Structural Investigation of the Alluaudite-Like Mixed-Valence Iron Phosphate: Na1.25Mg1.10Fe1.90 (PO4)3.

Abstract A new mixed-valence iron phosphate Na 1.25 Mg 1.10 Fe 1.90 (PO 4 ) 3 has been synthesized as single crystals by a flux technique and its structure has been refined from X-ray data to a residual R 1  = 0.032. The compound crystallizes in the monoclinic space group C 2/ c with the parameters: a  = 11.7831(3) A, b  = 12.4740(3) A, c  = 6.3761(2) A, β  = 113.643(2)° and Z  = 4. The structure belongs to the alluaudite structural type, and thus it obeys to the X(2)X(1)M(1)M(2) 2 (PO 4 ) 3 general formula. The X(2) and X(1) sites are occupied by sodium while the M(1) and M(2) sites feature a statistical distribution of iron and magnesium. Additional information about the cation distribution has been extracted from a Mossbauer spectroscopy study which confirmed the mixed valency of the compound. A magnetic susceptibility study has also been undertaken and has shown the compound to be antiferromagnetic with a Neel temperature of about 35 K.