AbstractThe new fluoride BaSr(NH4)Mg5F15 precipitates as a nanocrystalline powder from aqueous solutions of the alkaline earth ions and ammonium fluoride. The compound crystallizes in the tetragonal tungsten bronze structure type with lattice parameters of a,b = 12.4492(14) Å and c = 3.9421(4) Å (space group P4/mbm [Nr. 127], structure refinement from powder data, RBragg = 4.0 %). Corner‐linked magnesium‐fluoride octahedra form a channel structure which contains one empty triangular channel, one channel filled with Sr2+ [CN = 8+4] and one channel with disordered Ba2+and NH4+ (ratio 1:1) with CN = 7+8. Solid solutions between the composition limits Ba(Ba0.8Sr0.2)(NH4)Mg5F15 and (Ba0.6Sr0.4)Sr(NH4)Mg5F15 were obtained. The crystallite size increases from 15 to 75 nm with increasing barium concentration. The thermal stability between 350 to 450 °C depends on the amount of absorbed (NH4)F. Heated samples show luminescence and phosphorescence.