Journal of Nuclear and Radiochemical Sciences, Vol. 11, No.1, pp. A1-A3, 2010

Accounts

The Role of Emission Mössbauer Spectroscopy in the Study of Sophisticated Materials*
Amar Nath*
Department of Chemistry, University of North Carolina at Asheville, Asheville, NC, 28804, USA.
Received: December 1, 2009; In Final Form: January 15, 2010
Emission Mössbauer spectroscopy is about three orders of magnitude more sensitive than the conventional
absorption mode, thereby allowing parts per million substitution of 57Co at the Cu or Mn active site without
perturbing the finely-tuned material. 57 Co faithfully probes the behavior of Cu-based high temperature
superconductors and mixed valence magnetoresistive manganites. The application of emission Mössbauer
spectroscopy to biological systems is also highlighted.

1. Electronic Relaxation Following an Auger Event:
Whither Goes the Colossal Excitation Energy
A 57Co nucleus undergoes radioactive decay by capturing an
orbital electron – most frequently from the K-shell. It triggers
Auger ionization and thus the vacancy cascades traverse to thevalence shell, resulting in a multiply-charged ionic species.
Pollak1 estimated that the maximum charge build up for thedaughter 57 Fe as +8, viz Fe 8+. The Auger vacancy cascade
occurs in about 10 -16 s. This charge is rapidly neutralized bytransfer of electrons from neighboring atoms in the molecule.
This redistribution of charges in the molecule would occur
within 10 -15 s, and therefore without any change in the relative
positions of the individual constituent nuclei. In the gas phase,several researchers have shown that the multi-center Coulombic
repulsion explodes the molecule containing 80m Br, leading toformation of several highly charged fragments.2,3 A similar
picture of disruption of molecules in liquids has been observedby Libby,4 among others. We initiated Mössbauer investigations on a variety of cobalt complexes incorporating 57Co during 1966/1967 while I was visiting the Nobelist Libby’s
laboratory at UCLA and then continued these studies while
visiting Nobelist Calvin’s laboratory in UC Berkeley. Libby,
who was conditioned by the Auger after-effects in gas and liquid phase, was not particularly thrilled by our venture. He
commented, “What can you learn about a crystal vase using asledge hammer?” His concern was that all cobalt complexes
would succumb to the Auger event.
However, in a solid, things could be different. We found thatmost of the cobalt complexes fragment and show spectra of
Fe2+ /Fe3+. On the other hand, complex molecules with conjugated ring systems (having delocalized electrons), such ascobalt phthalocyanine, tris-bipyridyl Co(III) perchlorate, andvitamin B12 , escape fragmentation in 100% of the Auger
events.5-7 As a spin-off, we could follow directly the electronically-excited isotopic exchange of carrier-free 57Co ++ with complexed Co in Co(III)(bipy)3 (ClO4)3 using emission Mössbauer
spectroscopy (EMS).8 We also found that the probability offragmentation depends on the degree of conjugation in the molecule.9 It was not surprising that molecules of several cobalt
complexes fragment following the Auger event, as predicted by
Libby. What was amazing was that some molecules whichhave conjugated ligands with delocalized π-electrons escapefragmentation. We proposed the following model: There is alarge charge built up on the molecule following the Auger*Corresponding author. E-mail: anath@unca.edu

event. It is followed by electronic relaxation, with electrons
flowing from neighboring molecules depositing tens of eVelectronic excitation energy. The molecule has to get rid of thisenergy in less than 10 -14s to escape fragmentation. Energy dissipation through radiationless transitions is a slow process. We
proposed that the delocalized π-electrons undergo collective
excitation and that the plasmon decays in <10 -14 s, transferring
practically all of its energy to a single electron, which is emitted.9,10
The proposed novel mode of dispersal of colossal electronic
excitation (>50 eV) from a molecule remained unnoticed due tobenign neglect or skepticism until recent photoemission studiesof C60 and C70.11,12 The European scientists illuminated fullerenes with photon energies of 5 to 50 eV and observed that thephoto-ion yield is dominated by a strong resonance at about 20
eV. The results were interpreted invoking collective excitationof π-electrons either by a single or a high intensity multi-photon process. The plasmon is strongly coupled to single electron
continuum states, and the plasmon decay is associated with anejection of an electron, which overshadows the direct photoionization process. Our prior work was not cited. We submitthat the strong coupling is not confined to C60 and C70 molecules as alluded to by the authors, but that all molecules with alarge number of delocalized electrons would exhibit this novel
mode of self-protection.13 Why the plasmons are not coupled
to single electron continuum states in covalent compounds withno conjugation (and thereby offer no self-protection from fragmentation) is not known and remains a fertile area for investigation by theorists.
2. What Is the Spin-Off Arising from These Emission
Mössbauer Studies?
First, EMS can be very valuable in the study of biologicalprocesses, especially for systems having a fair degree of conjugation, e.g., vitamin B12, hemoglobin and zinc fingers. EMS isabout three orders of magnitude more sensitive than the conventional absorption mode. 57Co has a theoretical specificactivity of 310 GBq/mg, and so a couple of micrograms of 57Co
will suffice for measurements. Secondly, one can substituteminuscule amounts of 57Co for Cu in high temperature superconductors and for Mn in mixed valence magnetoresistive
manganites without perturbing the systems. Both systems havedelocalized electrons for plasmon formation followed by deexcitation with transfer of energy to an emitted electron, andthereby escaping damage arising from Auger after-effects. It
may be noted that for purely ionic systems with little covalence,
presence of delocalized electrons is not a prerequisite for EMS© 2010 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/18/2010

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J. Nucl. Radiochem. Sci., Vol. 11, No. 1, 2010

Amar

investigations.
To give readers a flavor as to how EMS can be used to study
biomolecules and to probe microscopic behavior in materials,
I will mention a few highlights of our findings.
3. Emission Mössbauer Studies of Cobalamins
Large conformational changes in the porphyrin ring, as
exhibited by isomer shift and quadrupole splitting, wereobserved just by hydrolysis of a couple of propionamide side
chains. We suspect that conformational isomerization precedeshydrolysis. Simple hydrolysis of a couple of side chains far
removed from 57Co sitting in the center of the porphyrin ringshould not bring about such drastic changes in Mössbauer
parameters.14 Modern fast techniques in NMR spectroscopy
can hopefully detect a short-lived conformational isomer preceding hydrolysis of Vitamin B12.
4. Emission Mössbauer Study of Cooperativity in Hemoglobin
Cobalt substitution for Fe in the porphyrin ring does notbring about any major change in the general function of hemoglobin (Hb), namely to pick up oxygen in the lungs and totransport it to the muscle tissues with high efficiency. Once a
sub-unit of Hb picks up oxygen , it transmits the message to theremaining three sub-units to do the same with higher affinity,
and then unloads the cargo with high efficiency where everrequired. This cooperative behavior has been a challenging
problem for biochemists and continues to be so even at present.
Research in the area of cooperativity of Hb was completely
dominated by Perutz’s model for several years. An importantfeature of Perutz’s trigger mechanism for cooperativity in thereversible oxygenation of Hb was the tension along the histidine-metal linkage in deoxy Hb and deoxy cobalt Hb (CoHb),
supposedly due to the pull exerted by the globin on the metal
atom. We attempted to verify the existence of this pull bystudying the emission Mössbauer spectra of deoxy-57CoHb,
oxy-57CoHb, oxy-57CoMb and 57Comet-Hb. For none of these
CoHb and myoglobin, the emission Mössbauer spectrum
matched with the absorption spectrum of the correspondingiron analog. These observations indicate that the daughter 57Fe
atom is “frozen” almost in the same spatial situation as that ofthe parent 57Co. The protein is apparently holding the cobalt
atom in position rather rigidly and after the electron capture
decay of the 57Co atom, the protein does not permit the daughter 57Fe to move to a position characteristic of the iron atom inthe time scale of Mössbauer emission.15 Our EMS investigations show the absence of tension in the Co/Fe bond as proposed by Perutz. However, this area of research is still
important especially due to the finding that that there are several intermediate states between the completely fully oxygenated “relaxed” state and the “tense” deoxygenated one. EMS
can also provide valuable information about the conformationalchanges occurring in sickle-cell anemia.
5. Emission Mössbauer Studies of Some Outstanding
Problems in High Temperature Superconductivity
Mössbauer spectroscopy provides a very valuable tool for
obtaining information on the nanoscopic level of a local site.
Emission Mössbauer spectroscopy is much more sensitive than
the regular absorption mode. For emission Mössbauer effectstudies, a few tens of parts-per-million of Cu are substituted
by 57Co with minimal perturbation of the system. 57Co faithfully microprobes subtle changes in the electron density, local
dynamics, presence of extraneous oxygen and magnetic behavior. A few examples of our studies, which led to a better understanding of some superconducting systems, are mentioned
below.

Figure 1. Diffraction studies could not decide positional disorder ofO2-.

A high resolution neutron powder diffraction study showedthat the Cu-O chain in YBa2Cu3O7 is zigzag with two potential
minima for the O2- ions at about 0.015 nm on either side of thechain. Neutron diffraction could not tell whether the zig-zag
configuration is static or dynamic with O2- oscillating between
the split-sites. It has been proposed earlier that should the oxygen ions be moving in a double-well potential that would resultin an extraordinary enhancement of the electron-lattice coupling which would explain the high superconducting transition
temperature. Our Mössbauer studies show that the oxygen ions
are not stationary but oscillate between the two potential
wells.16,17 We also find that partial substitution of yttrium bypraseodymium in the system results in dampening and finallykilling of this oscillation. This correlates well with the deleterious effect of Pr on superconductivity.16,17

Figure 2. Mössbauer spectra of as prepared and deoxygenated Nd257
xCexCu( Co)O 4.

The electron-doped material Nd2-xCexCuO4 becomes superconducting with a Ce 4+ content around 0.16, but only after
removal of a minuscule amount of extraneous oxygen. This
enigmatic behavior has attracted considerable attention. We
find that the presence of a minuscule amount of oxygen resultsin the localization of the static magnetic order in the Cu-O
plane. These microscopic spin clusters have variable size centered around 10 nm. Upon slight deoxygenation, a major fraction of spin clusters associated with a six line spectrum
vanishes, and converts to a paramagnetically relaxed species
with a doublet, which is the signature of the superconducting
phase. The Mössbauer parameters also indicate that Nd-CeCu-O is a truly electron-doped superconductor contrary to several reports in the literature.16-19
The Mössbauer studies of HoNi(57Co)B2C provide a direct
evidence for the electronic phase inhomogeneity in the material.20
6. Emission Mössbauer Studies of Magnetoresistive
Manganites
On substitution of Ca2+ for La3+ in LaMnO3, the compoundLa1-xCa x MnO3 becomes a ferromagnetic metal for 0.18 ≤ x <0.5. The hopping of an electron from Mn3+ to O2- is associatedwith a simultaneous hop from the latter to Mn4+. The probability of the hopping by this double exchange (DE) is the highest
when both hopping electrons have the same spin orientation
requiring Mn ions to be ordered ferromagnetically. The metal-

The Role of Emission Mösbauer Spectroscopy in the Study of

lic behavior is synergistically associated with the ferromagnetic
order through electron hopping. The resistivity increases considerably near the Curie temperature (TC). On application of amagnetic field near TC, there is a dramatic decrease in resistivity due to regeneration of metallicity. The origin of this colossal magnetoresistivity (CMR) is rather enigmatic and has
attracted lot of attention from researchers. In short, what was
baffling the scientific community was as to how a moderatemagnetic field could convert what was believed to be a paramagnetic state to a ferromagnetic one. We have addressed thisproblem using emission Mössbauer spectroscopy where thelocal probe 57Co/ 57Fe substituting minuscule amount of Mn
atoms senses the microscopic behavior of the material. Only afew parts per million substitution of Mn atoms with 57Co substituting minuscule amount of Mn atoms senses the microscopic behavior of the material. We find that, surprisingly thelong range ferromagnetic order starts breaking down below theCurie temperature (TC) and the material degenerates into nanosize spin clusters which fluctuate rapidly and exhibit superparamagnetic-like behavior. On a time scale of 10 -8 s, the spinclusters exhibit minimal lattice distortions. These spin clusters
survive well above TC, and would very effectively increase bulk
resistivity due to their rapid fluctuations. On application of an
external magnetic field, the small magnetic clusters coalesce toform larger ones with more ordered spins, and the materialshows enhanced conductivity through percolation. This provides a rationale for observation of magnetoresistivity in manganites.21-24 EMS investigations permit us to conclude that, thebreak up into small rapidly fluctuating spin clusters near Tc
and thereby exhibiting superparamagnetic-like behavior is aprerequisite for observing bulk colossal magnetoresistivity.

Figure 3. Mössbauer spectra of single crystal La0.9Ca0.1Mn(57Co)O3
at T=175K.

7. Conclusions
We find that complex molecules with conjugated systems
having delocalized electrons escape fragmentation following
the Auger event. The huge amount of electronic excitationenergy deposited during neutralization of the built-up charge
results in collective excitation and formation of plasmon. The
plasmon decays transferring all its energy to an electron, whichleaves the molecule unscathed. This opens up the possibilityof using emission Mössbauer spectroscopy to systems withdelocalized electrons like superconductors, and mixed valencemagnetoresistive manganites and some biological systems.

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Parts of this paper were presented as plenary talk at
APSORC 09
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