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 NaBrO 3 (sodium chlorate and sodium bromate, respectively) were reinvestigat ed by X-ray diffraction. Contrary to previous reports, NaClxBr1-xO3 is not cubic. Data from adjacent {100} growth sectors of crystals of var ying 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 add ress forgotten anomalies, first observed almost 150 years ago, which c ould have been a sufficient basis for earlier structural reinvestigati ons. The mixed-crystal structure speaks to the general nonapplicabilit y of the Law of Isomorphism to solid solutions. A link between optical anisotropy and nonstatistical guest site occupancy was achieved by an nealing crystals between 523 and 533 K. Na2S2O3 and Na2S2O6 were used as habit-modifying impurities to produce NaClxBr1-xO3 crystals with {1 11} and {(111) over bar} habits, respectivley. Diffraction data from { 111} and {(111) over bar} 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 BrO3-. Conoscopic optical inv estigations nevertheless showed that the crystals are biaxial with a s mall 2V (10-15 degrees), in marked contrast to the 90 degrees angle in (100) crystals. We failed to reconcile the optical and X-ray structur es by calculating the optical indicatrix with bond polarizability sum models. This led us to estimate the magnitude of other factors which c ontribute to the optical properties, including strain associated with dislocations which may exert its influence through combined piezoelect ric and linear electro-optic effects.