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15
MINICOURSES
Mössbauer spectroscopy for materials science: The aging effect in
superconductor CaLaBa{Cu 1−x Fe x } 3 O 7−δ with 0≤ x ≤0.07.
Angel Bustamante 1
1 Laboratorio de cerámicos y nanomateriales, Universidad Nacional Mayor de San Marcos,
Lima, Peru.
The Mössbauer effect is the emission and absorption resonant recoilless nuclear gamma
rays and corresponds to the zero-phonon transition. The term recoilless is interpreted as
the crystal as a whole suffers a recoil almost negligible. Mössbauer effect is likely to
happen and corresponds to the factor f which depends on the temperature. To this effect
and using the Doppler velocity hyperfine interactions detected as the isomer shift (IS)
which is due to the finite size of the nucleus and the term expansion monopolar
electrostatic interaction energy between nucleus, total electric charge Ze distributed
over the nuclear volume density N (r) at point r and an electrical potential V(r) due to
the loads on the environment. This interaction also displaces energy levels without
altering the transition energies of both the radioactive source which emits the gamma
ray absorber and provided in the oxidation state and coordination atom studied. The
second term in the expansion is the electric dipole between the electric dipole moment
and the electric field in the center of the core. This term is zero because the core does
not have electric dipole moment. The third term corresponds to the interaction of the
nuclear quadrupole moment with the electric field gradient (EFG) produced by an
asymmetric distribution around the nucleus charges (electrons, ions, dipoles, etc.) and is
the cause of the degeneration partially unfold 2I + 1 nuclear levels and for the case of
Fe 57 with fundamental level, I g =1/2, that unfolds, and excited, with I e =3/2, is split into
two sublevels with m 3/2 = 3/2 and 1/2, two lines shown in the Mössbauer spectrum
whose separation is called quadrupole splitting QS or E Q , and provides information on
the oxidation state and coordination number of the distortion of the crystal lattice.
Nuclei with nuclear magnetic moment can interact with a magnetic field H which can
be a field external or internal fields o hyperfine fields H hf that can be produced by the
spin and the unpaired electrons (dipole field), the angular moment produced by the
orbital electrons. Electrons "s" and those who are not full layers contribute to this
component (orbital field) and produced by electrons "s" that penetrate the nucleus
(Fermi contact field). The magnetic interaction completely breaks the degeneracy of the
nuclear spin levels I0 and unfolded each level 2I+1 equally spaced sublevels. The
allowed transitions between these sublevels are governed by the selection rules
m I =0,1 andI =1. The experimental arrangement for the case of iron considered a
radioactive Co 57 source in a matrix of Rhodium which emits 14.4 Kev gamma ray, an
absorber containing iron, a multichannel analyzer 512 or1024 channels and a software
for fitting spectra. In this talk, we will present an introduction to Mössbauer
spectroscopy an the long-term aging effect caused by Fe atoms in the superconductor
CaLaBa{Cu 1−x Fe x } 3 O 7−δ with 0 ≤ x ≤ 0.07. XRD confirms that this system has a
YBCO-like structure. The critical temperature (Tc) is strongly affected by aging and
depends on the amount of Fe in the structure. Room temperature Mössbauer
spectroscopy reveals the presence of the typical species A, B–B´, C and new species
E´and F. Interestingly; A, which corresponds to the Fe 3+ atom located in the Cu(1) of
the chains with spin S z = 3/2, shows a drastic reduction which means migration to the
species B, B´ and C. Species B and B´correspond to the Fe 3+ in the Cu(2) site forming
planar quasi-octahedral and planar square pyramidal, while the C specie is a square
pyramidal with O(5) respectively (spin S z = 3/2 in all these cases). Aging causes loss of
superconductivity in the samples with 5 and 7% of iron content.