Weiden, M., Grauel, A., Norwig, J., Horn, S., & Steglich, F. (1995). Crystalline structure of the strontium niobates Sr4Nb2O9 and Sr5Nb4O15. Journal of Alloys and Compounds, 218(1), 13-16. doi:10.1016/0925-8388(94)01461-2.