  The evolution of the polycrystalline pattern of electron paramagnetic
resonance (EPR) spectra of intrinsic and induced paramagnetic centers in an
ensemble of submicrometer diamond particles on diminishing average particle
size is considered. Recent experimental data unambiguously demonstrate
consistent reduction and then zeroing of typical hyperfine pattern from P1
centers on approaching a certain particle size and below it. These changes are
accompanied by appearance and strengthening of new structureless signals. In
small particles the electron wave function of a surface paramagnetic center is
delocalized over the whole nanoparticle. In result the electron spin
"experiences" the average field of all surrounding nuclei, which is zero. At
the same time, the paramagnetic centers localized in the bulk of a nanoparticle
are also cut off the hyperfine interaction due to the electron spin diffusion.
Thus, upon the nanodiamond particle size decreases the hyperfine related
features of the polycrystalline EPR spectrum become weaker and the spectrum
appear to be structureless.
