Spectroscopic investigation and colour change of natural topaz exposed to PbO and CrO3 vapour

The colourless natural topaz stones were surface modified to beautiful yellow and green colour after heat treatment with lead oxide (PbO) and chromium oxide (CrO3), respectively. The surface of topaz shows a uniform colouration over the whole structure, which is confirmed by the UV-vis spectroscopic study. The absorption bands in the range 390-440 nm and 646 nm in PbO and CrO3 treated samples are associated with the yellow and green colouration of the stone. The chemical composition and oxidation states of the components present in the un-treated and treated topaz samples were determined from X-ray photoelectron spectroscopy (XPS) study, which clearly signifies that the stone retains its structure after treatment. Also, shifts of prominent peaks were observed in XRD and Raman spectra after heat treatment. Pb and Cr incorporation promotes oxygen vacancies in the topaz matrix, which is confirmed from the Raman peak at 223 and 220 cm−1 respectively. As deduced from Raman and XPS spectra, defect centers produced due to the oxygen vacancy and electronic charge transfer between topaz matrix with metal ions are the prime factor for the observed colour on topaz surface.