Long-range superexchange in Cu₂A₂O₇ (A = P, As, V) as a key element of the microscopic magnetic model

A microscopic magnetic model for alpha-Cu₂P₂O₇ is evaluated in a combined theoretical and experimental study. Despite a dominant intradimer coupling J₁, sizable interdimer couplings enforce long-range magnetic ordering at T_N = 27 K. The spin model for a-Cu₂P₂O₇ is compared to the models of the isostructural beta-Cu₂V₂O₇ and alpha-Cu₂As₂O₇ systems. As a surprise, coupled dimers in a-Cu₂P₂O₇ and alternating chains in alpha-Cu₂As₂O₇ contrast with a honeycomb lattice in beta-Cu₂V₂O₇. We find that the qualitative difference in the coupling regime of these isostructural compounds is governed by the nature of AO₄ side groups: d elements (A = V) hybridize with nearby O atoms forming a Cu-O-A-O-Cu superexchange path, while for p elements (A = P, As) the superexchange is realized via O-O edges of the tetrahedron. Implications for a broad range of systems are discussed.