Hydrothermal Synthesis, Structure, and Characterization of a Mixed-Valent Iron(II/III) Phosphate, NaFe3.67(PO4)3: A New Variation of the Alluaudite Structure Type

A mixed-valent Fe(II/III) phosphate, NaFe3.67(PO4)3, has been synthesized by high-temperature/high-pressure hydrothermal methods and characterized by single-crystal X-ray diffraction, EDAX, magnetic susceptibility, Mössbauer, infrared, and diffuse reflectance spectroscopies, DTA, and bond valence sum calculations. The compound crystallizes in the monoclinic space groupC2/c(No. 15) witha=11.881(4) Å,b=12.564(3) Å,c=6.522(2) Å,β=115.09(2)°,V=881.7(5) Å3, andZ=4 withR/Rw=0.029/0.033. It exhibits a three-dimensional channel-like structure with an open framework that is similar to the naturally occurring iron-containing mineral alluaudite, NaFe3(PO4)3. In alluaudite there are two distinct channels, one which contains the sodium cations and the second which remains empty. In the title phase, this second channel is partially occupied by a square-planar iron atom. The Mössbauer spectral parameters are consistent with the structure and indicate that the fully occupied iron sites are octahedral Fe(II) and the square-planar iron is Fe(III). Structural comparisons to a number of phosphates and arsenates that also possess the alluaudite-type structure, including NaCu4(AsO4)3and NaMn4(AsO4)3are discussed.