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Fig. 2.22 Mössbauer spectra of [Fe(bpym)(NCSe) 2 ] 2 bpym at: a 4.2 K without applied magnetic
field and b 4.2 K in applied magnetic field of 5 T. LS in [HS-LS] and [LS-LS] pairs (black), HS
in [HS-LS] pairs (grey), HS in [HS-HS] pairs (white)[ 34 , 35 ]
Fe II ions [ 34 , 35 ]. The effective hyperfine field H eff at the iron nuclei of a para-
magnetic non-conducting sample in an external field H ext may be estimated as
H eff & H ext - [220 - 600(g - 2)] h S i where h S i is the expectation value of the
atomic spin moment and g the Landé splitting factor [ 36 , 37 ]. The difference
between the expectation values of S for the Fe II atom in the LS and in the HS states
in [HS-LS] and [HS-HS] pairs enables one to distinguish unambiguously between
the dinuclear units consisting of two possible spin states in an external magnetic
field. To do so, the strength of the external magnetic field should be sufficiently
high and the temperature sufficiently low in order to avoid magnetic relaxation
taking place within the characteristic time window of a Mössbauer experiment.
The zero-field spectrum of the [Fe(bpym)(NCSe) 2 ] 2 bpym recorded at 4.2 K
reflects, in agreement with the magnetic measurements, the nearly ''one-half'' ST
according to the area fractions of the HS (48.0 %) and LS (52.0 %) components
with parameters d HS ¼ 0 : 86 ðÞ mms 1 ; DE H Q ¼ 3 : 11 ðÞ mms 1 ; and d LS ¼
0 : 22 ðÞ mms 1 ; DE L Q ¼ 0 : 36 ðÞ mms 1 ; respectively (Fig. 2.22 a). The measure-
ment in a magnetic field of 50 kOe (5 T) at 4.2 K yields a spectrum that consists of
three components as can be seen in Fig. 2.22 b. One of them (shown in black) with
relative intensity (area fraction) 52.0 % and with isomer shift and quadrupole
splitting indicative of Fe II in LS state shows an expected effective hyperfine field
that is practically the same as the applied field, H eff & H ext (because S = 0). This
subspectrum arises from LS-Fe II ions in [HS-LS] and [LS-LS] pairs. The second
component (shown in white) with area fraction of ca. 4.0 % is a doublet with
isomer shift and quadrupole splitting values characteristic of Fe II in HS state. The
resonance lines are magnetically broadened through magnetic dipole interaction
with a local effective hyperfine field of H eff = 14 kOe, which is rather small and
originates from a small h S i value as a result of antiferromagnetic coupling in [HS-
HS] pairs. The third component (shown in grey) with relative intensity of ca.
44.0 % and isomer shift and quadrupole splitting values indicative of Fe II in HS
state can be unambiguously assigned to [HS-LS] pairs; because the measured
effective magnetic field at the iron nuclei of 81 kOe clearly stems from a spin
quintet ground state of Fe II (S = 2). As a result, the complete distinction between
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