with the nitrogen \[Table \[tab:spin\_hamiltonian\_parameters\]\], indicating a low-symmetry defect with essentially zero unpaired electron spin density on the nitrogen nucleus [@Green2015].

A large number of purely nitrogen-related defects have been identified by EPR in diamond, including [$\mathrm{N_{s}^{0}}$]{} [@Smith1959a], [${\ensuremath{\mathrm{NV}}\xspace}^{-}$]{} [@Loubser1977; @Loubser1978], interstitial nitrogen [@Felton2009], and even substitutional nitrogen pairs [@Nadolinny1999a]. It is thus unlikely that a new defect which involves only nitrogen would be identified in material which is novel due to its simultaneously high concentration of nitrogen and silicon. Therefore, we hypothesise that this defect must also contain silicon, whose natural abundance of $^{28}\mathrm{Si}$ ($I=0$) makes it difficult to identify without a high defect concentration.

![a) EPR spectrum of [Sample G]{}with $B\|\hkl<110>$. Experiment in black; simulation in red. b) Angular variation (in a plane) of measured EPR transition fields (circles) overlaid with a simulation produced using the spin Hamiltonian parameters given in Table \[tab:spin\_hamiltonian\_parameters\]. To improve clarity, only transitions with a theoretical intensity of the most intense transition are shown.[]{data-label="fig:110_and_roadmap"}](graphs_epr_principal_directions_110_and_roadmap_combined){width="\columnwidth"}

![EPR spectrum of [Sample G]{}with $B\|\hkl<111>$. Experimental data in black; simulation in red. Additional panels show the $^{29}\mathrm{Si}$ hyperfines on each side of the primary spectrum. As expected, their intensity is of the primary spectrum. Simulation generated by EasySpin 