BaSr(NH₄)Mg₅F₁₅, a Tetragonal Tungsten Bronze Structure with Ammonium Barium Disorder and its Solid Solutions Ba_xSr_2−x(NH₄)Mg₅F₁₅ (x = 1.8−0.6)

Abstract

The new fluoride BaSr(NH 4 )Mg 5 F 15 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 ϭ 12.4492(14) Å and c ϭ 3.9421(4) Å (space group P4/mbm [Nr. 127], structure refinement from powder data, R Bragg ϭ 4.0 %). Corner-linked magnesium-fluoride octahedra form a channel structure which contains one empty triangular channel, one channel filled with Sr 2ϩ [CN ϭ 8ϩ4] and one channel with disordered Ba 2ϩ and NH 4 ϩ (ratio 1:1) with CN ϭ 7ϩ8. Solid solutions

Crystal Chemistry and Selected Physical Properties of Inorganic Fluorides and Oxide-Fluorides

“…As for the HTB structure, the TTB structure involves triangular cycles of MX 6 octahedra, lying in the (001) planes. Mixed valence compounds include K 0.6 MF 3 phases (M = Fe, Cr); mixed-metal bronzoids are represented by BaSr­(NH 4 )­Mg 5 F 15 , K 0.38 Mg 0.38 In 0.62 F 3 , K 3 Fe 3 Cr 2 F 15 , or K 3 Cu 3 Fe 2 F 15 . , …”

Structural Studies on Ba2SrMg4F14 and Ba2MM´Mg4F14 (M= Ca, Sr, Ba)

“…Two types of connections can be observed: Mg-Mg distances for edge-sharing octahedra are between 2.9 -3.1Å as observed for Ba 2 Mg 3 F 10 [3], Ba 6 Mg 7 F 26 [4] and Ba 6 Mg 11 F 34 [5] or MgF 2 [6]. The distances (4) for corner-sharing octahedra in these and other compounds such as BaMgF 4 [3], BaSr(NH 4 )Mg 5 F 15 [7] and SrMgF 4 [8] vary from 3.5 to 4.2Å. The Mg-Mg distance increases as expected with increasing Mg-FMg angles from ∼ 130 to 180…”