Heat capacity and neutron diffraction studies on the frustrated magnetic Co2(OH)(PO4)1−x(AsO4)x [0≤x≤1] solid solution

The Co2(OH)(PO4)1−x(AsO4)x [0≤x≤1] solid solution exhibits a complex magnetic behaviour due to the bond-frustration in its magnetic structure. Heat capacity measurements of the (x=0.1–0.5) phases show a three-dimensional magnetic ordering (λ anomaly) that shifts to lower temperatures and becomes broader as the AsO43− content increases. For x=0.75, no significant feature was observed whereas for higher arsenate ion content, x=0.9 and 1, a small maximum was detected. The magnetic structures of solid solution are consistent with the existence of predominant antiferromagnetic superexchange interactions through the |OH| and |XO4| (X=P and As) groups between the Co+2 ions. The substitution of PO43− by AsO43− anions by more than 90% substantially modifies the magnetic exchange pathways in the solid solution, leading to an incommensurate antiferromagnetic structure in Co2(OH)(PO4)1−x(AsO4)x [x=0.9 and 1] phases.