Transformation of as-grown phosphorus-related centers in HPHT treated synthetic diamonds

This communication presents new data on phosphorus-containing centers in synthetic diamonds grown in the P-C system by high-pressure high-temperature (HTHP) method and annealed in the temperature range of 2,073-2,573 K. The electron paramagnetic resonance (EPR) study has shown that as-grown at 1,873 K diamonds contain single substitutional nitrogen (P1) and single substitutional phosphorus (MA1) centers. The main part of the spin density in the MA1 center locates on the carbon atom C(1) separated from phosphorus by one carbon atom. HPHT annealing (7 GPa, 2,073-2,273 K) results in aggregating substitutional nitrogen and phosphorus atoms. On the first step of annealing (2,073 K) of as-grown diamonds nitrogen-phosphorus NIRIM8 (NP1) centers are created. It is supposed that nitrogen and phosphorus atoms in this center are separated by two carbons. Further temperature increasing shifts the nitrogen atom toward phosphorus and creates two new nitrogen-phosphorus centers NP2 and NP3 with the supposed structures C(1)-N-C-P and N-P-C(1), respectively. The main part of the spin density in MA1, NIRIM8 (NP1), NP2 and NP3 is located on the carbon atom C1. Annealing these samples in the temperature range of 2,073-2,273 K has shown vanishing of NIRIM8 and increasing of NP2 and NP3 centers. HPHT annealing of diamonds at 2,573 K significantly changes the electron paramagnetic resonance (EPR) spectra: all previous nitrogen-phosphorus centers disappear and two new phosphorus centers NP4 and NP5 are created. Features of these centers are g approximate to 2.001 and high spin density located on the phosphorus atoms. The NP5 center is sensitive to X-ray irradiation and low-temperature annealing. The EPR spectra of both these centers are due to the hyperfine structure of one phosphorus atom. The structures of all phosphorus-containing centers are discussed