STUDY OF THERMAL SPIN CROSSOVER IN Mn(III) COMPOUNDS
MARIANNA ROMAN
Institute of Applied Physics of A.S.M.
Keywords: spin-crossover materials; manganese (III); magnetic properties.
The phenomenon of spin-crossover (SCO) is one of the best examples of molecular bistability
that means that in some transition metal complexes the spin state of the complexes changes due to
external perturbation such as temperature, pressure, light illumination. The bistable compounds could be
useful for the construction of memory and sensory molecular-based devices. The most often this
phenomenon is found for Fe(II) compounds, it is rarely observed for Mn(III) compounds. Recently the
spin-crossover phenomenon was revealed for the Mn(III) compound with the chemical formula
[MnL2]NO3 , where L2 notes the methylene groups. [1] Here we present a microscopic approach to the
SCO phenomenon in crystals containing manganese (III) ions as structural units and reveal the main
mechanisms governing this effect. The model includes the interaction of the manganese ions with the
crystal field of the surrounding ligands and the cooperative electron-deformational interaction. The model
also takes into account that the crystal field of C2v symmetry splits the cubic 3T1 term of the MnIII ion into
an orbital singlet and an orbital doublet . In this field the 5 E term is split into two singlets . The range of
the crystal field parameters for which the low-lying part of the energy spectrum of the MnIII ion consists
of two non-degenerate levels arising from the states 3T1 and 5 E with the spin S=1 state being the
ground one is determined. It is shown that in this case the ground level mainly originates from the
3
T1 (t24 ) term i.e. the situation facilitating SCO takes place. The ls-hs transition is considered as a
cooperative phenomenon driven by the interaction of the electronic shells of the Mn III ions with the allround full symmetric deformation that is extended over the crystal lattice via the acoustic phonon field.
The SCO phenomenon is treated within the molecular field approximation. Quite good agreement has
been obtained between the calculated and experimental values of the magnetic susceptibility and the
effective magnetic moment.
4,4
4,2
eff/B
4,0
Fig.1 Temperature dependence of magnetic
momentum calculated with the set of the
-1
-1
parameters:J=95 cm ; g=5.99;1=395.44 cm .
theory calculated
experimental data.
3,8
3,6
3,4
3,2
3,0
2,8
50
100
150
200
250
300
T,K
22000
18000
16000
Fig.2 Temperature dependence of the magnetic
susceptibility calculated with the set of the
-1
-1
parameters:J= 95 cm ; g=5.99; 1=395.44 cm .
theory calculated;
experimental data [1].
-6
3
M/10 cm mol
-1
20000
14000
12000
10000
8000
6000
50
100
150
200
T, K
250
300
1.G.G.Morgan, K.D.Murnaghan, H.Müller-Bunz, V.McKee, C.J.Harding, Angew.Chem.Int.Ed., 2006,
45, 7192-7195.