typically orders of magnitude higher than the grown-in NV concentrations [@Edmonds2012], we presume the majority of the silicon is incorporated as substitutional silicon [@DHaenens-Johansson2011], as discussed in §\[subsec:annealing\]. Therefore, there is a substantial source of silicon available within the sample itself from growth.

We have not identified [$\mathrm{SiVN}^0$]{} in any as-grown samples (putting an upper limit on the as-grown concentration of approximately ). We first observe [$\mathrm{SiVN}^0$]{} upon HPHT annealing at (for one hour, or at ), and its concentration increases up to the maximum temperature \[Fig \[fig:annealing\]\]. As is typical for vacancy-containing defects in diamond, we assume [$\mathrm{SiVN}$]{} production must occur via vacancy-assisted migration of impurities, as the energy required for direct diffusion of substitutional nitrogen ( [@Goss2006]) and silicon is significantly higher than the vacancy-assisted mechanisms [@Mainwood1994]. Furthermore, the diffusion barrier for [${\ensuremath{\mathrm{NV}}\xspace}^{-}$]{}( [@Pinto2012]) is significantly lower than for [$\mathrm{SiV}^{0}$]{}( [@Gali2013; @Goss2007]). At high temperatures where [[$\mathrm{NV}$]{}]{}is unstable, nitrogen may diffuse through the lattice by concerted exchange with a vacancy before the NV pair breaks up [@Pinto2012]. We therefore understand [$\mathrm{SiVN}$]{}production to occur via the diffusion of vacancies and subsequent capture by [$\mathrm{Si_s}$]{}, producing [$\mathrm{SiV}$]{}; and the vacancy-assisted diffusion of nitrogen to [$\mathrm{SiV}$]{}centers producing [$\mathrm{SiVN}$]{}.

Recent reports of 