2 Synchrotron-Radiation-Based Energy-Domain Mössbauer …
81
Using this method, the site-specific PDOS in magnetite (Fe 3 O 4 ) was measured.
In magnetite, which is a well-known mixed-valence compound, the iron atoms are
located in two nonequivalent positions in the unit cell. One-third of the Fe ions
(Fe
3+ ) occupy the A sites and are tetrahedrally surrounded by four oxygen ions. The
remaining two-thirds of the Fe ions (Fe
3+ and Fe
2+ ) occupy the B sites and are octahedrally surrounded by six oxygen ions. Magnetite is ferrimagnetic, and the magnetic
moments of the A sites are aligned antiparallel to the magnetic moments of the B
sites below T N =858 K. Nuclear magnetic resonance spectroscopy and Mössbauer
spectroscopy cannot distinguish the Fe
3+ or Fe
2+ ions on the B sites, indicating the
delocalized nature of the charge carriers with a formal average valence of Fe
2.5+ at
room temperature [106]. The sample measured in this experiment was prepared such
that oxidization is prevented and was doped with 7 mol% Ni to replace Fe because
some B sites are probably oxidized to Fe
3+ in air [109]. The enrichment of
57 Fe in the
used sample was 95.5%. The energy spectrum of NRIS of
57 Fe in Fe 3 O 4 is shown
in Fig. 2.14a. Examples of the incoherent time spectra (measured at 16 meV and
35 meV) are shown in Fig. 2.14b [6]. From these time spectra, the ratio of A site
and B site at each phonon energy was obtained by least-squares fitting each time
spectrum with two exponential functions accompanied by sinusoidal quantum beats
corresponding to A and B sites. These ratios and the PDOS of all Fe atoms give the
partial PDOSs of A site and B site. The obtained spectra of the partial PDOS for all
Fe sites and the site-specific PDOSs of the A and B sites are shown in Fig. 2.15a. The
difference between the partial PDOS s of the A and B sites is clearly observed. In
Fig. 2.15b, the PDOSs of Fe in the ideal Fe 3 O 4 obtained from ab initio band calculations are shown. The PDOS of all Fe is shown as a black line, and the calculated
PDOSs of states of the A and B sites are shown as dashed green and dashed-anddotted dark yellow lines, respectively. The overall character of the calculated PDOS
of all Fe sites in Fe 3 O 4 agrees well with the PDOS of all Fe sites measured by
NRIS spectroscopy. Neutron inelastic scattering, X-ray inelastic scattering, Raman
scattering, and infrared absorption methods are well-known and very useful for the
study of atomic dynamics. It is, however, generally impossible to discern the atomic
motions of the same element in different environments. Therefore, this method is
considered to be unique.
2.3.4 Summary
NRIS spectroscopy is a method used for investigating the vibrational states in
substances. Since NRIS spectroscopy uses the nuclear resonant excitation accompanied by phonon creation and annihilation, unique and very effective measurement
is possible as shown above. The development of spectroscopic methods is ongoing,
and remarkable progress has been achieved in relation to the optics, detectors, and
methodologies used. Because this spectroscopy covers a broad range of scientific
areas, such as physical, chemical, biological, and earth sciences, recent developments
and further improvements of nuclear resonant scattering spectroscopy that will solve
81
Using this method, the site-specific PDOS in magnetite (Fe 3 O 4 ) was measured.
In magnetite, which is a well-known mixed-valence compound, the iron atoms are
located in two nonequivalent positions in the unit cell. One-third of the Fe ions
(Fe
3+ ) occupy the A sites and are tetrahedrally surrounded by four oxygen ions. The
remaining two-thirds of the Fe ions (Fe
3+ and Fe
2+ ) occupy the B sites and are octahedrally surrounded by six oxygen ions. Magnetite is ferrimagnetic, and the magnetic
moments of the A sites are aligned antiparallel to the magnetic moments of the B
sites below T N =858 K. Nuclear magnetic resonance spectroscopy and Mössbauer
spectroscopy cannot distinguish the Fe
3+ or Fe
2+ ions on the B sites, indicating the
delocalized nature of the charge carriers with a formal average valence of Fe
2.5+ at
room temperature [106]. The sample measured in this experiment was prepared such
that oxidization is prevented and was doped with 7 mol% Ni to replace Fe because
some B sites are probably oxidized to Fe
3+ in air [109]. The enrichment of
57 Fe in the
used sample was 95.5%. The energy spectrum of NRIS of
57 Fe in Fe 3 O 4 is shown
in Fig. 2.14a. Examples of the incoherent time spectra (measured at 16 meV and
35 meV) are shown in Fig. 2.14b [6]. From these time spectra, the ratio of A site
and B site at each phonon energy was obtained by least-squares fitting each time
spectrum with two exponential functions accompanied by sinusoidal quantum beats
corresponding to A and B sites. These ratios and the PDOS of all Fe atoms give the
partial PDOSs of A site and B site. The obtained spectra of the partial PDOS for all
Fe sites and the site-specific PDOSs of the A and B sites are shown in Fig. 2.15a. The
difference between the partial PDOS s of the A and B sites is clearly observed. In
Fig. 2.15b, the PDOSs of Fe in the ideal Fe 3 O 4 obtained from ab initio band calculations are shown. The PDOS of all Fe is shown as a black line, and the calculated
PDOSs of states of the A and B sites are shown as dashed green and dashed-anddotted dark yellow lines, respectively. The overall character of the calculated PDOS
of all Fe sites in Fe 3 O 4 agrees well with the PDOS of all Fe sites measured by
NRIS spectroscopy. Neutron inelastic scattering, X-ray inelastic scattering, Raman
scattering, and infrared absorption methods are well-known and very useful for the
study of atomic dynamics. It is, however, generally impossible to discern the atomic
motions of the same element in different environments. Therefore, this method is
considered to be unique.
2.3.4 Summary
NRIS spectroscopy is a method used for investigating the vibrational states in
substances. Since NRIS spectroscopy uses the nuclear resonant excitation accompanied by phonon creation and annihilation, unique and very effective measurement
is possible as shown above. The development of spectroscopic methods is ongoing,
and remarkable progress has been achieved in relation to the optics, detectors, and
methodologies used. Because this spectroscopy covers a broad range of scientific
areas, such as physical, chemical, biological, and earth sciences, recent developments
and further improvements of nuclear resonant scattering spectroscopy that will solve