Abstract
Raman microscopy has been used to investigate the reactions between the chemical dosants in scandium metal halide discharge lamps and their silica lamp envelopes; such lamps are typically dosed with Hg, NaI, ScI<sub>3</sub>, and sometimes, additionally, excess Sc metal. Raman measurements were made both on operated lamps and dosed silica ampoules that had been furnace heat-treated. The ampoules mimic closely the dose-envelope interactions of lamps in a convenient manner while avoiding the obscuring and complicating effects in whole-lamp studies resulting from the reactions and mobility of electrode materials. <i>In situ</i> Raman analyses of deposits in the envelopes and ampoules, supported by an extensive database of the Raman spectra of lamp materials, and <i>ex situ</i> X-ray diffraction (XRD) analyses of refractory deposits to confirm independently the Raman assignments, have demonstrated that: (1) Sc metal reacts with envelope silica to produce Sc<sub>2</sub>O<sub>3</sub> and elemental Si; (2) Sc metal in the presence of ScI<sub>3</sub> reacts with the envelope silica to produce Sc<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>; and (3) Sc metal reacts with envelope silica in the presence of NaI alone to produce Sc<sub>2</sub>O<sub>3</sub> and not Sc<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>. The results confirm and extend previous studies and demonstrate the value of Raman microscopy as a nondestructive investigative tool for lamp chemistry.
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