Structure and properties of Na₅FeSi₄O₁₂ crystallized from 5Na₂O-Fe₂O₃-8SiO₂ glass

The phase Na₅FeSi₄O₁₂ [pentasodium iron(III) silicate] crystallizes readily from the Na₂O-Fe₂O₃-SiO₂ glass system in a relatively large compositional range. However, its crystal structure and properties have not been studied in detail since its discovery in 1930. In this work, the Na₅FeSi₄O₁₂ phase was crystallized from a host glass with 5Na₂O·Fe₂O₃·8SiO₂ stoichiometry, and both the glass and the crystal were studied. It was found that the Na₅FeSi₄O₁₂ phase crystallizes at ∼720 °C from the glass and melts at ∼830 °C when heated at a rate of 10 °C min^-1. The crystal structure was solved using single-crystal X-ray diffraction and the refined data are reported for the first time for the Na₅FeSi₄O₁₂ phase. It exhibits trigonal symmetry, space group R-3c, with a = 21.418 and c = 12.2911 Å. The Na atoms located between adjacent structural channels exhibit positional disorder and splitting which was only refined by using low-temperature data collection (150 K). While ∼7% of the total Fe cations occur as Fe²⁺ in the glass, four-coordinated Fe³⁺ constitutes ∼93% of the total Fe cations. However, iron in the crystal, which exhibits a paramagnetic behavior, is solely present as six-coordinated Fe³⁺. The magnetic and vibrational properties of the glass and crystal are discussed to provide additional insight into the structure.