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(BF4)(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 (P6(3)/mmc) revealing highly disordered A-site cations. Upon cooling, 1 undergoes two-step P6(3)/mmc <-> P (3) over bar c1. P2(1)/n transitions at 344 and 338 K, respectively, including a ferroelastic one ((3) over bar 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 P6(3)/mmc <-> P2(1)/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.