On the crystallochemical origin of the disordered form of Ba₇^(EuII)F₁₂Cl₂ and the structural changes induced at high temperature

Abstract

The crystal structure of the disordered modification of Ba 7 F 12 Cl 2 has been carefully re-examined on several new crystals prepared under different conditions of synthesis. All single crystal structure refinements reveal a residual electron density of ∼ 3 e -/Å 3 in the 0,0,0 position which is explained by the introduction of a small amount of sodium ions in the crystal. The title compound transforms from a disordered to an ordered modification at ~800°C. New structural data show a change in space group from P6 3 /m to P6. During this process, a slight chemical change and the formation of nano-channels in the crystals is observed by electron microscopy. These changes were further studied by electron microprobe analysis, by spectroscopic methods and thermal analysis.

Crystallochemical Studies in the Family of Crystals Ba7−xNayF12Cl2−zBrz (x < 0.1, y < 0.2, z < 1.5)

“…The presence of Na was detected, as discussed previously [11], as a residual interstitial electron density at the 0 0 1 / 4 position. In order to quantify the amount of sodium present, all the populations in the different 0 0 z positions (Ba3, Ba33 and Na) were refined without constraints.…”

“…Preliminary solid state reactions revealed that Figure 2 collects lattice parameters of Ba 7Ϫx Na y F 12 Cl 2Ϫz Br z single crystals obtained in this work together with previous data reported in [1,11,12]. The full line connects the pure 1042 chlorides (i.e.…”

“…However, when substituting Ba by Ca, it was found that the Ca ion will not occupy the Ba site on 0 0 0 or 0 0 0.5 (in the P6 3 /m structure), but rather the 0 0 0.25 site (see Figure 1) with a sixfold fluoride coordination [10]. In a further study we were able to establish that the disordered structure is in fact stabilized by the presence of small, but significant amounts of Na on the 0 0 0.25 site [11], leading to the approximate composition Ba ϳ6.9 Na ϳ0.2 F 12 Cl 2 . We have also studied CaϪcontaining and CaϪfreebromide substi-tuted samples of the disordered modification [10,12].…”

Crystal Chemistry in the Barium Fluoride Chloride System

“…49 The optimum F mol fraction was found to be close to 75% (i.e., 2 NaCl + 3 BaF 2 ). Higher sodium chloride content in the mixture leads to progressively increasing formation of BaFCl.…”

BaSr(NH4)Mg5F15, a Tetragonal Tungsten Bronze Structure with Ammonium Barium Disorder and its Solid Solutions BaxSr2−x(NH4)Mg5F15 (x = 1.8−0.6)

“…Thermal stability is very important in the preparation of phosphors with rare earth doping, as reduction of Eu III to Eu II needs elevated temperature, so systems have to be coated to stabilize the compounds even at higher temperature. At 500°C, samples decompose to Ba 0.55 Sr 0.45 MgF 4 [8], SrF 2 these phases continue to react to other Ba/Sr/Mg-fluoride phases. Infrared spectroscopy measurements (see Fig.…”