ИСТИНА

Alkali halides (MeHal) are widely applied in laboratory studies because of their dual importance - both technological (radioactive waste storage) and purely scientific, as solids modeling other materials. In particular, grain boundary (GB) wetting in NaCl has been used to find out a number of general features characterizing this important phenomenon [1]. Adding an excessive amount of alkali metals to their salts is well known to result in formation of coloured units ranging from point defects (F-centers etc) to metal nanocrystalline particles localized along dislocation lines. GB enrichment with metal particles or films has been, to our knowledge, practically not documented. The goal of our work has been to work out an experimental method for introducing an alkali metal into its polycrystalline salts preferentially along GB planes. We have modified for this purpose the technique proposed earlier to decorate dislocations. Coarse grained polycrystals of NaCl, KCl and KBr were obtained by hot flattening of single crystals and subsequent recrystallization annealing. Polycrystalline samples were heated in sodium vapours. For each salt, a combination of temperature, heating duration, quenching and annealing regimes was used in order to maximize the colour contrast between grains and GBs. Absorption spectra showed that the diameter range of Na and K inclusions was of 30 to 80 nm. The average alkali metal content in the salts was of 10-2 to 10-1 %. The results may be summarized as follows: 1) Only certain GBs are colour-bearing. 2) The colourless GBs are the low energy ones. 3) The average percentage of coloured GBs is in a rough agreement with the known values for the average free energy of MeHal GBs , Na-NaCl interfacial energy [2] and GB energy distribution width. 1. V.Traskine, Z. Skvortsova, et al. // Colloid Journal, 2015. V.77. P.226. 2. A. Sugonyako, Doctoral Thesis, Groningen 2007