Magnetism in the series (FexMn1-x)(5)(PO4)(2)(PO4H)(2)(H2O)(4)

Differential magnetic susceptibility measurements in synthetic polycrystals of the hureaulite series (FexMn1-x)(5)(PO4)(2)(PO4H)(2)(H2O)(4) (0 less than or equal to x less than or equal to 1) are reported for the first time. Its structure consists essentially of a condensation of pentamers formed by distorted edge-sharing octahedra whose centres are occupied by Mn2+ or Fe2+ ions and disposed in three different crystallographic sites in the proportion (III):(II):(I) = 2:2:1. The results from high temperature susceptibility measurements show an effective magnetic moment ranging from 11.89 to 13.53 mu(B)/f.u., suggesting that the spins in the pentamers are all parallel except one, which was attributed to the central one, that would have its spin reversed. The compound with x = 1 (ferrous compound) behaves as a normal canted antiferromagnet down to the Neel temperature, 12.7 K. Below this temperature the susceptibility tends to zero with the temperature. It was shown that the replacement of Fe2+ by Mn2+ introduces ferromagnetic correlations in the neighbourhood of the critical temperature due to anisotropic exchange interactions (weak ferromagnetism). The compound for x = 0 (manganous compound) is a weak ferromagnet with Neel temperature of 6.0 K. It was also observed that for x less than or equal to 0.40 a second phase transition begins to appear due, possibly, to a rearrangement of spins.