Crystal and magnetic structures of the oxyphosphates MFePO5 (M = Fe, Co, Ni, Cu). Analysis of the magnetic ground state in terms of superexchange interactions

We have studied in detail the crystal and magnetic structures of the oxypho sphates MFePO5 (M: divalent transition metal) using neutron powder diffract ion as a function of temperature. All of them are isomorphic to the mixed v alence compound alpha -Fe2PO5 with space-group Pnma. No disorder exists bet ween the two metallic sites. The M2+O6, octahedra share edges between them and faces with Fe3+O6 octahedra building zigzag chains running parallel to the b-axis that are connected by PO4 tetrahedra. The topology of this struc ture gives rise to a complex pattern of super-exchange interactions respons ible of the observed antiferromagnetic order. The magnetic structures are a ll collinear with the spin directed along the b-axis except for M = Co. The experimental magnetic moments of Cu+2 and Ni2+ correspond to the expected ionic value, on the contrary the magnetic moment of Fe3+ is reduced, probab ly due to covalence effects, and that of Co2+ is greater than the spin-only value indicating a non negligible orbital contribution. Using numerical ca lculations we have established a magnetic phase diagram adapted for this ty pe of crystal structure and determined the constraints to be satisfied by t he values of the exchange interactions in order to obtain the observed magn etic structure as the ground state.