Incommensurate structures of the [CHNH][Co(COOH)] compound.

The present article is devoted to the characterization of the structural phase transitions of the [CHNH][Co(COOH)] () perovskite-like metal-organic compound through variable-temperature single-crystal neutron diffraction. At room temperature, compound crystallizes in the orthorhombic space group (phase ). A decrease in temperature gives rise to a first phase transition from the space group to an incommensurate phase (phase ) 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 ). At lower temperatures ( 78 K), the crystal structure again becomes commensurate and can be described in the monoclinic space group 2/ (phase ). Although phases and have been reported previously [Boča (2004). C, m631-m633; Gómez-Aguirre (2016). , 1122-1125; Mazzuca (2018). , 388-399], phases and corresponding to the (00γ)00 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)] framework, but also the reorganization of the [CHNH] 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 .