  Detailed 57Fe Mossbauer spectroscopy measurements on superconducting NaFeAs
powder samples have been performed in the temperature range 13 K < T < 300 K.
The 57Fe spectra recorded in the paramagnetic range (T > TN ~ 46 K) were
discussed supposing that most of the Fe2+ ions are located in distorted (FeAs4)
tetrahedral of NaFeAs phase, while additional minor (< 10%) component of the
spectra corresponds to impurity or intergrowth NaFe2As2 phase with a nominal
composition near "NaFe2As2". Our results reveal that the structural transition
(TS ~ 55K) has a weak effect on the electronic structure of iron ions, while at
T < TN the spectra show a continuous distribution of hyperfine fields HFe .
Shape of these spectra was analyzed in term of two models: (i) an
incommensurate spin density wave modulation of iron magnetic structure, (ii) a
formation of a microdomain structure or phase separation. It was shown that the
hyperfine parameters obtained using these two methods have very similar values
over the whole temperature range. The analysis of the temperature dependence
HFe(T) with the Bean-Rodbell model leads to ksi = 1.16 +/- 0.05, suggesting
that the magnetic phase transition is first-order in nature. A sharp evolution
of the VZZ(T) and etha(T) parameters of the full Hamiltonian of hyperfine
interactions near T ~ (TN, TS), were interpreted as a manifestation of the
anisotropic electron redistribution between the dxz, dyz- and dxy-orbitals of
the iron ions.
