Chemistry Reference
In-Depth Information
Fig. 2.20 LIESST effect at A sites of [Fe(mtz)
6
](BF
4
)
2
(LS to metastable HS conversion with green
light, 514 nm) and at B sites (HS to metastable LS conversion with red light, 820 nm). Thermal
relaxation from HS to LS at A sites and from LS to HS at B sites sets in at ca. 65 K in both cases [
31
]
Fig. 2.21 a View of the crystal structure of [Fe(bpym)(NCS)
2
]
2
bpym [
32
]. b Plot of |
M
T vs. T for
[Fe(bpym)(NCSe)
2
]
2
bpym, where |
M
is the magnetic susceptibility corrected for diamagnetic
contributions [
33
]
v
M
T value of ca. 3.5 cm
3
mol
-1
K which indicates that about 50 % of the Fe
II
ions
have undergone ST to the LS state (Fig.
2.21
b) [
33
]. The question, however, arises
concerning the nature of the plateau region: Does the plateau originate from only
[HS-LS] pairs or from a 1:1 mixture of [HS-HS] and [LS-LS] pairs?
Neither can this question be answered by magnetic susceptibility measurements
nor by conventional Mössbauer spectroscopy. The latter would yield identical
spectra for the HS state of Fe
II
atoms in [HS-LS] and [HS-HS] spin pairs as seen
in Fig.
2.22
a. However, the Mössbauer spectrum recorded in applied magnetic
field can give a conclusive answer based on the different effective hyperfine
magnetic fields arising from different expectation values of the spin state of the
