the group-IV-vacancy centres (SiV [@Evans2018; @Rose2018; @Green2017c], SnV [@Iwasaki2017], and PbV [@Trusheim2019]) have recently emerged as potential candidates in QIP applications.

Unlike bulk nitrogen-doped diamond, where a significant effort stretching over decades has identified many nitrogen-related point defects [@Zaitsev2001; @Dischler2012], relatively little experimental study has been performed on high-quality single crystal diamond which is bulk-doped with silicon. The only definitive assignments of optical centers to silicon are the well-known [$\mathrm{SiV}^{-}$]{} [@Goss1996; @Rogers2014a; @Hepp2014a] and [$\mathrm{SiV}^{0}$]{} [@DHaenens-Johansson2011; @Green2017c; @Green2019]. Additionally, electron paramagnetic resonance (EPR) studies have identified [$\mathrm{SiVH}^0$]{}[@Iakoubovskii2003a; @Edmonds2008a] and [$\mathrm{SiV_2H}^0$]{}[@DHaenens-Johansson2010], while a tentative assignment has been made to [$\mathrm{SiB}^0$]{}[@Nadolinny2016]. Density functional theory (DFT) studies of silicon-related point defects indicate that isolated substitutional silicon, [$\mathrm{Si_s}$]{}, is stable though aggregates are energetically unfavorable [@Goss2007]. Some silicon-related multi-vacancy, multi-hydrogen, nitrogen-related complexes are theoretically stable [@Goss2007; @Thiering2015] but most have yet to be identified experimentally.

In this work, we have studied silicon and nitrogen co-doped single-crystal synthetic diamond from as-grown to a treatment temperature of using a combination of optical absorption spectroscopies and EPR. We identify the neutrally-charged silicon-vacancy-nitrogen complex [$\mathrm{SiVN}^0$]{} through combined experimental measurements and theoretical modeling, and tentatively assign an infrared absorption mode at to substitutional silicon.

Charge transfer {#subsec:charge_transfer}
---------------

It 