Room-temperature ferroelectric and ferroelastic orders coexisting in a new tetrafluoroborate-based perovskite

Coexistence of multiferroic orders attached increasing attentions on its potential applications in multiple-state memory, switch, and computing, but it is still challenging to design the single-phase crystalline materials hosting multiferroic orders at above room temperature. By utilizing versatile ABX3-type perovskite as structural model, we judiciously introduced a polar organic cation with easy changeable conformations into a tetrafluoroborate-based perovskite system, and successfully obtained an unprecedented molecular perovskite, (homopiperazine-1,4-diium)[K(BF4)3], hosting both ferroelectricity and ferroelasticity at above room temperature. By using the combined techniques of variable-temperature single-crystal X-ray structural analyses, differential scanning calorimetry, dielectric, second harmonic generation, and piezoresponse force microscopy measurements, we demonstrated the domain structures for ferroelectric and ferroelastic orders, and furthermore disclosed how the delicate interplay between stepwise changed dynamics of organic cations and cooperative deformation of inorganic framework induce ferroelectric and ferroelastic phase transitions at 311 K and 455 K, respectively. This instance, together with its underlying mechanism of ferroic transitions, provides important clues for designing advanced multiferroic materials based on the organic-inorganic hybrid crystals.