Kamel Boukheddaden

Spin crossover in transition metal compounds

C.N.R. Rao, M.M. Seikh, C. Narayana: Spin-State Transition in LaCoO3 and Related Materials .- H.A. Goodwin: Spin Crossover in Cobalt(II) Systems .- Y. Garcia, P.Gutlich: Thermal Spin Crossover in Mn(II), Mn(III) Cr(II) and Co(III) Coordination Compounds .- D.N. Hendrickson, C.G. Pierpont: Valence Tautomeric Transition Metal Complexes .- P. Guionneau, M. Marchivie, G.Bravic, J.-F. Letard, D. Chasseau: Structural Aspects of Spin Crossover. Example of the [Fe(II)Ln(NCS)2] Complexes .- J. Kusz, P. Gutlich, H. Spiering: Structural Investigations of Tetrazole Complexes of Iron(II) .- A. Hauser: Light-Induced Spin Crossover and the High-Spin Low-Spin Relaxation .- F. Varret, K. Boukheddaden, E. Codjovi, C. Enachescu, J. Linares: On the Competition Between Relaxation and Photoexcitations in Spin Crossover Solids under Continuous Irradiation .- P. Gutlich: Nuclear Decay Induced Excited Spin State Trapping (NIESST) .- M.-L. Boillot, J. Zarembowitch, A. Sour: Ligand-Driven Light-Induced Spin Change (LD-LISC): A Promising Photomagnetic Effect

Electric-Field-Induced Charge-Transfer Phase Transition: A Promising Approach Toward Electrically Switchable Devices

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb(0.8)Mn[Fe(CN)(6)](0.93).1.62H(2)O and Co(3)[W(CN)(8)](2)(pyrimidine)(4).6H(2)O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices.

Simple two-dimensional model for the elastic origin of cooperativity among spin states of spin-crossover complexes.

We study the origin of the cooperative nature of spin crossover (SC) between low-spin and high-spin (HS) states from the viewpoint of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states and apply constant temperature molecular dynamics based on the Nosé-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.

Electronic and structural aspects of spin transitions observed by optical microscopy. The case of [Fe(ptz)6](BF4)2.

The colorimetric analysis of images recorded with an optical microscope during the onset of the spin crossover transformation allows monitoring separately the involved electronic and structural aspects, through the separation of resonant absorption and scattering effects. Complementary information can also be obtained by using the polarized modes of the microscope. These potentialities are illustrated by the observation of [Fe(ptz)(6)](BF(4))(2) single crystals during the onset of the thermal transitions in the 110-140 K range. We characterized the interplay between the electronic (HS <--> LS) and structural (order <--> disorder) transformations. Elastic stresses and mechanical effects (hopping, self-cleavage) generated by the volume change upon electronic transition are also illustrated, with their impact on the photoswitching properties of the crystals.

Structures of Metastable States in Phase Transitions with a High-Spin Low-Spin Degree of Freedom

The difference in the degeneracy of low-spin (LS) and high-spin (HS) states causes interesting entropy effects on spin-crossover phase transitions and charge transfer phase transitions in materials composed of spin-crossover atoms. Mechanisms of the spin-crossover (SC) phase transitions have been studied using the Wajnflasz model, where the degeneracy of the spin states (HS or LS) is taken into account, and the cooperative nature of the spin-crossover phase transitions has been well described. Recently, a charge transfer (CT) phase transition due to electron hopping between LS and HS sites has been studied using a generalized Wajnflasz model. Systems with SC and CT both have a high temperature structure (HT) and a low temperature structure (LT), and the transition between them can be a smooth crossover or a discontinuous first-order phase transition, depending on the parameter values of the systems. Although, apparently, the standard SC system and the CT system are very different, it lias been shown that the two models are equivalent under a certain transformation of variables. In both systems, the structure of the metastable state at low temperatures is a matter of interest. We study the temperature dependence of the fraction of HT systematically in a, unified model and find several structures of equilibrium and metastable states of the model as functions of the system parameters. In particular, we find a reentrant-type metastable branch of HT in a low temperature region, which sould play an important role in the study of the photo-irradiated processes of related materials.

Examples of molecular switching in inorganic solids, due to temperature, light, pressure, and magnetic field

We describe various molecular switching processes occurring in several types of inorganic solids: spin cross-over (SC) compounds, photomagnetic Prussian blue analogs (PBAs), and valence-tautomeric system. Their thermo-, photo-, piezo-, and magneto-chromic properties are illustrated by recent examples. A common description of their static properties by a two-level model is given.

Spin transition with a large thermal hysteresis near room temperature in a water solvate of an iron(III) thiosemicarbazone complex

The magnetic properties of the monohydrated ferric complex Li[Fe(5Brthsa)2]·H2O (H2-5Brthsa = 5-bromosalicylaldehyde thiosemicarbazone) have been investigated by SQUID and Mossbauer measurements. The S = 1/2 ↔ S = 5/2 spin transition of the ferric ion is accompanied by a quite broad hysteresis (ΔT = 39 K) centred around 313 K. The spin states involved are characterised by the quadrupole splittings ΔEQ(2T2) = 2.584 ± 0.002 mm s−1 and ΔEQ(6A1) = 0.338 ± 0.006 mm s−1 and the isomer shifts δIS (2T2) =+0.262 ± 0.001 mm s−1 and δIS (6A1) =+0.294 ± 0.002 mm s−1 at 77 and 360 K, respectively. A powder X-ray diffraction study at various temperatures demonstrates the occurrence of a crystallographic first-order phase transition of the lattice coupled to the spin conversion. The enthalpy and entropy variations associated with the transition have been estimated from DSC measurements at ΔH = 5.7 ± 0.5 kJ mol−1 and ΔS = 18 ± 2 J mol−1 K−1. The existence of a crystallographic first-order phase transition associated to the spin crossover is consistent with the cooperative character of the process. This phase transformation might originate from the modification of the extended hydrogen-bond network.

Intrinsic effects of the boundary condition on switching processes in effective long-range interactions originating from local structural change

Masamichi Nishino, Cristian Enachescu, Seiji Miyashita, Kamel Boukheddaden, and François Varret Computational Materials Science Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Japan Department of Physics, Alexandru Ioan Cuza University, Iasi, Romania Department of Physics, Graduate School of Science, The University of Tokyo, Bunkyo-Ku, Tokyo, Japan Groupe d’Etudes de la Matière Condensée, CNRS-Université de Versailles/St. Quentin en Yvelines, 45 Avenue des Etats Unis, F78035 Versailles Cedex, France CREST, JST, 4-1-8 Honcho Kawaguchi, Saitama, 332-0012, Japan (Dated: December 29, 2009)

Light-induced phase separation in the [Fe(ptz)6] (BF4)2 spin-crossover single crystal

We present novel insight on like-spin domains (LSD) in cooperative spin transition solids by following the photo-transformation and the subsequent relaxation of a [ Fe(ptz)6] (BF4)2 single crystal in the vicinity of the light-induced instability. Self-organization under light is observed, accompanied by Barkhausen-like noise and jumps which reveal the presence of elastic interactions between LSDs. The light-induced phase separation process is discussed in terms of a dynamic potential providing spinodal instability in the corresponding temperature range. This useful concept is applicable to all types of switchable molecular solids.

Elastic Frustration Causing Two-Step and Multistep Transitions in Spin-Crossover Solids: Emergence of Complex Antiferroelastic Structures

Two-step and multistep spin transitions are frequently observed in switchable cooperative molecular solids. They present the advantage to open the way for three- or several-bit electronics. Despite extensive experimental studies, their theoretical description was to date only phenomenological, based on Ising models including competing ferro- and antiferro-magnetic interactions, even though it is recognized that the elastic interactions are at the heart of the spin transition phenomenon, due to the volume change between the low- and high-temperature phases. To remedy this shortcoming, we designed the first consistent elastic model, taking into account both volume change upon spin transition and elastic frustration. This ingredient was revealed to be powerful, since it was able to obtain all observed experimental configurations in a consistent way. Thus, according to the strength of the elastic frustration, the system may undergo first-order transition with hysteresis, gradual, hysteretic two-step or multistep transitions, and incomplete transitions. Furthermore, the analysis of the spatial organization of the HS and LS species in the plateau regions revealed the emergence of complex antiferro-elastic patterns going from simple antiferro-magnetic-like order to long-range spatial modulations of the high-spin fraction. These results enabled us to identify the elastic frustration as the fundamental mechanism at the origin of the very recent experimental observations showing the existence of organized spatial modulations of the high-spin fraction inside the plateau of two-step spin transitions.