OPTICAL AND THERMODYNAMIC BASICITIES

PHILLIP DALE BENNETT, Purdue University

Abstract

For many years, scientists who have worked in the area of glasses and molten salts have been aware of the strong environmental dependence of the ('3)P(,1) (<---) ('1)S(,0) transition characteristics of Tl('+), Pb('2+) and Bi('3+) ions that are doped, in trace quantities, into oxide glasses. The dopant or probe ion, is sensitive to its environment in such a way that as the electrons on their nearest neighbors become more available, the energy required to cause the electronic transition becomes lower. Researchers who have investigated these effects have suggested that these ions were sensing the thermodynamic basicity of the medium in which they were immersed. The goal of this project was to determine how closely the spectroscopically measured basicity (termed "optimal basicity"), using the above probe ions, follows the thermodynamic basicity as measured by the more conventional techniques. To accomplish the goal, the molten chloroaluminate systems were used as the solvents for the probe ions because precise thermodynamic data were available for these molten salts and these data could be compared with the optical basicity data. Furthermore, the chloride ion activity is known to change many orders of magnitude in a small composition region near the AlCl(,4)('-) stoichiometry, so the relationship between the optical basicity and thermodynamic basicity (chloride ion activity) can be studied under sufficiently stringent conditions. Many difficulties were encountered in performing the spectroscopic measurements because of the problem in obtaining AlCl(,3)-based melts sufficiently clean to be transparent in the near U.V., where the probe ions absorb radiation. In fact, no U.V. study of AlCl(,3)-based melts has ever appeared in the literature. Spectroscopic shifts occurring in the composition range where the large thermodynamic activity changes occur have been found for both Pb('2+) and Bi('3+) probe ions. The magnitude of the spectroscopic shifts for Pb('2+) scale with the magnitude of the thermodynamic changes in the melts; therefore, these shifts reflect the changes in nature of the anions which are nearest neighbor to the Pb('2+) probe ions. A small composition range exists in the ethylammonium chloride-AlCl(,3) system, where a Pb('2+) complex precipitates out of solution. The spectral shift in this system is not seen to occur continuously as originally hoped, but instead occurs in a sudden step as the chloride activity quickly changes. The magnitude of the spectral shifts did not scale with the magnitude of the thermodynamic changes when Bi('3+) was used as the probe ion. However, in the composition region where the chloride ion activity changes many orders of magnitude, the absorption spectrum of Bi('3+) doped in an ethylammonium chloride-AlCl(,3) melt systematically changed as the composition of the melt changed. The absorption spectrum of Tl('+) did not change as the composition of the melt changed in NaCl-AlCl(,3) and KCl-AlCl(,1) melts.

Degree

Ph.D.

Subject Area

Chemistry

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