Orthorhombic and triclinic modifications of the arsenate Cu4O(AsO4)2 and isotypic phosphates Cu4O(PO4)2: strongly frustrated antiferromagnetics

Abstract

Abstract A structural-magnetic models of the orthorhombic and triclinic modifications of the arsenates Cu4O(AsO4)2 and isotypic phosphates Cu4O(PO4)2 has been built and analyzed. Their base elements are the complicated ribbons composed of antiferromagnetic Cu4 tetrahedra. Structurally, these tetrahedra have no shared copper atoms; however, there are strong antiferromagnetic (AFM) and ferromagnetic (FM) couplings between them, both within the complicated ribbons and between them. It has been established that both modifications are strongly frustrated 3D antiferromagnetics due to competition between the nearest AFM interactions along the edges of the Cu4 tetrahedra and competition between interactions and a multiplicity of long-range secondary AFM and FM interactions. Additionally, a large number of weaker long-range interactions are competing among each other. However, there is a possibility of the ordering Dzyaloshinskii-Moriya (DM) interaction in the centrosymmetric orthorhombic modification (Pnma), because two of the three types of magnetic ions, Cu1 and Cu3, are in the partial position 4c, where the ions are not related by the inversion center. In the triclinic modification (P −1) of Cu4O(As(P)O4)2, all four copper ions are in the centrosymmetric equivalent position 2i, which prevents DM interactions. This centrosymmetry will allow magnetic interactions in the triclinic modification of Cu4O(As(P)O4)2 to be still frustrated at lower temperature. It is possible that the triclinic modification of these compounds is a quantum spin liquid.