The structural, phonon and optical properties of [CH3NH3]M0.5CrxAl0.5−x(HCOO)3 (M = Na, K; x = 0, 0.025, 0.5) metal–organic framework perovskites for luminescence thermometry

Abstract

We report the structural, phonon and luminescence studies of six heterometallic perovskite-type metal–organic frameworks (MOFs) templated by methylammonium cations (CH₃NH₃⁺ and MeA⁺) with the following formulae: [MeA]Na_0.5Cr_0.5(HCOO)₃ (MeANaCr), [MeA]K_0.5Cr_0.5(HCOO)₃ (MeAKCr), [MeA]Na_0.5Al_0.5(HCOO)₃ (MeANaAl), [MeA]K_0.5Al_0.5(HCOO)₃ (MeAKAl), [MeA]Na_0.5Cr_0.025Al_0.475(HCOO)₃ (MeANaAlCr, 5 mol% of Cr³⁺ ions) and [MeA]K_0.5Cr_0.025Al_0.475(HCOO)₃ (MeAKAlCr, 5 mol% of Cr³⁺ ions). All of them crystallise in a monoclinic system (P2₁/n space group) with one MeA⁺ cation in an asymmetric unit forming four medium-strength hydrogen bonds (HBs) with a metal-formate framework. The DSC measurements and XRD single-crystal studies show that the studied crystals do not undergo structural phase transitions in the 100–440 K range. The high tolerance factors indicate that there is not enough space for the re-orientational motions of the MeA⁺ cations, explaining the lack of the structural phase transitions. We also present IR and Raman studies supported by the factor group analysis together with luminescence properties. We have shown that the strength of the crystal field parameter (D_q/B) varies in the 2.13–2.56 range depending on the chemical composition of the studied compounds. We show that perovskite-like formate MOFs with Na⁺ (K⁺) ions can be considered as potential candidates for non-contact thermometry in the 225–400 K (325 K) range.