Novel Phase-Transition Materials Coupled with Switchable Dielectric, Magnetic, and Optical Properties: [(CH3)4P][FeCl4] and [(CH3)4P][FeBr4]

Two inorganicorganic hybrid compounds with zero-dimensional crystal structures, tetramethylphosphonium tetrachloroferrate(III) (compound 1, [(CH3)(4)P][FeCl4]) and tetramethylphosphonium tetrabromoferrate(III) (compound 2, [(CH3)(4)P][FeBr4]), are discovered as multifunctional materials exhibiting simultaneously switchable dielectric, magnetic, and optical properties. Despite the analogue chemical formulas, compounds 1 and 2 crystallize in the different noncentrosymmetric space groups, that is, P63mc and F4(-)3m, and exhibit distinct responses in the three above-mentioned physical channels, especially for the magnetic property. Compound 1 undergoes dielectric anomalies which could be tuned in three distinct dielectric states and switched by the sequential phase transitions around 362 and 436 K, respectively. The symmetry breaking occurring during the first phase transition is confirmed by the switchable temperature-dependent second harmonic generation (SHG) effect. Weak antiferromagnetic interactions are also found in compound 1 below room temperature. In contrast, the continuous phase transitions occur at 353 and 359 K in compound 2, together with the steplike dielectric anomalies which also could be tuned in three distinct dielectric states. Except for the switchable SHG effect and the antiferromagnetic interactions stronger than compound 1, compound 2 displays magnetic bistability in the vicinity of the second phase transition, with a thermal hysteresis of 6 K.