Kinetic ordering of atoms in sodium chlorate-bromate solid solutions

摘要

Kinetic ordering of anions in Na((ClO3)(1-x)(BrO3)(x)) solid solutions with anomalous birefringence was studied by means of single crystal X-ray diffraction. The crystal structures of the two samples (x = 0.10 and 0.255) cut out from the {100} growth sectors were refined in the ideal space group P2(1)3 and in two of its subgroups-monoclinic P2(1) and triclinic P1 with R = 0.030-0.038. The final choice of monoclinic symmetry was confirmed by the analysis of the diffraction pattern and Cl/Br distribution over the halogen crystallographic sites. The Cl/Br ordering reflects the geometrical difference of these sites with respect to the growth front orientation. The observed optical indicatrix was successfully reproduced in the point-dipole approximation. According to our data and the data of Crundwell et al. (Acta Crystallogr. Sect. B 53 (1997) 189), the degree of ordering increases in the middle of the isomorphous series along with the birefringence. The obtained results confirm the hypothesis of growth dissymmetrization. (C) 2004 Elsevier Inc. All rights reserved.