Nanocomposites and nanomaterials

Spin-crossover behavior of iron molecular chain
Gudyma Iu.V.
Physical, Technical and Computer Sciences Institute, Yuriy Fedcovich Chernivtsi National University, Chernivtsi 58012, Ukraine. 
E-mail:yugudyma@gmail.com

The spin-crossover compounds are the class of inorganic coordination molecular complexes of the chemical elements with d4-d7 electronic configurations of the outer orbital which form the ligand environment with transient metal ion centered in octahedral ligand field. These materials are characterized by two degenerate spin states: low-spin (LS) state with diamagnetic properties and high-spin (HS) state with paramagnetic behavior. The LS and HS spin configuration can be interchanged by the action of external controlling fields such as temperature, pressure, or light intensity.
In this work, an examination of the one-dimensional molecular chain with two single-particle degenerate states is presented [1]. We determined the connection of the molecular chain with the Ising model with phononic interactions and investigated properties of the model using a transfer-matrix method. This method offers a promising pathway for simulating such materials properties. The role of the degeneracy of states and phononic interaction is made explicit. We analyzed regimes of the system and parameters of the occurring crossover. Here, we presented exact results for the magnetization per spin, the correlation function, and the effective volume of the system. We showed the possibility of the existence of two peaks in the specific heat capacity thermal behavior.
The thermodynamic properties of a one-dimensional spin-crossover molecular chain under constant external pressure are investigated also. The effective compressible degenerate Ising model is used as a theoretical basis. Analytical results for the crossover from low to high spin are obtained using the transfer matrix formalism. Exact expressions are derived for the fraction of molecules in the HS state, the correlation function, and the heat capacity. 
The analysis of the interval of parameters in which the spin-crossover occurs was carried out, and it was shown how the pressure changes the position of the crossover.

1.	Gudyma A., Gudyma Iu. 1D spin-crossover molecular chain with degenerate states // J Appl Phys.-2021.-129, 123905.