Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Nitrogen-Containing Species in the Structure of the Synthesized
  Hydroxyapatite   Synthesized by the wet chemical precipitation technique hydroxyapatite
powders (HAp) with the sizes of the crystallites of (20-50) nm and 1000 nm were
analyzed by different analytical methods. By means of electron paramagnetic
resonance (EPR) it is shown that during the synthesis process nitrate anions
from the reagents (by-products) could incorporate into the HAp structure. The
concentration of the stable NO32- radicals detected after X-ray irradiation of
the product at room temperature does not decrease even after annealing up to
400 oC. The relaxation times and EPR parameters of the axially symmetric NO32-
paramagnetic centers are measured with high accuracy. Increased sensitivity and
resolution of high-frequency EPR (95 GHz) allows to reveal additional
paramagnetic centre(s) which origin is discussed. Analyses of electron-nuclear
double resonance (ENDOR) spectra from 1H and 31P nuclei and ab-initio density
functional theory (DFT) calculations allow suggesting that the paramagnetic
centers and nitrate anions as the precursors of NO32- radicals preferably
occupy PO43- site in the HAp structure.

32
The categories of this paper are: atom-ph
35
The categories of this paper are: mtrl-th
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
24
The categories of this paper are: dg-ga
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
0
The categories of this paper are: hep-ph
20
The categories of this paper are: acc-phys
13
The categories of this paper are: quant-ph
1
The categories of this paper are: cs