Applications of Mössbauer Spectroscopy

340 Pages·1976·17.831 MB·English

by  Richard L. Cohen (Eds.)

Contributors
Richard L. Cohen
Peter G. Debrunner
P. K. Gallagher
N. H. Gangas
V. I. Goldanskii
B. Keisch
L. A. Korytko
A. Kostikas
George Lang
Henry Leidheiser, Jr.
W. T. Oosterhuis
L. H. Schwartz
Gary W. Simmons
A. Simopoulos
K. Spartalian
APPLICATIONS OF
MÖSSBAUER
SPECTROSCOPY
Volume I
Edited by
Richard L. Cohen
Bell Laboratories
Murray Hill, New Jersey
@
ACADEMIC PRESS New York San Francisco London 1976
A Subsidiary of Harcourt Brace Jovanovich, Publishers
COPYRIGHT © 1976, BY BELL TELEPHONE LABORATORIES, INC.
ALL RIGHTS RESERVED.
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PERMISSION IN WRITING FROM THE PUBLISHER.
ACADEMIC PRESS, INC.
Ill Fifth Avenue, New York, New York 10003
United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1
Library of Congress Cataloging in Publication Data
Main entry under title:
Applications of Mössbauer spectroscopy.
Bibliography: v. 1, p.
Includes index.
1. Mössbauer spectroscopy. I. Cohen, Richard Lewis,
(date)
QC491.A66 537.5'352 75-26349
ISBN 0-12-178401-0 (v. 1)
PRINTED IN THE UNITED STATES OF AMERICA
List of Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
Richard L. Cohen (1), Bell Laboratories, Murray Hill, New Jersey
Peter G. Debrunner (171), Physics Department, University of Illinois,
Urbana, Illinois
P. K. Gallagher (199), Bell Laboratories, Murray Hill, New Jersey
Ν. H. Gangas (241), University of Ioannina, Ioannina, Greece
V. I. Goldanskii (287), Institute of Chemical Physics, USSR Academy of
Science, Moscow, USSR
B. Keisch (263), National Gallery of Art Research Project, Carnegie-Mellon
Institute of Research, Pittsburgh, Pennsylvania
L. A. Korytko (287), Institute of Chemical Physics, USSR Academy of
Science, Moscow, USSR
A. Kostikas (241), Nuclear Research Center Democritos, Athens, Greece
George Lang (129), Department of Physics, The Pennsylvania State Uni-
versity, University Park, Pennsylvania
Henry Leidheiser, Jr. (85), Center for Surface and Coatings Research,
Lehigh University, Bethlehem, Pennsylvania
W. T. Oosterhuis (141), t Physics Department, Carnegie-Mellon University,
Pittsburgh, Pennsylvania
L. H. Schwartz (37), Materials Science and Engineering Department,
Northwestern University, Evanston, Illinois
Gary W. Simmons (85), Center for Surface and Coatings Research, Lehigh
University, Bethlehem, Pennsylvania
A. Simopoulos (241), Nuclear Research Center Democritos, Athens, Greece
K. Spartalian (141), î Physics Department, Carnegie-Mellon University,
Pittsburgh, Pennsylvania
f
Present address: Division of Materials Research, National Science Foundation, Washing-
ton, D.C.
% Present address: Physics Department, The Pennsylvania State University, University
Park, Pennsylvania.
ix
Preface
The technique of Mössbauer spectroscopy, now 15 years old, is
beginning to be used in many analytical and engineering research areas.
Many of these efforts in applied science have been highly successful, and we
can expect them to form an increasing proportion of the research performed
using Mössbauer spectroscopy. The utilization and propagation of these
results in new areas has been somewhat hampered by the fact that the
experimental techniques, the interpretation of the results, and the significance
of the findings are relatively foreign to, for example, an archaeologist or a
protein chemist.
The main goal of this series will be to make available to scientists and
engineers in varied disciplines, in their own terms, a discussion of what
results have been achieved by Mössbauer spectroscopy to date, and what
additional advances are likely. The emphasis will be on fields often con-
sidered in the grouping "materials science." Basic physics and chemistry
studies performed by Mössbauer spectroscopy have been extensively re-
viewed elsewhere and will be described here only to the extent necessary
for the understanding of the applied science.
In this first volume, the emphasis is on metallurgy, solid state and in-
terface chemistry, and structure of iron-containing proteins. In the second
volume, the topics will be extended to include catalysis, studies of disordered
systems, and diffusion.
xi
/
Elements of
Mössbauer Spectroscopy
Richard L. Cohen
Bell Laboratories
Murray Hill, New Jersey
I. Introduction 1
II. Interpretation of Mössbauer Spectra 4
A. What Does Mössbauer Spectroscopy Measure? 4
Β. Isomer Shift 7
C. Quadrupole Coupling and Magnetic Hyperfine
Interaction 10
D. Dynamic Effects—Paramagnetic Hyperfine
Structure and Superparamagnetism 18
E. Lineshape and Resonance Intensity 23
III. Experimental Techniques 28
A. Mössbauer Spectrometers, Detectors, and Data
Collection 28
B. Data Analysis 29
IV. Limitations and Future Possibilities 30
A. New Isotopes 30
B. Absorber Size and Geometry 31
C. Source Experiments 31
General Bibliography 32
References 32
I. Introduction
"Mössbauer spectroscopy" is the name given to a technique of
studying the absorption of y rays by the nuclei of atoms. The nuclear pro-
cesses producing this effect were first observed and reported by Rudolf L.
Mössbauer in 1958 (Mössbauer, 1958). This work immediately attracted wide
interest because of the unprecedented sharpness of the resonance observed,
which held out great promise for studies of gravitation, relativity, and certain
1
2 Richard L. Cohen
areas of nuclear physics. The technique would have been relatively little used,
however, if those were the only possible topics for study—the vast majority
of experiments performed are in the fields of solid state physics and chemistry,
metallurgy, geochemistry, and biophysics. These topics also form the basis
of most of the applications-oriented work reviewed in this book.
It is easy to describe the processes occurring in nuclear y-ray-resonance
spectroscopy. Nuclei are the heavy cores of atoms and are generally con-
sidered to be composed of protons and neutrons. The number of protons in
the nucleus is the atomic number of the atom, and it determines the chemical
properties of the atom. For each element (atomic number), a number of
different isotopes, corresponding to different numbers of neutrons in the
nucleus, may be stable. These are the naturally occurring isotopes of that
element, identified by their mass number (sum of protons and neutrons).
Unstable (radioactive) nuclei undergo decay, or transformation, with the
emission of various kinds of radiation.
Even stable nuclei, however, have excited states, configurations in which
the nucleus has some discrete, well-defined quantity of added energy over that
present in the stable, or ground state, configuration. These excited states often
decay to the nuclear ground state, with the extra energy being emitted in the
form of a y ray. Gamma rays are electromagnetic radiation, identical in prop-
erties to χ rays. They have no electric charge and cannot be deflected by
electric or magnetic fields. When y rays pass through matter, they are ab-
sorbed or scattered primarily by occasional energetic collisions with elec-
trons. Thus, a beam of y rays that initially have the same energy loses its
intensity primarily due to the absorption of individual y rays. Those y rays
that are not absorbed or scattered out of the beam continue to propagate with
their original energy.
These facts are important for the present discussion because the basis of
the Mössbauer effect is the emission of y rays by radioactive nuclei, and the
subsequent reabsorption of these gamma rays by other nuclei of the same
type. The nuclear emission and absorption energies are slightly affected by
the solids in which the nuclei are incorporated. Using the Mössbauer effect,
these tiny energy changes can be measured, and used to deduce information
about the surroundings of the nucleus.
Figure 1 shows a schematic drawing of the nuclear decay and excitation
process. It is worthwhile to emphasize here a number of features of the y-ray
resonance absorption process that are implicit in the above description and
provide some of the distinctive advantages of the technique.
1. Since nuclear energy levels in the range involved here are so narrow
and sharply defined (see Section ΙΙ,Ε,Ι), y rays from any nucleus (e.g., 57Fe)
can only be reabsorbed by nuclei of the same type, since any other isotope
will have absorption energies (corresponding to excited states) in a different
1 Elements of Mössbauer Spectroscopy 3
NUCLEAR
1 τ <«- EXCITED
X
n
A STATE
r
Y
EMMITTED ABSORBED
c
w NUCLEAR
w
» 1 GROUND
STATE
RADIOACTIVE SOURCE Δ RQOR RPB
EMITTING / RAY ABSORBER
Fig. 1. Schematic indication of the events occurring in Mössbauer spectroscopy. The
horizontal lines represent the nuclear states. The diagram shows (left) the source nucleus going
from the excited state to the ground state, emitting a y ray. The y ray is subsequently absorbed
(right), raising the absorber nucleus to its excited state. The resonance absorption can be de-
tected either by the decreased transmission of the absorber, or by the subsequent decay of the
absorber nucleus out of the excited state.
energy region. Thus, experiments are absolutely specific to the particular
isotope involved, and no cross-interference from other isotopes or elements
ever arises.
2. It is possible to incorporate the radioactive source atoms in the mate-
rial to be studied and thus combine the advantages of radioactive tracer
experiments with those of the Mössbauer technique. Samples containing as
few as ~ 1012 probe atoms can thus be studied.
3. Since the resonance absorption is a purely nuclear process, its exis-
tence is inherently independent of the properties of the host (e.g., symmetry,
metallic character), which sometimes interfere with the use of other resonance
techniques.
4. The nuclear energy level perturbations observable using the Möss-
bauer effect arise only from the first few nearest-neighbor shells of an ion.
Thus, short-range order, over as little as 10-15 Â, is adequate to provide
sharp Mössbauer spectra. Glassy materials, disordered alloys, and very finely
divided samples can all produce well-defined spectra.
5. Sample preparation is usually very simple—single crystals are not
normally necessary, and no special polishing or surface treatment is required.
Power samples can be readily utilized.
6. The dependence of the recoil-free fraction (Debye-Waller factor, dis-
cussed in Section II,E,2) on the properties of the host lattice allows investi-
gation of the Debye temperature and anharmonic binding forces via the
temperature dependence of the resonance intensity.
7. Although the technique is in principle limited to studying nuclei in
solids, it is often possible to investigate dissolved molecules and com-
plexes by freezing the solutions and making measurements on the resulting
solid.
4 Richard L. Cohen
8. The existence of chemically, crystallographically, or magnetically in-
equivalent sites is generally revealed by the appearance of distinct com-
ponents, arising from the different sites, in the Mössbauer spectrum.
II. Interpretation of Mössbauer Spectra
A. What Does Mössbauer Spectroscopy Measure F
A Mössbauer spectrum is normally produced by varying the source y-ray
energy and measuring the (nuclear) resonance absorption as a function of
y-ray energy, as shown in Fig. 2. At y-ray energies that match the possible
excitation energies in nuclei in the absorber, the nuclear resonance will result
SOURCE OF
/
•••• • I
MOTION
1 Κ RESONANT
ABSORBER
COUNTER
NO
ABSORPTION \ /
*t \/
0- MAXIMUM \J
£ "ABSORPTION
D
ο
ο
1
- 0—> +
SOURCE VELOCITY
Fig. 2. Basic arrangement for measuring a Mössbauer spectrum in transmission geometry.
The source is moved to Doppler modulate the γ-ray energy. When the y rays have the proper
energy to be resonantly absorbed, the increased absorption produces a decrease in the number
of y rays transmitted through the absorber, and the counting rate decreases.
1 Elements of Mössbauer Spectroscopy 5
in increased absorption, and an absorption "line" will occur. This dip (or
series of dips) is the Mössbauer spectrum.
Chapters 3 and 9 describe resonance scattering experiments in which
backscatter geometry (as shown in Fig. 3) is used, rather than transmission
SOURCE OF \
Ύ RAYS \
MOTION •
Y///M
i ff
r
/^g^S. \J\S^— COUNTER
^ ^ ^ ^ O T ^ -I
^ ^ /^ RESONANT
SCATTERER
I
UJ
I
ο -~ +
SOURCE VELOCITY
Fig. 3. Mössbauer spectrometer for backscatter experiments. When the y rays incident
on the absorber are the proper energy to be resonantly absorbed, absorber nuclei are raised
into their excited states. They subsequently decay, emitting radiation that is detected by the
counter.
geometry. There are two advantages to this approach. (1) It is possible to
study thick samples in situ without the special preparation of a thin layer of
the material ; the advantages for the study of a valuable painting or locomotive
boiler should be obvious. (2) If the scattering is observed via the emission of
secondary radiation of low penetrating power, only the atoms near the
surface of the sample will contribute to the observed resonance effect. De-
pending on the isotope used and the sample material, surface layers from 0.1
to 1 μιη thick can be studied. The backscatter technique is also useful in cases
where the absorber consists of only a very thin layer of material, as in cor-
rosion research.