Effect of pre-annealing on NO super(2)- sub(3) centers in synthetic hydroxyapatite

Effect of pre-annealing of synthetic hydroxyapatite (HAP) on properties of gamma - and UV- induced NO super(2)- sub(3) centers was studied by electron paramagnetic resonance (EPR). Nitrate-containing hydroxyapatite powders ((N)HAP)) and the powders with an admixture of carbonate and nitrate ions ((C,N)HAP) were annealed in the temperature range T sub(ann) = 20 degree C - 600 degree C before irradiation. It was found that pre-annealing of (N)HAP samples changes the parameters of NO super(2)- sub(3) centers while no changes took place in (C,N)HAP. Moreover, at the pre-annealing temperatures Tann > 200 degree C two new NO super(2)- sub(3) centers were observed in (N)HAP samples; they are characterized by larger value of A sub(L) (3.67 and 4.41 mT) as compared to the known centers. It was also found that the dependence of NO super(2)- sub(3) centers amount on T sub(ann) is non-monotonous in both types of samples. Presumably this is caused by the escape of water molecules from HAP during the annealing and essential modification of the defect subsystem of HAP.