Optimization of the coherence properties of diamond samples with an intermediate concentration of NV centers oleh O.R. Rubinas, V.V. Soshenko, S.V. Bolshedvorskii, A.I. Zeleneev, A.S. Galkin, S.A. Tarelkin, S.Y. Troschiev, V.V. Vorobyov, V.N. Sorokin, A.A. Sukhanov, V.G. Vins, A.N. Smolyaninov, A.V. Akimov