Phase transitions and chromium(iii) luminescence in perovskite-type [C2H5NH3][Na0.5CrxAl0.5−x(HCOO)3] (x = 0, 0.025, 0.5), correlated with structural, dielectric and phonon properties

Abstract

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C₂H₅NH₃][Na_0.5Cr_0.5(HCOO)₃] (EtANaCr), [C₂H₅NH₃][Na_0.5Al_0.5(HCOO)₃] (EtANaAl) and [C₂H₅NH₃][Na_0.5Al_0.475Cr_0.025(HCOO)₃] (EtANaAlCr). These compounds crystallize in non-centrosymmetric monoclinic polar structures (space group Pn) and undergo order–disorder phase transitions upon heating to the monoclinic centrosymmetric structure (space group P2₁/n) at 369 (EtANaAl) and 373 K (EtANaCr). In principle, they are ferroelectric below these temperatures. In the high-temperature phase, ethylammonium (EtA⁺) cations are dynamically disordered over two symmetrically independent positions while upon cooling they begin to order. The ordering is accompanied by distortion of the metal formate framework. The hydrogen bonds (HBs) between the NH₃⁺ group and NaO₆ octahedral units are more robust than between the NH₃⁺ group and CrO₆ (AlO₆) octahedral units and this feature explains a much stronger distortion of the former units and a weak effect of a trivalent cation type on the phase transition temperature. The dielectric studies have confirmed the occurrence of phase transitions of dipolar character and dipole relaxation processes. The optical studies show that EtANaCr and EtANaAlCr exhibit efficient Cr(III)-based emission characteristics for intermediate-ligand field strength.