Nuclear-Magnetic-Resonance Studies of Bonding in Semiconducting Tl-As-Se Glasses

Abstract

$^{205}\mathrm{Tl}$ NMR measurements have been carried out on glasses of the semiconducting chalcogenide system Tl-As-Se. Chemical shifts and linewidths were measured as a function of composition and temperature. For a series of glasses in the pseudobinary system ${({\mathrm{Tl}}_{2}\mathrm{Se})}_{x}\ensuremath{-}{({\mathrm{As}}_{2}{\mathrm{Se}}_{3})}_{1\ensuremath{-}x}$ the isotropic component of the observed chemical shift ranges from - 0.163% for $x=0.33$ to - 0.194% for $x=0.66$. These relatively large paramagnetic shifts indicate that the thallium bonding configuration in this range of glass composition is highly covalent. For $x\ensuremath{\le}0.3$ the chemical shift drops (in magnitude) precipitously to about - 0.09%, signaling the introduction of a more ionic-bonding configuration for lower thallium content. The $^{205}\mathrm{Tl}$ NMR linewidths increase with increasing Larmor frequency as a result of anisotropic-chemical-shift contributions to the linewidth; these contributions are also an indication of the covalent nature of the thallium bonding. The observed chemical shifts and linewidths remain unchanged as the temperature is increased through the glass transition temperature and into the molten range. These unchanged shifts and linewidths demonstrate that the local structural order which involves the thallium bonding is preserved in the molten glass. Correlation times ${\ensuremath{\tau}}_{c}$ for the thallium bonds in these molten glasses are longer than 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ sec at temperatures up to 200 \ifmmode^\circ\else\textdegree\fi{}C. These results are contrasted with those for selenium in which the $^{77}\mathrm{Se}$ NMR becomes motionally narrowed in the melt, and the resulting correlation times are two orders of magnitude shorter than those for Tl-As-Se glasses at equivalent viscosity.