The NMR of205Tl was observed in the thallium silicate, thallium germanate, and thallium borate glass systems at six operating frequencies spanning the range 8ndash;26.5 MHz. Two sets of glasses were used for each system spanning the glasshyphen;forming region, one using205Tl in the natural abundance (70.5percnt;) and the other using thallium enriched to 99.5percnt;205Tl. Large distributions in the values of the chemical shift tensor and a large Tlndash;Tl exchange interaction were observed in each of the glass systems. The results for the thallium silicate glasses indicate a pairing or clustering of the thallium atoms. A variety of complicated and distorted structural groupings appear to be present in an SiO2matrix; the groupings change in size and structure with composition and link together at compositions having more than 20 molar percent Tl2O. Data for the germanate and borate glasses are similar to each other and indicate the following conditions. (1) The environment for the thallium atoms in low thallium content glasses is ionic. (2) The thallium environment gradually changes to a covalent environment with increasing thallium content, and there appears to be a variety of thalliumhyphen;rich structural units in the region of 20ndash;35 molar percent Tl2O. (3) The thallium atoms are paired or clustered in similar covalent structural units for high thallium contents. This situation produces a strong exchange interaction and a reduction in the size of the distribution of the values of the chemical shift tensor.
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