Showing papers by "Bart Kahr published in 1993"

Reevaluating structures for mixed crystals of simple isomorphous salts: sodium chlorate bromate (NaClxBr1-xO3)

Abstract: Evidence has accumulated recently supporting the notion that structures of solid solutions have been misunderstood since the inception of X-ray diffraction.’ Flags abound in the crystallographic record that signal caution when examining mixed crystals grown under kinetic control.2 This is particularly true of the optical descriptions of the many crystals whose morphologies are at variance with their optical properties.3 Therefore, we began structural studies of mixed crystals of simple isomorphous salts,4 including NaC103 and NaBr03.5 NaC1,Brl_,03, previously studied by X-ray diffraction and formerly treated as ideal, cubic, solid solutions, reveal secondary low-symmetry structures heretofore unappreciated. The structures question the general applicability of the Law of Isomorphism to mixed crystals. Anomalous birefringence in cubes of NaC103 was first observed by Mitscherlich in 18466 and commented on subsequently’ for NaC1,Brl.,03. Mitscherlich’s greatest contribution, the Law of Isomorphism,8 was based on the observation that some simple crystalline salts could be mixed without affecting habit; ions were exchanged willy-nilly, seemingly without affecting s t r ~ c t u r e . ~ Our intuition about solid solutions is inherited from him-the solute should be distributed randomly. I The optical anisotropy in NaC1,BrI_,O3 implies a reduction from cubic symmetry, but unfortunately it has been hard to interpret. Brauns said that the crystals look “so complicated, that I have not obtained a clear p i c t ~ r e ” . ~ ~ Recent observers treated the birefringence as a nuisaxe. l o We reproduced birefringent NaC1,Brl_,03 crystals by evaporating water solutions of varying composition. Crystals were selected from large reservoirs. Compositional homogeneity was confirmed by analyzing for Br03-. Solutions made from frag-