Long-Range Order in Layered Perovskite Salts – Structure and Magnetic Properties of [(CH3)2CHCH2NH3]2CuX4 (X = Cl, Br)

The synthesis, structure, and magnetic properties of two new layered perovskite-type copper halide salts are reported. Dominant ferromagnetic coupling is observed with J/k = 14.58(1) and 21.5(4) K for the Cl and Br salts, respectively. For [(iBA)2CuCl4] (1); (iBA = isobutylammonium): P21/c with a = 13.9481(8) A, b = 7.5061(4) A, c = 7.5133(4) A and β = 102.520(1)° with Z = 4. In this structure, planar CuCl42– ions are linked together into layers of corner-sharing distorted octahedra. Ferromagnetic ordering occurs at Tc = 6.5 K for 1 with spin canting. Hysteresis loops show a coercive field of 2 mT with rapid saturation at 2 K that persists up to T = 9 K. The bromide salt is not isomorphous with the chloride salt, since an antiferromagnetic interaction is also found to be present. A structure consisting of a perovskite-type ribbon decorated with distorted tetrahedral CuBr42– ions is proposed. In this structure, there is dominant ferromagnetic coupling within the central ribbons and antiferromagnetic coupling with the decorated CuBr42– ions. The long-range order at 12.2 K is assumed to be within the ribbons, leading to a ferrimagnetic state with the decorated ribbons. A second ordering transition observed at Tc2 ≈ 6.8 K may be due to the onset of 3d ordering between the decorated ribbons. At 2 K, hysteresis is observed with a coercive field of 6.3 mT and a maximum magnetization value of about 0.45 μB, which is consistent with the proposed structure and magnetic coupling. Above 2 K, the coercive field decreases smoothly to zero at Tc2, then increases to a value of about 2 mT at Tc1, before gradually decreasing to zero near 30 K. The presence of hysteresis in the paramagnetic phases of the compounds is presumed to be due to large regions of highly correlated spins.