Hydration and dehydration (on calcination) of SrCl 2 , YOCl, and HoOCl powders were studied.Single-crystal chloride materials are of interest for photonics owing to their transparence in the IR range and a [soft] photon spectrum. The main problem in both growing the crystals and working with them is their hydration. In particular, chlorides of rare-earth elements are very hygroscopic; hydrated rare-earth chlorides are readily hydrolyzed on heating [13 6]. This substantially complicates the use of such materials, though the spectroscopic characteristics, e.g., of LaCl 3 : R 3+ single crystals are highly promising [7]. Among matrices resistant to moisture are PbCl 2 and KPb 2 Cl 5 [8310].In this study we examined the hydration of SrCl 2 powders and also of oxychlorides of rare-earth elements. Strontium chloride crystallizing in a cubic lattice of the fluorite type has a high isomorphous capacity with respect to chlorides of rare-earth elements [11]. Properties of SrCl 2 single crystals and of Sr 1 !x R x Cl 2 + x solid solutions have been studied repeatedly [12314]. Oxychlorides of rare-earth elements are more resistant to moisture than the chlorides [15]. Therefore they could be a convenient form for introducing dopants.We used ultrapure 73 4 grade SrCO 3 , chemically pure grade ammonium chloride NH 4 Cl, and chemically pure grade hydrates of rare-earth elements RCl 3 . 6H 2 O as starting substances. The initial chemicals were characterized by X-ray diffraction. The X-ray phase analysis was carried out on a DRON-2 diffractometer (CuK = radiation, focusing monochromator from pyrolytic graphite). We recorded variations in sample weights on an Acculab V-200 electron balance. The humidity was measured with a psychrometer. The hydration was carried out at 19 + 2oC and the relative humidity of 38 + 5%.We prepared strontium chloride by the reaction SrCO 3 + 2NH 4 Cl = SrCl 2 + H 2 O8 + CO 2 8 + 2NH 3 8. (1) We used a fourfold excess of NH 4 Cl. The reaction was carried out in an alundum crucible at 150 3350oC for 6 h. The yield of SrCl 2 was 99%. The resulting SrCl 2 had a cubic fluorite-type lattice with a = 6.977 A, which agrees with the published data (PDF card no. 38-0496). On exposure to air, the sample weight continuously increased; the process gradually decelerated with time (Fig. 1). The increase in the substance weight in the first hour was 7%. The increase in the sample weight was accompanied by changes in its X-ray pattern. Along with the SrCl 2 lines, an additional set of reflections appeared, and their intensities rapidly increased, whereas the lines of SrCl 2 weakened (Fig. 2). Analysis of the pattern using the PDF database revealed mixture of mono-, di-, and hexahydrates of SrCl 2 . The final hydration product was SrCl 2 . 6H 2 O. On calcination of strontium chloride hexahydrate at 350oC for 2 h, the weight loss corresponded to complete dehydration. The subsequent Dm, g t, h Fig. 1. Kinetics of SrCl 2 hydration. (,m) Change in the weight of the sample and (t) time.
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