The effect of K+ cations on the phase transitions, and structural, dielectric and luminescence properties of [cat][K0.5Cr0.5(HCOO)3], where cat is protonated dimethylamine or ethylamine

Abstract

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of two novel heterometallic perovskite-type metal–organic frameworks (MOFs) of the following formula: [(CH₃)₂NH₂][K_0.5Cr_0.5(HCOO)₃] (DMAKCr) and [C₂H₅NH₃][K_0.5Cr_0.5(HCOO)₃] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R space group) and undergoes an order–disorder phase transition to the monoclinic system (P space group) at about 190 K. The dielectric studies confirm the presence of first-order relaxor-like structural transformation. In the high-temperature phase, the dimethylammonium cations are dynamically disordered over three equal positions and upon cooling the dynamical disorder evolves into a two-fold one. This partial ordering is accompanied by a small distortion of the metal–formate framework. EtAKCr crystallizes in a monoclinic structure (P2₁/n space group) with ordered EtA⁺ cations and does not experience any phase transition. The differences in the thermal behavior caused by the substitution of Na⁺ ions by larger K⁺ ions in the [cat]M^IM^III (cat = DMA⁺, EtA⁺, M^I = Na⁺, K⁺ and M^III = Cr³⁺ and Fe³⁺) heterometallic MOF family are discussed taking into account the impact of the hydrogen bond (HB) pattern and other factors affecting the stability of metal–formate frameworks. The optical studies show that DMANaCr and EtAKCr exhibit Cr³⁺-based emission characteristics for intermediate ligand field strength.