with studies of silicon-doped HPHT-grown samples [@Palyanov2017; @Palyanov2015b], we tentatively assign the peak to substitutional silicon. Difference spectra reveal essentially no change between the as-grown and samples, with subsequent anneals reducing the strength of the mode \[Fig. \[fig:ir\_comparison\](c)\]— this is consistent with the increase in observed Si-related defects from [Sample A]{}to [Sample F]{}\[Fig \[fig:annealing\]\].

The concentration of [$\mathrm{SiV}^{0}$]{}$^{/-}$ increases by over an order of magnitude from [Sample A]{}to [Sample F]{}. We conclude that the majority of the silicon was originally incorporated in other forms (assumedly substitutionally) during growth, with the subsequent production of [$\mathrm{SiV}$]{} proceeding by vacancy capture during the HPHT treatment, analagous to the production of the $\mathrm{N}_n\mathrm{VH}$ defects. EPR measurements of samples annealed at and higher reveal the presence of a previously-unidentified silicon-containing defect. We identify this defect as a silicon-vacancy center decorated with a nitrogen atom ([$\mathrm{SiVN}^0$]{}): the defect is discussed further in §\[sec:sivn\]. The concentration of [$\mathrm{SiV}^{0}$]{}and [$\mathrm{SiVN}^0$]{} measured in [Sample F]{}\[Fig. \[fig:annealing\]\] indicates that at least of silicon must have been incorporated during growth.

Between samples A and F, approximately of substitutional nitrogen has been lost in addition to of [${\ensuremath{\mathrm{NVH}}\xspace}^{0}$]{}$^{/-}$, and is accompanied by the production of approximately and of [${\ensuremath{\mathrm{N_2VH}}\xspace}^{0}$]{} and [${\ensuremath{\mathrm{N_3VH}}\xspace}^{0}$]{}, respectively. 