Pressure and temperature induced high spin–low spin phase transition: Macroscopic and microscopic consideration

• We have installed the relationship between thermodynamic and microscopic parameters.
• For the first time the microscopic theory for describing of the experimental HL transition is used.
• It was established that for TIST the value of inelastic interactions dominate over the elastic.
• At the PIST the elastic interactions become dominant with pressure increase.

The behavior under pressure of the high spin–low spin phase transition in the coordination compounds containing 3d ions is analyzed using thermodynamic and microscopic approaches. For thermodynamic approach the mean field model with interactions between spin-crossover molecules is considered. Microscopic model takes into account the interaction of d electrons of the transition metal ions with full symmetric distortions of the ligands. The relationship of the thermodynamic interaction parameters with microscopic ones is installed and shown how the quantum–mechanical interactions form the cooperativity of the system. Within the microscopic model the temperature and pressure dependences of the high spin fraction in 2-D compounds {Fe(3-Fpy)2[M(CN)4]} (M=Pd, Pt) are simulated and microscopic parameters are evaluated. It is concluded that different experimental behaviors of the temperature and pressure induced spin transitions are determined by different variations of the inelastic and elastic energies under pressure, and vibrational component of the free energy drives the ST equally with electronic part.

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