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

The phase Na5FeSi4O12 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 is crystallized from a host glass with 5Na2O·Fe2O3·8SiO2 stoichiometry, and both the glass and crystal are studied. It is 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. The crystal structure was solved using single crystal X-ray diffraction, and the refined data as well as the Crystallographic Information File (CIF) are reported for the first time for the Na5FeSi4O12 phase. It exhibits trigonal symmetry, space group R3 ̅c with a = 21.418, c = 12.2911 Å. The sodium atoms located between adjacent structural channels (Na5, Na51, Na52, Na53) 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, 4-coordinated Fe3+ constitutes ~ 93% of the total Fe cations. However, iron in the crystal, which exhibits a paramagnetic behavior, is solely present as 6-coordinated Fe3+. The magnetic and vibrational properties of the glass and crystal are discussed to provide additional insight into the structure.