Effect of Habit Modification on Optical and X-ray Structures of Sodium Halate Mixed Crystals: The Etiology of Anomalous Double Refraction

Structures of mixed crystals of the isomorphous salts NaClO₃ and NaBrO₃ (sodium chlorate and sodium bromate, respectively) were reinvestigated by X-ray diffraction. Contrary to previous reports, NaCl_xBr_1-xO₃ is not cubic. Data from adjacent {100} growth sectors of crystals of varying composition were refined in the triclinic space group P1; halate ions occupy nominally symmetry-related sites nonstatistically. Optical measurements showed that six asymmetric sectors in cubes are disposed to give an object with approximate tetrahedral point symmetry. We address forgotten anomalies, first observed almost 150 years ago, which could have been a sufficient basis for earlier structural reinvestigations. The mixed-crystal structure speaks to the general non-applicability of the Law of Isomorphism to solid solutions. A link between optical anisotropy and nonstatistical guest site occupancy was achieved by annealing crystals between 523 and 533 K. Na₂S₂O₃ and Na₂S₂O₆ were used as habit-modifying impurities to produce NaCl_xBr_1-xO₃ crystals with {111} and {1̄1̄1̄} habits, respectivley. Diffraction data from {111} and {1̄1̄1̄} growth sectors were refined in the trigonal space group R3. In each case the pyramidal halate ion that was located on the special position was depleted in BrO^-₃. Conoscopic optical investigations nevertheless showed that the crystals are biaxial with a small 2V (10-15°), in marked contrast to the 90° angle in {100} crystals. We failed to reconcile the optical and X-ray structures by calculating the optical indicatrix with bond polarizability sum models. This led us to estimate the magnitude of other factors which contribute to the optical properties, including strain associated with dislocations which may exert its influence through combined piezoelectric and linear electro-optic effects.