Eur. Phys. J. B (2012) 85: 205
https://doi.org/10.1140/epjb/e2012-30132-1

Regular Article

Vibration-driven thermal transition in binuclear spin crossover complexes

¹ Laboratoire de Physique et Chimie des Matériaux (LPCM), Université de M’sila, 28000 M’sila, Algeria
² Département de Physique, Faculté des Sciences, Université de Batna, 05000 Batna, Algeria
³ Laboratoire de Chimie de Coordination, CNRS UPR-8241 and Université de Toulouse, UPS, INP, 31077 Toulouse, France

^a e-mail: azzedine.bousseksou@lcc-toulouse.fr

Received: 13 February 2012
Received in final form: 26 March 2012
Published online: 20 June 2012

Abstract

An Ising-like model including the effect of intramolecular vibrations is proposed to investigate Fe(II) binuclear spin crossover complexes, especially in the situation where it allows the assessment of the conditions under which cooperative exchange interactions are weak enough to favor a vibration-driven mechanism for the thermal transition. In particular, two spin states (LS-LS, LS-HS or HS-HS where LS and HS stand for low-spin and high-spin respectively) must be quasi-degenerate. Whatever the equi-energy scenario, the effect of molecular vibrations on the groundstate appears in the low temperature region, where a deviation from the low frequency approximation of the standard Ising-like model is observed. Thirteen quasi-equi-energy situations lead to three possible vibronic groundstates at T = 0 K, from which the thermal evolution of the HS fraction presents a similar trend as mononuclear systems. However, binuclear systems permit to generate a HS-LS vibronic groundstate. For this latter case the application of pressure, known to drastically affect the spin transition, is suggested to reveal a two-step behavior according to a LS-LS, HS-LS HS-HS scheme. Intramolecular coupling between the two metallic centres and intermolecular cooperativity few influence the spin transition curves due to a difference of the temperature ranges between the effects of molecular vibrations and spin crossover phenomenon.