Chemistry Reference
In-Depth Information
Fig. 2.38 Schematized molecular structure of a trinuclear cationic complex which accommo-
dates LS-Fe
III
(red) and LS-Fe
II
(blue) ions separated by bridging LS-Co
III
ion. At low
temperatures two well resolved spectra are observed assigned to the localized LS-Fe
III
and the
LS-Fe
II
species. With increasing temperature the intensities of these two doublets decrease at the
favor of a new doublet (green) from a time-averaged species with intermediate oxidation state of
the two iron sites. This species is the only one remaining at room temperature [
69
]
intensities of these two resonance signals decrease at the favor of a new resonance
signal, another quadrupole doublet (blue), arising from a time-averaged species
where both iron centers are in an intermediate oxidation state as a result of fast
electron fluctuation through the bridging diamagnetic central Co
III
ion (Fig.
2.38
).
2.3.3.4 Valence Fluctuation in EuNiP
Mixed-valency and thermally induced transition between localized and delocalized
valence states was observed with
151
Eu Mössbauer spectroscopy of the interme-
tallic compound EuNiP [
70
] (Fig.
2.39
).
High temperature
151
Eu Mössbauer measurements between room temperature
and ca. 500 K provide proof for mixed-valent behavior in the pnictide EuNiP. Two
well resolved signals are observed at room temperature, one being typical for Eu
2+
(dark grey) and the other one for Eu
III
(light grey). With increasing temperature the
electron fluctuation between Eu
II
and Eu
III
becomes faster than the time-window
of
151
Eu Mössbauer spectroscopy and above ca. 450 K the resonance signals of
localized Eu
II
(dark grey) and Eu
III
(light grey) gradually disappear at the favor of
a new signal with time-averaged oxidation state of Eu
2.5+
(non shaded) [
70
].
