7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
339
= 1.33 mm s
−1 . The asymmetry of the doublet is due to the Goldanskii-Karyagin
effect arising from the vibrational anisotropy of Sn. The isomer shift of SnO is close
to that of βSn (2.56 mm s
−1 ) but must be assigned to Sn(II) oxidation state. Its low
value, compared to other Sn(II) species, reflects the hybridization of the Sn valence
orbitals that reduces the Sn 5s electron population. The rather high quadrupole splitting is due to the asymmetry of the Sn 5p electrons arising from the SnO 4 local
structure that can be related to the existence of a Sn 5p lone pair along the fourfold
symmetry axis of the SnO 4 square based pyramid. This axis corresponds to the Z
axis of the EFG principal axis system, which leads to η = 0. According to Eq. (7.10),
V ZZ is therefore the only electronic contribution to the quadrupole splitting and
reflects the activity of the Sn 5p lone pair.
The area of the resonance absorption line is of primary interest for the application
of Mössbauer spectroscopy to batteries. For thin samples containing Mössbauer
atoms with the same local environment, the area is proportional to t a , and therefore,
to f a and n a . However, the Mössbauer atoms have generally different environments,
i.e., different Mössbauer sites as encountered, for example, in crystals with different
crystallographic sites, in amorphous phases, or in the different phases of a composite.
The different Mössbauer sites, labelled i, give different subspectra that contribute to
the overall Mössbauer spectrum. The total area is the sum of the subspectrum areas
that are each proportional to f i n i and a quantitative analysis requires the knowledge
or the determination of the different f i , which cannot be usually achieved.
Many electrode materials are transformed into a multiphase system during
the electrochemical reactions and a particular attention must be paid to the evaluation of the recoil-free fractions for a quantitative analysis. For instance, the relative
amounts of reactants and products in a Fe
2+ /Fe
3+ two-phase reaction can often be
obtained by considering the same value of f for the different phases, as in the case of
LiFePO 4 /FePO 4 discussed in Sect. 7.4.2. But this simple assumption fails for some
reactions such as the first lithiation of FeSn 2 ending with Li 7 Sn 2 . These two phases
contain Sn(0) in different chemical environments, leading to significantly different
recoil-free fractions at room temperature f(FeSn 2 ) and f(Li 7 Sn 2 ). In that case, the relative amounts of FeSn 2 and Li 7 Sn 2 can only be evaluated at each stage of lithiation
from the Mössbauer spectra by considering the ratio f(FeSn 2 )/f(Li 7 Sn 2 ) as shown in
Sect. 7.6.1.
The existence of a large number of overlapping subspectra can make the distinction between reactants and products difficult, especially for
119 Sn Mössbauer spectroscopy. Thus, it is often impossible to determine the Mössbauer parameters for
each Mössbauer site in each phase. In that case, more advanced fitting procedures of the Mössbauer data that include hyperfine parameter distributions should
be considered, but simplified fitting strategies combined with additional information provided by complementary techniques are often more interesting for the
analysis of the electrochemical mechanisms. Such a situation is often encountered with electrode materials in batteries. For instance, Li or Na insertion reactions in positive electrode materials (solid solution) come with the occupation of
different vacant sites that can affect selectively the Mössbauer atoms. Another
complex situation concerns alloying reactions in tin based negative electrodes
339
= 1.33 mm s
−1 . The asymmetry of the doublet is due to the Goldanskii-Karyagin
effect arising from the vibrational anisotropy of Sn. The isomer shift of SnO is close
to that of βSn (2.56 mm s
−1 ) but must be assigned to Sn(II) oxidation state. Its low
value, compared to other Sn(II) species, reflects the hybridization of the Sn valence
orbitals that reduces the Sn 5s electron population. The rather high quadrupole splitting is due to the asymmetry of the Sn 5p electrons arising from the SnO 4 local
structure that can be related to the existence of a Sn 5p lone pair along the fourfold
symmetry axis of the SnO 4 square based pyramid. This axis corresponds to the Z
axis of the EFG principal axis system, which leads to η = 0. According to Eq. (7.10),
V ZZ is therefore the only electronic contribution to the quadrupole splitting and
reflects the activity of the Sn 5p lone pair.
The area of the resonance absorption line is of primary interest for the application
of Mössbauer spectroscopy to batteries. For thin samples containing Mössbauer
atoms with the same local environment, the area is proportional to t a , and therefore,
to f a and n a . However, the Mössbauer atoms have generally different environments,
i.e., different Mössbauer sites as encountered, for example, in crystals with different
crystallographic sites, in amorphous phases, or in the different phases of a composite.
The different Mössbauer sites, labelled i, give different subspectra that contribute to
the overall Mössbauer spectrum. The total area is the sum of the subspectrum areas
that are each proportional to f i n i and a quantitative analysis requires the knowledge
or the determination of the different f i , which cannot be usually achieved.
Many electrode materials are transformed into a multiphase system during
the electrochemical reactions and a particular attention must be paid to the evaluation of the recoil-free fractions for a quantitative analysis. For instance, the relative
amounts of reactants and products in a Fe
2+ /Fe
3+ two-phase reaction can often be
obtained by considering the same value of f for the different phases, as in the case of
LiFePO 4 /FePO 4 discussed in Sect. 7.4.2. But this simple assumption fails for some
reactions such as the first lithiation of FeSn 2 ending with Li 7 Sn 2 . These two phases
contain Sn(0) in different chemical environments, leading to significantly different
recoil-free fractions at room temperature f(FeSn 2 ) and f(Li 7 Sn 2 ). In that case, the relative amounts of FeSn 2 and Li 7 Sn 2 can only be evaluated at each stage of lithiation
from the Mössbauer spectra by considering the ratio f(FeSn 2 )/f(Li 7 Sn 2 ) as shown in
Sect. 7.6.1.
The existence of a large number of overlapping subspectra can make the distinction between reactants and products difficult, especially for
119 Sn Mössbauer spectroscopy. Thus, it is often impossible to determine the Mössbauer parameters for
each Mössbauer site in each phase. In that case, more advanced fitting procedures of the Mössbauer data that include hyperfine parameter distributions should
be considered, but simplified fitting strategies combined with additional information provided by complementary techniques are often more interesting for the
analysis of the electrochemical mechanisms. Such a situation is often encountered with electrode materials in batteries. For instance, Li or Na insertion reactions in positive electrode materials (solid solution) come with the occupation of
different vacant sites that can affect selectively the Mössbauer atoms. Another
complex situation concerns alloying reactions in tin based negative electrodes