Incommensurate structures of the [CH₃NH₃][Co(COOH)₃] compound

The present article is devoted to the characterization of the structural phase transitions of the [CH₃NH₃][Co(COOH)₃] (1) perovskite-like metal-organic compound through variable-temperature single-crystal neutron diffraction. At room temperature, compound 1 crystallizes in the orthorhombic space group Pnma (phase I). A decrease in temperature gives rise to a first phase transition from the space group Pnma to an incommensurate phase (phase II) at approximately 128 K. At about 96 K, this incommensurate phase evolves into a second phase with a sharp change in the modulation vector (phase III). At lower temperatures (ca 78 K), the crystal structure again becomes commensurate and can be described in the monoclinic space group P2₁/n (phase IV). Although phases I and IV have been reported previously [Boča et al. (2004). Acta Cryst. C60, m631-m633; Gómez-Aguirre et al. (2016). J. Am. Chem. Soc. 138, 1122-1125; Mazzuca et al. (2018). Chem. Eur. J. 24, 388-399], phases III and IV corresponding to the Pnma(00γ)0s0 space group have not yet been described. These phase transitions involve not only the occurrence of small distortions in the three-dimensional anionic [Co(HCOO)₃]^- framework, but also the reorganization of the [CH₃NH₃]⁺ counter-ions in the cavities of the structure, which gives rise to an alteration of the hydrogen-bonded network, modifying the electrical properties of compound 1.