Reevaluating Structures for Mixed Crystals of Simple Isomorphous Salts, BaxPb1-x(NO3)2

✍ Scribed by Prakash Gopalan; Bart Kahr

• Book ID: 102604788
• Publisher: Elsevier Science
• Year: 1993
• Tongue: English
• Weight: 201 KB
• Volume: 107
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1993.1382

✦ Synopsis

Structures for mixed crystals of simple isomorphous salts, (\mathrm{Ba}\left(\mathrm{NO}{3}\right){2}) and (\mathrm{Pb}\left(\mathrm{NO}{3}\right){2}), were reinvestigated by X-ray diffraction. Contrary to previous reports, (\mathrm{Ba}{x} \mathrm{~Pb}{1-x}\left(\mathrm{NO}{3}\right){2}) is not cubic. Uniaxial crystallites are aligned along axes normal to each ({111}) face. The crystals suffer from a hierarchy of twinnings. Within each of these growth sectors, an obverse/reverse twinning of the parent cell is directed by the growing faces making one body diagonal unique. The selective expression of degenerate twinnings leads to symmetry reduction. Preferential exclusion of (\mathrm{Pb}^{2+}) from special positions also contributes to the symmetry reduction. The best structure conformed to space group R3. We address forgotten anomalies, first observed almost 150 years ago, which could have been a sufficient basis for earlier structural reinvestigations. The structures speak to the general nonapplicability of the Law of Isomorphism to solid solutions, and the limitations of Vegard's Rule as a predictor of mixed crystal structure. 1993 Academic Press, Inc.