HYDROTHERMAL SYNTHESIS, STRUCTURE, AND CHARACTERIZATION OF A MIXED-VALENT IRON(II III) PHOSPHATE, NAFE3.67(PO4)(3) - A NEW VARIATION OF THEALLUAUDITE STRUCTURE TYPE/

A mixed-valent Fe(II/III) phosphate, NaFe3.67(PO4)(3), has been synthe sized by high-temperature/high-pressure hydrothermal methods and chara cterized by single-crystal X-ray diffraction, EDAX, magnetic susceptib ility, Mossbauer, infrared, and diffuse reflectance spectroscopies, DT A, and bond valence sum calculations. The compound crystallizes in the monoclinic space group C2/c (No. 15) with a = 11.881(4) Angstrom, b = 12.564(3) Angstrom, c = 6.522(2) Angstrom, beta = 115.09(2)degrees V= 881.7(5) Angstrom(3), and Z = 4 with R/R-w = 0.029/0.033. It exhibits a three-dimensional channel-like structure with an open framework tha t is similar to the naturally occurring iron-containing mineral alluau dite, 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 sq uare-planar iron atom. The Mossbauer spectral parameters are consisten t with the structure and indicate that the fully occupied iran sites a re 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)(3) and NaMn4(AsO 4)(3) are discussed. (C) 1998 Academic Press.