Spin model of volborthite Cu₃V₂O₇(OH)₂.2H₂O revisited. Coupled trimers instead of zigzag chains

Motivated by recent experiments on volborthite single crystals showing a wide (1)/(3)-magnetization plateau, we adopt the structural data and perform microscopic modeling by means of density functional theory (DFT). Using DFT+U, we find four leading magnetic exchanges: antiferromagnetic J and J₂, as well as ferromagnetic J^' and J₁. Simulations of the spin Hamiltonian show good agreement with the experiment for J:J^':J₁:J₂ = 1: -0.2: -0.5: 0.2 with J ≅ 252 K. The (1)/(3)-plateau phase pertains to polarized magnetic trimers formed by strong J bonds. An effective J → ∞ model shows a tendency towards condensation of magnon bound states preceding the plateau phase.