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

✍ Scribed by Michael B. Korzenski; George L. Schimek; Joseph W. Kolis; Gary J. Long

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 398 KB
• Volume: 139
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1998.7823

✦ Synopsis

A mixed-valent Fe(II/III) phosphate, NaFe 3.67 (PO 4 ) 3 , has been synthesized by high-temperature/high-pressure hydrothermal methods and characterized by single-crystal X-ray diffraction, EDAX, magnetic susceptibility, Mo] ssbauer, infrared, and diffuse reflectance spectroscopies, DTA, and bond valence sum calculations. The compound crystallizes in the monoclinic space group C2/c (No. 15) with a ‫؍‬ 11.881(4) A s , b ‫؍‬ 12.564(3) A s , c ‫؍‬ 6.522(2) A s , ‫؍‬ 115.09(2)°, V ‫؍‬ 881.7(5) A s 3 , and Z ‫؍‬ 4 with R/R w ‫؍‬ 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, NaFe 3 (PO 4 ) 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 squareplanar iron atom. The Mo¨ssbauer 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 NaCu 4 (AsO 4 ) 3 and NaMn 4 (AsO 4 ) 3 are discussed.