Experimental Evidence of the Coexistence of Proper Magnetic and Structural Incommensurability on the [CH₃NH₃][Ni(COOH)₃] Compound

The present work is dedicated to characterization of the structural phase transition and incommensurate magnetic structure of the [CH 3 NH 3 ][Ni(COOH) 3 ] ( 1 ) perovskite-like metal–organic compound. The structural and magnetic characterization has been performed through variable-temperature single-crystal and powder neutron diffraction. Compound 1 crystallizes in the orthorhombic Pnma space group at room temperature. Below 84 K, a new phase has been observed. The occurrence of new reflections, which can be indexed with a wavevector along the c axis [ q = 0.1426(2) c *], suggests the occurrence of an incommensurately modulated crystal structure. The structure was determined using the superspace group formalism on the Pnma (00γ)0 s 0 space group. This incommensurate phase remains unchanged with a decrease of the temperature up to the base temperature (ca. 2 K). Moreover, the magnetic susceptibility data, collected under zero-field-cooled and field-cooled conditions at different applied magnetic fields, show that compound 1 exhibits antiferromagnetic behavior below 34 K. In the current paper, we have confirmed that compound 1 presents the coexistence of nuclear and proper magnetic incommensurability below T N .