Chemistry Reference
In-Depth Information
µ
eff
(
b
)
x
0.00
0.25
0.31
0.41
0
100
200
300
T (K)
µ
eff
(T)dependences forCu(hfac)
2
L
48
x
L
49
1-x
(thecurveswerenotfittedtothesingle
Figure13.9
Experimental
ordinateaxis).
the interchain space and involved only in van der Waals interactions with heterospin chains, significantly
affected both the character of the
µ
eff
(T) dependence and the temperature range of the magnetic anomaly.
The introduction of certain solvent molecules in breathing crystals actually proved a highly effective
method of control over the physical parameters of the magnetic anomaly.
220
The relationship between the
chemical step and the physical property was studied on specially selected series of breathing crystals. As a
result, it was found that “mild” modification of T
c
in Cu(hfac)
2
L
40
5Solv required a substantially smaller
structural step than modification by one methylene unit typically used in organic chemistry. X-ray cinema
demonstrated mutual coherence of the structural dynamics of
·
0
.
N-
•
O-Cu(II)-O
•
-N
>
<
heterospin exchange
clusters and the structural dynamics of interchain diamagnetic fragments.
Quantum chemical calculations using X-ray diffraction data allowed researchers to trace the character of
changes in the parameters of exchange interactions in exchange clusters in the range of the phase transition.
It was found that in the temperature range corresponding to the phase transition, the exchange parameter
considerably changed in magnitude and occasionally in sign, a non-trivial finding. The smooth change in
the magnetic moment on cooling, observed in the majority of experiments near T
c
, was the result of the
gradual increase in the fraction of the low temperature phase in the high temperature phase.
A pronounced change in the exchange integral was also recorded in a radiospectroscopic study.
221
For heterospin breathing crystals based on copper(II) complexes with nitroxides, electron paramagnetic
resonance (EPR) spectroscopy can be a specific and extremely powerful experimental technique, which
allows determination of the sign of the exchange integral and evaluation of the exchange parameter
J
by
analyzing the temperature dependence of the g factor.
222 - 225
The unusual magnetic properties of breathing
crystals also stimulated the development of the phenomenological theory of spin transitions for multispin
exchange-coupled systems.
226,227
Recently, the possibility of a LIESST (Light-Induced Excited Spin State
Trapping) effect was demonstrated on a breathing crystal.
228
Both initiation (which suggested preliminary
preparation of a fine film or finely disperse particles of a heterospin complex on the surface of an inert
and transparent support with further irradiation) and EPR recording of the effect differed substantially in
the experimental technique from the classic variant.
210
To summarize the discussion of these surprising
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