Characterization of the Structural Environment of Dithionate Ions Associated with Their Role in the Crystal Habit Modification of Sodium Chlorate

Sodium chlorate (NaClO₃) crystals change from a cuboid to a tetrahedron of {1̅1̅1̅} morphology when crystallized in the presence of sodium dithionate (Na₂S₂O₆) impurity. Polarized extended X-ray absorption fine structure at the S K-edge, used to probe the local structure around this impurity with respect to its orientation within the bulk crystal lattice, reveals that the S–S bond of the S₂O₆²‾ ions is closely aligned along the <111>/<1̅1̅1̅> lattice direction. High resolution diffraction studies using X-ray multiple diffraction reveal growth-induced anisotropy in the doped crystals associated with subtle lattice distortions in the symmetry-independent {1̅1̅1̅} and {111} growth sectors. The data are consistent with a mechanistic model involving the creation of lattice vacancies and the substitution of one of the SO₃ anionic groups of the dopant ion for a host ClO₃ ion when incorporated at the {1̅1̅1̅} growth interface with the other SO₃ group substituting for one or more anionic sites in the succeeding growth layer depending on the degree of impurity concentration within the crystallization solution. This mechanism is also fully consistent with the formation of twinning at higher impurity concentrations previously reported by Lan et al. Cryst. Growth & Des. 2014, 14, 6084–6092.