Multiple strongly coupled antiferromagnetic spin S = 1/2 dimers in liroconite Cu2Al(As,P)O4(OH)4·4H2O

Abstract

We report on the magneto-structural properties of the rare copper aluminum hydroxo-arsenate mineral liroconite with chemical composition Cu₂AlAs_1−x P _x O₄(OH)₄·4H₂O (x ≈ 0.2). In order to characterize the natural mineral sample chemical analyses, X-ray single crystal and powder diffraction, heat capacity and crystal water desorption, anisotropic thermal expansion and Raman scattering and magnetic susceptibility investigations have been carried out. The magnetic properties are well described by two discrete oxygen bridged Cu²⁺ spin S = 1/2 dimers with antiferromagnetic spin exchange ranging between −320 K and −136 K, depending on to which group-15 five-valent cation, As⁵⁺ or P⁵⁺, the dimer bridging oxygen atoms coordinate to. Accordingly the temperature dependence of the magnetic susceptibilities can be well fitted to a sum of two Bleaney–Bowers type spin S = 1/2 dimer susceptibilities suggesting that the dimers show negligible mixed coordination to (AsO₄)³⁻/(PO₄)³⁻ tetrahedra. DFT + U calculation confirm the ratio of the spin exchange parameters of the (AsO₄)³⁻ or (PO₄)³⁻ coordinated Cu²⁺ – Cu²⁺ dimers. Inter dimer spin exchange is about two orders of magnitude smaller than intra dimer exchange.