Habit modification and structural analysis of nearly-perfect S₂O₆ doped NaBrO₃ single crystals by n-beam X-ray diffraction and absorption

Crystallization processes are continuously developed to meet the increasing demand of the chemical industry in optimizing characteristics such as good fluidity, compaction, dissolution, among others. One property that plays an essential role in these aspects is the crystalline habit, which is defined as the description of the final shape of the crystal in terms of the importance of its different exposed faces. In that sense, the intentional modification of the crystalline habit, by adding impurities (additives) during the crystallization process, can enhance such desired characteristics. Furthermore, crystal habit modification via impurity incorporation allows studies on fundamental aspects on the role of the surface in the crystal growth mechanism, since even small concentrations cause substantial changes in the final morphology of the crystal. These impurities can either block the surface sites movement or disrupt the molecular packing of the next growing layers. However, structural changes in the crystalline lattice caused by the incorporation of additives are still poorly investigated, and then, it can bring new perspectives for both fundamental science and industrial crystallization processes. In this work, large nearly perfect single crystals of sodium bromate (NaBrO₃) were grown with and without the presence of habit modifier ion, the dithionate (S₂O₆ 2-) in concentrations of 50 to 2000 ppm in molar ratio. At moderate saturation, the pure NaBrO₃ crystallizes in the spatial group P213 (a = 6.70717(10) Å) and has a habit governed mainly by "positive" tetrahedral faces of the form-{111},(defined by faces (111), (11̅1 ̅) , (1̅11̅) and (1̅1 ̅1 )), with a small presence of the "negative" tetrahedral faces of the form-{1̅1 ̅1 ̅} (with (1̅1 ̅1 ̅) , (1̅11), (11̅1) and (111̅) faces). These symmetrical (non-equivalent) faces have a polar behavior given by group P213, and therefore, their distinct surface chemistry dictates the impurity incorporation selectivity between such faces. In this sense, changes in the NaBrO₃ final habit were observed, and the form-{1̅1 ̅1 ̅} prevails over the form-{111} for S₂O₆ concentrations higher than 100 ppm. X-ray powder diffraction has shown very similar patterns between the pure and doped samples, indicating very good incorporation of the dopant by the host lattice, as confirmed by Rietveld analysis. Surface microtopography revealed an increase in roughness and the presence of growth hillocks as the dopant level increases. Results of X-ray multiple diffraction, reciprocal space mapping, and extended X-ray absorption fine structure, however, revealed: 1) a subtle but noticeable change in the lattice parameters dependence on the S₂O₆ concentration; 2) the transition between strain/relaxation in the parallel direction to the {1̅1 ̅1 ̅} - form planes; 3) different sizes of mosaic blocks in both parallel and perpendicular directions to the [1̅1 ̅1 ̅] growing front, and its dependence on the S₂O₆ concentration of; 4) the dithionate S-S bond forms ~ 49° with the {1̅1 ̅1 ̅} -form faces; v) a steep change between the profiles of the (002̅)/(1̅1 ̅1 ̅) and (002̅) /(111̅) BSD peaks in doped crystals, i.e., symmetry breaking between Friedel pairs, even outside the resonant scattering range. Experimental results exhibited a strong structural modification in NaBrO₃ with 1000 ppm of dithionate, and that above this concentration, the S-S bond of S₂O₆ makes 49° with the (1̅1 ̅1 ̅) -growing planes, a value confirmed by molecular modelling. (author)