  Moessbauer transmission spectra for the 14.41-keV resonant line in 57Fe have
been collected at room temperature by using 57Co(Rh) commercial source and
alpha-Fe strain-free single crystal as an absorber. The absorber was magnetized
to saturation in the absorber plane perpendicular to the gamma-ray beam axis
applying small external magnetic field. Spectra were collected for various
orientations of the magnetizing field, the latter lying close to the [110]
crystal plane. A positive electric quadrupole coupling constant was found
practically independent on the field orientation. One obtains the following
value Vzz=+1.61(4)x10^19 V/m^2 for the (average) principal component of the
electric field gradient (EFG) tensor under assumption that the EFG tensor is
axially symmetric and the principal axis is aligned with the magnetic hyperfine
field acting on the 57Fe nucleus. The nuclear spectroscopic electric quadrupole
moment for the first excited state of the 57Fe nucleus was adopted as +0.17 b.
Similar measurement was performed at room temperature using as-rolled
polycrystalline alpha-Fe foil of high purity in the zero external field.
Corresponding value for the principal component of the EFG was found as
Vzz=+1.92(4)x10^19 V/m^2. Hence, it seems that the origin of the EFG is
primarily due to the local (atomic) electronic wave function distortion caused
by the spin-orbit interaction between effective electronic spin S and
incompletely quenched electronic angular momentum L. It seems as well that the
lowest order term proportional to the product L.LAMBDA.S dominates, as no
direction dependence of the EFG principal component is seen. The lowest order
term is isotropic for a cubic symmetry as one has LAMBDA=lambda.1 for cubic
systems with the symbol 1 denoting unit operator and lambda being the coupling
parameter.
