Optical anomalies in solid solutions of simple isomorphous salts
Abstract
Mixed Crystals of $\rm NaBr\sb{x}Cl\sb{1-x}O\sb3,\ Ba\sb{x}Pb\sb{1-x}(NO\sb3)\sb2,$ and, $\rm (NH\sb4)\sb{x}K\sb{1-x}Al(SO\sb4)\sb2{\cdot}12H\sb2O$ (as well as other mixed alums), while usually treated as isotropic crystals like their parent salts, have optical symmetries which are incompatible with their morphological symmetries. Although the anomalous optical behavior of these materials has been known since the turn of the century, modern scientists have almost completely neglected the structural information afforded by optical crystallography. Therefore reinvestigations of each mixed crystal system was undertaken in attempts to correlate the optical properties with the structures of the mixed crystals. $\rm NaBr\sb{x}Cl\sb{1-x}O\sb3$ and $\rm Ba\sb{x}Pb\sb{1-x}(NO\sb3)\sb2$ crystals routinely show a non-statistical incorporation of guest that differs with changing morphology and is a consequence of the kinetics of crystal growth. In $\rm \{111\}\ NaBr\sb{x}Cl\sb{1-x}O\sb3$ crystals, X-ray diffraction studies reveal that guest ion incorporation preserves the symmetry element normal to the tetrahedral growth faces however cube faces on $\rm\{100\}\ NaBr\sb{x}Cl\sb{1-x}O\sb3$ crystals do not show any preferential inclusion of guest. The desymmetrization caused by the selective inclusion of guest qualitatively accounts for the observed birefringence. Inhomogeneous and homogeneous $\rm\{111\}\ Ba\sb{x}Pb\sb{1-x}(NO\sb3)\sb2$ crystals show distinct chemical shift regions for different distributions of Ba and Pb ions in the first coordination sphere as evidenced by $\sp{207}$Pb MAS NMR. However despite differences in cation distribution throughout the bulk, $\rm Ba\sb{x}Pb\sb{1-x}(NO\sb3)\sb2$ crystals show similar magnitudes of birefringence. In addition $\rm \{100\}\ Ba\sb{x}Pb\sb{1-x}(NO\sb3)\sb2$ crystals incorporate lead preferentially from solutions containing both pure salts despite a larger literature solubility for Pb(NO$\sb3)\sb2.$ Pure alum and dithionate (MS$\rm\sb2O\sb6{\cdot}4H\sb2O;$ where M = Ca, Pb, and Sr) crystals are structurally disordered and the incorporation of isomorphous impurities enhances it. The correlation between mixed crystal structure and optical behavior is unlikely without considering contributions from other kinetic effects of crystal growth such as dislocations and their associated strain fields.
Degree
Ph.D.
Advisors
Kahr, Purdue University.
Subject Area
Chemistry|Chemistry
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