The effect of K + cations on the phase transitions, and structural, dielectric and luminescence properties of [cat][K 0.5 Cr 0.5 (HCOO) 3 ], where cat is protonated dimethylamine or ethylamine

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: [(CH ) NH ][K Cr (HCOO) ] (DMAKCr) and [C H NH ][K Cr (HCOO) ] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R3[combining macron] space group) and undergoes an order-disorder phase transition to the monoclinic system (P1[combining macron] 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 (P2 /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 M (cat = DMA , EtA , M = Na , K and M = Cr and Fe ) 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 Cr -based emission characteristics for intermediate ligand field strength.