80
M. Seto et al.
the dynamics of Fe ions in Nafion membranes [104, 105] were measured using the
aforementioned method, which are used as ion-exchange membranes.
This method gives element (isotope)-specific phonons but there are compounds
containing two or more different atomic states of a particular element; for example,
magnetite, which is a mixed-valence Fe compound, is well-known [106]. Although
this method cannot distinguish the “site-specific” vibrational properties, information
on the difference of the sites is sometimes required because the properties of the
individual atomic motion in nonequivalent positions in a compound are not necessarily equivalent; in fact, mixed valence systems, such as iron oxides with a phase
transition accompanied by charge splits [107], and spin crossover materials [108],
are well known. Furthermore, even in an ideal material containing only one atomic
site, there may be atoms with different environments due to imperfections or impurities. Materials with different sites occupied by the same element are not unusual.
A method capable of distinguishing the vibrational properties of a specific site is
valuable and crucial.
2.3.3 Advanced NRIS Method
Thus far, the measurement of electronic and phonon states has been studied independently, except for the information on the recoilless fraction of the Mössbauer
effect. The recoilless fraction sometimes gives important insights for the lattice
dynamics study. However, PDOS, which allows us to calculate the recoilless fraction
as discussed above, has quite rich information. The combination of the measurement
methods of phonon energy spectra and incoherent time spectra through the hyperfine
interactions permits the observation of the site-specific PDOS [6]. Since the observed
NRIS consists of scattering from individual excited nuclei, the PDOS obtained from
the scattering spectrum is the sum of the partial PDOS of individual atoms. Therefore, a partial PDOS of each atom is obtained from the measured NRIS spectrum
by discerning the contribution of each atom. The hyperfine interactions between the
nuclei and the surrounding electronic states, which split the nuclear energy level,
allow this procedure. In that case, quantum beats whose oscillating cycles reflect
the splitting energies are observed in the time-domain measurement of incoherently
emitted γ-rays during the decay of an excited nucleus. The incoherently emitted γrays obey an exponential law with a finite lifetime. In the case of multiple electronic
states of a certain element in a compound, the hyperfine interactions for those states
are different, and the beat patterns in the time spectrum are expected to be different.
The measurement of the quantum beat patterns enables the identification of the
respective electronic states and gives the component ratios of the sites. If the nuclear
resonant excitation accompanied by phonon excitation occurs at a certain incident
radiation energy, the analysis of the quantum beat pattern in the time spectrum reveals
the respective phonon contributions from those sites with different electronic states.
Performing this procedure at different energies allows us to observe the partial PDOS
as distinguished by electronic states.
M. Seto et al.
the dynamics of Fe ions in Nafion membranes [104, 105] were measured using the
aforementioned method, which are used as ion-exchange membranes.
This method gives element (isotope)-specific phonons but there are compounds
containing two or more different atomic states of a particular element; for example,
magnetite, which is a mixed-valence Fe compound, is well-known [106]. Although
this method cannot distinguish the “site-specific” vibrational properties, information
on the difference of the sites is sometimes required because the properties of the
individual atomic motion in nonequivalent positions in a compound are not necessarily equivalent; in fact, mixed valence systems, such as iron oxides with a phase
transition accompanied by charge splits [107], and spin crossover materials [108],
are well known. Furthermore, even in an ideal material containing only one atomic
site, there may be atoms with different environments due to imperfections or impurities. Materials with different sites occupied by the same element are not unusual.
A method capable of distinguishing the vibrational properties of a specific site is
valuable and crucial.
2.3.3 Advanced NRIS Method
Thus far, the measurement of electronic and phonon states has been studied independently, except for the information on the recoilless fraction of the Mössbauer
effect. The recoilless fraction sometimes gives important insights for the lattice
dynamics study. However, PDOS, which allows us to calculate the recoilless fraction
as discussed above, has quite rich information. The combination of the measurement
methods of phonon energy spectra and incoherent time spectra through the hyperfine
interactions permits the observation of the site-specific PDOS [6]. Since the observed
NRIS consists of scattering from individual excited nuclei, the PDOS obtained from
the scattering spectrum is the sum of the partial PDOS of individual atoms. Therefore, a partial PDOS of each atom is obtained from the measured NRIS spectrum
by discerning the contribution of each atom. The hyperfine interactions between the
nuclei and the surrounding electronic states, which split the nuclear energy level,
allow this procedure. In that case, quantum beats whose oscillating cycles reflect
the splitting energies are observed in the time-domain measurement of incoherently
emitted γ-rays during the decay of an excited nucleus. The incoherently emitted γrays obey an exponential law with a finite lifetime. In the case of multiple electronic
states of a certain element in a compound, the hyperfine interactions for those states
are different, and the beat patterns in the time spectrum are expected to be different.
The measurement of the quantum beat patterns enables the identification of the
respective electronic states and gives the component ratios of the sites. If the nuclear
resonant excitation accompanied by phonon excitation occurs at a certain incident
radiation energy, the analysis of the quantum beat pattern in the time spectrum reveals
the respective phonon contributions from those sites with different electronic states.
Performing this procedure at different energies allows us to observe the partial PDOS
as distinguished by electronic states.