,4-diium (Hmdabco2+) for 1 and 1-methylpiperazine-1,4-diium (H2mpz2+) for 2. They have high-temperature phases in the same space group (P63/mmc) revealing highly disordered A-site cations. Upon cooling, 1 undergoes two-step P63/mmc ↔ P3̅c1 ↔ P21/n transitions at 344 and 338 K, respectively, including a ferroelastic one (3̅mF2/m) accompanied by a spontaneous strain of 0.013. In contrast, the smaller H2mpz2+ cation with more adoptable conformations induces a one-step sharp P63/mmc ↔ P21/c ferroelastic transition (6/mmmF2/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.
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