Mechanism of isosymmetric polar order-disorder phase transition in pyroelectric [CH₃CH₂NH₃]₂NaGa(HCOO)₆ double perovskite

Recently, hybrid double-perovskite structures have attracted attention due to their versatile multifunctional properties originating from the variety of different constituent units in these materials. Here, we report the synthesis and comprehensive multitechnique characterization of a novel hybrid double-perovskite formate-based material [CH3CH2NH3](2)NaGa(HCOO)(6). The heat capacity measurements indicate that this compound has a structural phase transition at 379 K. In the low-temperature phase, the compound crystallizes in the non-centrosymmetric polar Pn structure, which exhibits a long-range order of ethylammonium (EA(+)) cations. Surprisingly, the Pn space group is not affected by the transition to the high-temperature disordered phase, which indicates that the transitions are isosymmetric making this compound a unique member of the formate-based double-perovskite family. The presence of the second-harmonic generation response in both phases confirms their non-centrosymmetric nature, while the dielectric spectroscopy experiments reveal that the transitions have a continuous order-disorder character. The observation of the pyroelectric current, but the absence of the electric polarization switching indicate that this compound is not properly ferroelectric. The electron paramagnetic resonance experiments of a compound slightly doped with paramagnetic Fe3+ impurities confirm the continuous character of the transition and allow us to probe the EA(+) cation and framework dynamics in this system. The vibrational spectroscopy data confirm that the phase transition is primarily driven by the progressive ordering of EA(+) cations and the resulting changes in the hydrogen bond strength. The temperature-dependent luminescence studies show that the [EA](2)NaCr0.931Ga0.069(HCOO)(6) perovskite is a promising material for noncontact temperature monitoring.