Strong spin frustration from isolated triangular Cu(ii) trimers in SrCu(OH)3Cl with a novel cuprate layer

Abstract

Strontium oxocuprates such as Sr2CuO2Cl2 have been extensively investigated for their two-dimensional (2D) planar CuO2 square layers that are typical of high-temperature cuprate superconductors. A new synthetic strontium cuprate SrCu(OH)3Cl features a novel cuprate layer with isolated triangular Cu(II) trimers. The crystal structure of the title compound can be derived from breaking the cuprate planar layer to form isolated, nonplanar triangular copper trimers ({[Cu3(μ3-Cl)(OH)3](OH)6Cl2}6−), which are connected by strontium coordination spheres to form a novel framework structure. The copper trimers exhibit an unprecedented array that has triangular Cu3-planes perpendicular not only to their own layer but also to their counterparts in the next layers, which is thus different from the parallel array in the Kagomé layer of ZnCu3(OH)6Cl2. This cross-orientation arrangement of the triangular Cu(II) trimers is proposed to be responsible for a strong antiferromagnetic exchange (J = −233(2) cm−1) in the triangles characterized by a Weiss temperature θcw = −135 K but virtually zero (or negligible) magnetic coupling between the triangles. Such remarkable magnetic properties make the title compound a Mott insulator – an ordered inorganic solid with Cu spin-1/2 spins forming an intrinsic “molecular magnet”.