Chapter 27
Cooperative Phenomena in
Spin-Crossover Molecular Crystals
Iurii Gudyma, Artur Maksymov, and Kateryna Polonska
27.1 Introduction
Spin-crossover (SCO) systems are relatively rare class of transition metal complexes that display molecular magnetic bistability and interconverted reversibility
upon external stimuli. Under normal conditions, these complexes can exhibit spin
crossover, that is, the entropy-driven thermal transition from a low-spin (LS) state,
populated at low temperatures, to a high-spin (HS) state, populated at higher
temperatures. The observed spin state depends on the balance between maximizing
the number of parallel spins/unpaired electrons (Hund’s rule of maximum multiplicity) and having the lowest electronic energy by populating the lowest energy
orbitals. For a free atom or ion, or ionic compound, the normal spin state is that
of maximum multiplicity, in accordance to the first Hund’s rule; however, for
sufficiently large ligand field, one obtains a low-spin ground state. By the influence
of external physical field (light irradiation, temperature, pressure, and others), it is
possible to bring into the system additional energy that could increase spin-pairing
repulsion of the electrons. Due to equalization between spin-pairing energy and
ligand field splitting energy, the favorable conditions for transition are established.
Besides the changes of magnetic moment of the molecule, the spin transition is
also accompanied by the changes in the molecular volume for different states. For
spin-crossover materials are found a great variety of spin transition curves that
were characterized as follows: sharp transitions often accompanied by hysteresis,
more or less gradual transitions, and sometimes incomplete transitions with residual
Iu. Gudyma () · K. Polonska
Department of General Physics, Technical and Computer Sciences, Yuriy Fedkovych Chernivtsi
National University, Chernivtsi, Ukraine
A. Maksymov
Marian Smoluchowski Institute of Physics, Jagiellonian University, Kraków, Poland
© Springer International Publishing AG, part of Springer Nature 2018
O. Fesenko, L. Yatsenko (eds.), Nanochemistry, Biotechnology, Nanomaterials,
and Their Applications, Springer Proceedings in Physics 214,
https://doi.org/10.1007/978-3-319-92567-7_27
427
Cooperative Phenomena in
Spin-Crossover Molecular Crystals
Iurii Gudyma, Artur Maksymov, and Kateryna Polonska
27.1 Introduction
Spin-crossover (SCO) systems are relatively rare class of transition metal complexes that display molecular magnetic bistability and interconverted reversibility
upon external stimuli. Under normal conditions, these complexes can exhibit spin
crossover, that is, the entropy-driven thermal transition from a low-spin (LS) state,
populated at low temperatures, to a high-spin (HS) state, populated at higher
temperatures. The observed spin state depends on the balance between maximizing
the number of parallel spins/unpaired electrons (Hund’s rule of maximum multiplicity) and having the lowest electronic energy by populating the lowest energy
orbitals. For a free atom or ion, or ionic compound, the normal spin state is that
of maximum multiplicity, in accordance to the first Hund’s rule; however, for
sufficiently large ligand field, one obtains a low-spin ground state. By the influence
of external physical field (light irradiation, temperature, pressure, and others), it is
possible to bring into the system additional energy that could increase spin-pairing
repulsion of the electrons. Due to equalization between spin-pairing energy and
ligand field splitting energy, the favorable conditions for transition are established.
Besides the changes of magnetic moment of the molecule, the spin transition is
also accompanied by the changes in the molecular volume for different states. For
spin-crossover materials are found a great variety of spin transition curves that
were characterized as follows: sharp transitions often accompanied by hysteresis,
more or less gradual transitions, and sometimes incomplete transitions with residual
Iu. Gudyma () · K. Polonska
Department of General Physics, Technical and Computer Sciences, Yuriy Fedkovych Chernivtsi
National University, Chernivtsi, Ukraine
A. Maksymov
Marian Smoluchowski Institute of Physics, Jagiellonian University, Kraków, Poland
© Springer International Publishing AG, part of Springer Nature 2018
O. Fesenko, L. Yatsenko (eds.), Nanochemistry, Biotechnology, Nanomaterials,
and Their Applications, Springer Proceedings in Physics 214,
https://doi.org/10.1007/978-3-319-92567-7_27
427