  Spin-lattice relaxation within the nitrogen-vacancy (NV) center's electronic
ground-state spin triplet limits its coherence times, and thereby impacts its
performance in quantum applications. We report measurements of the relaxation
rates on the NV center's $|m_{s}=0\rangle \leftrightarrow |m_{s}=\pm 1\rangle$
and $|m_{s}=-1\rangle \leftrightarrow |m_{s}=+1\rangle$ transitions as a
function of temperature from 9 to 474 K in high-purity samples. We show that
the temperature dependencies of the rates are reproduced by an ab initio theory
of Raman scattering due to second-order spin-phonon interactions, and we
discuss the applicability of the theory to other spin systems. Using a novel
analytical model based on these results, we suggest that the high-temperature
behavior of NV spin-lattice relaxation is dominated by interactions with two
groups of quasilocalized phonons centered at 68.2(17) and 167(12) meV.
