Magnetic transition in K4Cu4OCl10: A model system of three-dimensional spin-1/2 tetrahedra

Isolated spin tetrahedral systems with weak intertetrahedral couplings, as have been reported for Cu2Te2O5X2 (X = Cl, Br) and the related compound Cu4Te5O12Cl4, have received much attention recently because they represent an interesting class of magnets that consist of weakly coupled magnetic clusters and, in particular, they can directly demonstrate the interplay of intertetrahedral couplings with built-in tetrahedral frustration. However, there is much debate about the structural low dimensionality of the Cu-Te-O-Cl(Br) compounds and its effect on the magnetism of the material. Here, we present a model spin tetrahedral system K4Cu4OCl10, with almost isotropic magnetic coupling within the tetrahedron and three-dimensional connection of the tetrahedra. The system enters a spin-singlet state with a susceptibility maximum at T-max = 11 K, and then enters an antiferromagnetic order at T-N = 4.4 K. The ratio T-N/T-max = 0.40 is close to the T-N/T-max = 0.38 for Cu2Te2O5Br2, which is viewed as an indicator of closeness to quantum criticality. Evidence in muon-spin rotation or relaxation suggests an incommensurate ordering. This work shows that the previously revealed ground state in anisotropically structured Cu2Te2O5X2 compounds also exists in an isotropic spin tetrahedral system. DOI: 10.1103/PhysRevB.87.144425