to a greater reduction of brown color [@Martineau2004; @Khan2009; @Khan2013]. The present samples display the same behavior, with samples E and F ( and , respectively) appearing near-colorless by eye in the heated state. Contrary to previous studies, the present samples treated at remain heavily photochromic, varying from a deep grey-blue to near-colorless in the UV and heated states, respectively.

UV-Vis measurements of [Sample D]{}in the UV state show strong absorption from both [$\mathrm{SiV}^{-}$]{} () and [$\mathrm{SiV}^{0}$]{} () \[Fig. \[fig:UV-vis-ch\]\]. The spectrum of the former reveals the optical structure associated with the second excited state of [$\mathrm{SiV}^{-}$]{} [@Rogers2014a; @Haußler2017] which has been reported previously in photoluminescence excitation [@Ekimov2017a; @Ekimov2018]. Comparison of the absorption spectra in the UV and heated states confirms that the photo/thermochromism is dominated by dramatic changes in the concentration of [$\mathrm{SiV}^{-}$]{}and [$\mathrm{SiV}^{0}$]{} \[Fig. \[fig:UV-vis-ch\]\]: this is the case for samples A–F. The visible photochromic color change in the present samples is much more extreme than the color change reported in nitrogen-doped CVD samples [@Khan2009] due to the incredibly broad absorption band of [$\mathrm{SiV}^{0}$]{} compared to the relatively broad and weak absorption bands associated with [$\mathrm{NVH}$]{} in purely nitrogen-doped material \[Fig \[fig:UV-vis-ch\]\].

The processes employed during the charge 