High-spin Phases Research Articles

Lattice Defects in Sub-Micrometer Spin-Crossover Crystals Studied by Electron Diffraction.

Spin-crossover particles of [Fe(Htrz)2trz](BF4) with sizes of some hundred nanometers are studied by in situ electron microscopy. Despite their high radiation sensitivity, it was possible to analyze the particles by imaging and diffraction so that a detailed analysis of crystallographic defects in individual particles became possible. The presence of one or several tilt boundaries, where the tilt axis is the direction of the polymer chains, is detected in each particle. An in situ exposure of the particles to temperature variations or short laser pulses to induce the spin crossover shows that the defect structure only changes after a high number of transformations between the low-spin and high-spin phases. The observations are explained by the anisotropy of the atomic architecture within the crystals, which facilitates defects between weakly linked crystallographic planes.

Order-Disorder, Symmetry Breaking, and Crystallographic Phase Transition in a Series of Bis(trans-thiocyanate)iron(II) Spin Crossover Complexes Based on Tetradentate Ligands Containing 1,2,3-Triazoles.

We report herein a series of neutral trans-thiocyanate mononuclear spin crossover (SCO) complexes, [FeIIL(NCS)2] (1-4), based on tetradentate ligands L obtained by reaction of N-substituted 1,2,3-triazolecarbaldehyde with 1,3-propanediamine or 2,2-dimethyl-1,3-diaminopropane [L = N1,N3-bis((1,5-dimethyl-1H-1,2,3-triazol-4-yl)methylene)propane-1,3-diamine/-2,2-dimethylpropane-1,3-diamine, 1/2 and N1,N3-bis((1-ethyl/1-propyl-1H-1,2,3-triazol-4-yl)methylene)-2,2-dimethylpropane-1,3-diamine, 3/4]. The thermal-induced SCO behavior is characterized by abrupt transitions with an average critical temperature (ΔT1/2)/hysteresis loop width (ΔThyst) in the range 190-252/5-14 K, while the photo-generated metastable high-spin (HS) phases are characterized by TLIESST temperatures in the range 44-59 K. Single crystal analysis shows that except 1, all compounds experience reversible symmetry breaking coupled with the thermal SCO. Furthermore, 4 experiences an additional phase transition at ca. 290 K responsible for the coexistence of two HS phases quenched at 10 K through LIESST and TIESST effects. The molecules form hexagonally packed arrays sustained by numerous weak CH···S and C···C/S···C/N···C bonds involving polar coordination cores, while non-polar pendant aliphatic substituents are segregated inside, occupying hexagonal channels. Energy framework analysis of complexes with one step SCO transition (1, 2, and 4) shows a correlation between the cooperativity and the amplitude of changes in the molecule-molecule interactions in the lattice at the SCO transition.

Variation of Spin-Transition Temperature in the Iron(III) Complex Induced by Different Compositions of the Crystallization Solvent

We crystallized the Schiff-base iron(III) spin-crossover complex [Fe(3,5Cl-L5)(NCSe)] from different two-component solvent mixtures containing methanol and chloroform (Φ = V(CH3OH)/V(solvent) = 0.05, 0.25, 0.50, 0.83, and 1.00). The obtained crystalline products were characterized by X-ray diffraction, and it was confirmed that they are all composed of the same crystalline phase, and they do not contain any crystal solvent. However, significant differences in magnetic properties were observed, and thermal hysteresis changed from (in K) 121T↓ and 134T↑ for Φ = 0.05 and 0.25, down to 72T↓ and 96T↑ for Φ = 1.00. The crystal structures of the low-spin and high-spin phases were studied theoretically and experimentally.

Excitonic Correlations, Spin-State Ordering, and Magnetic-Field Effects in One-Dimensional Two-Orbital Hubbard Model for Spin-Crossover Region

The electronic properties of excitonic insulators have been examined precisely in recent years. Pictures of exciton condensation may be applied to the spin-state transition observed in perovskite cobalt oxides. We examine the crystal-field and magnetic-field dependences of spatial spin structures on the basis of the density matrix renormalization group method using an effective model for the one-dimensional two-orbital Hubbard model in strong-coupling limit. We find an excitonic insulating (EI) phase and a spin-state ordering (SSO) phase in the intermediate region between low-spin and high-spin phases. In the EI phase, spin-triplet excitons are spatially fluctuating due to quantum effects, and an incommensurate spin correlation realizes. The analyses of a spin gap and degeneracy of entanglement spectra suggest the realization of the Haldane-like edge state in the EI phase. In the SSO phase, 3-fold or incommensurate SSO structures realize depending on the crystal-field splitting. These structures are stabilized as a result of the competition of exchange interactions between spin states.

Tutorial on elastic interaction models for multistep spin-crossover transitions

Spin-crossover (SC) compounds are fascinating materials that exhibit colorful phase transitions induced by temperature, pressure, photoirradiation, etc. From the microscopic point of view, the electronic (spin) state of a molecule changes between the low-spin (LS) and high-spin (HS) states by such stimuli, and a variation of the molecular size follows through the vibronic coupling between the electronic state and structure in the molecule. This causes an elastic distortion and then an elastic interaction. The elastic interaction is essential in cooperative properties of SC phenomena. In this paper, we present a tutorial study on elastic interaction models for SC phenomena, which are the recent trend of modeling of SC compounds. We focus on multistep transitions, which are a topic of SC phenomena. We analyze the phase diagrams including the metastable phases for several SC systems, in which antiferromagnetic-like and ferrimagnetic-like phases are associated in addition to the LS and HS phases. Making use of the phase diagrams, we show various patterns of thermally induced SC transition with steps. We also investigate SC transitions with steps in a core-shell SC nanocomposite composed of two different SC compounds. We focus on two cases: the core has a lower transition temperature than the shell, and the core has a higher transition temperature. We show characteristic features of difference in the two systems.

Thermally induced spin transition in Fe(pyrazine)[Fe(CN)5NO

Transition metal nitroprussides form a family of coordination polymers with interesting physical properties. From their three-dimensional (3D) phases, by inducing rupture of the axial T–NC bond through organic molecules (L) with a high ability to form a T–L coordination bond, pillared transition metal nitroprussides can be obtained. For monodentate molecules, for example, pyridine and its derivatives, the pillars are formed in the interlayer region by pairs of molecules coupled through their dipole moments, and the resulting solid has a two-dimensional (2D) structure with T(L)2[Fe(CN)5NO] as the formula unit. In this contribution, we report the use of a bidentate molecule, pyrazine, to obtain pillared ferrous nitroprusside. According to the refined crystal structure, this solid has a 3D framework, which results from the pyrazine molecule (L) coordinating to two iron atoms to afford Fe(pyrazine)[Fe(CN)5NO]. This material shows spin-crossover (SCO) behavior with thermal hysteresis of about 40 K, which is associated with relatively large structural changes, for instance, a reversible variation of 12% in the unit cell volume. The thermally induced spin transition is accompanied by a notable color change of the material. The main features of this thermally activated spin transition in the material under study are discussed herein on the basis of its structural characterization at 100 and 300 K, magnetic and differential scanning calorimetry (DSC) measurements, and Raman and Mössbauer spectra for both the low- and high-spin phases. The reversibility of this thermally activated spin transition has been verified by magnetic, Mössbauer, X-ray powder diffraction (XRD), and infrared (IR) data. To the best of our knowledge, this is the first report on the preparation and characterization of Fe(pyrazine)[Fe(CN)5NO] and its thermally induced SCO behavior.

Magnetism, Conductivity and Spin-Spin Interactions in Layered Hybrid Structure of Anionic Radicals [Ni(dmit)2] Alternated by Iron(III) Spin-Crossover Complex [Fe(III)(3-OMe-Sal2trien)] and Ferric Moiety Precursors.

In this study, crystals of the hybrid layered structure, combined with Fe(III) Spin-Crossover (SCO) complexes with metal-dithiolate anionic radicals, and the precursors with nitrate and iodine counterions, are obtained and characterized. [Fe(III)(3-OMe-Sal2trien)][Ni(dmit)2] (1), [Fe(III)(3-OMe-Sal2trien)]NO3·H2O (2), [Fe(III)(3-OMe-Sal2trien)]I (3) (3-OMe-Sal2trien = hexadentate N4O2 Schiff base is the product of the condensation of triethylenetetramine with 3-methoxysalicylaldehyde; H2dmit = 2-thioxo-1,3-dithiole-4,5-dithiol). Bulk SCO transition was not achieved in the range 2.0–350 K for all three compounds. Alternatively, the hybrid system (1) exhibited irreversible segregation into the spatial fractions of Low-Spin (LS) and High-Spin (HS) phases of the ferric moiety, induced by thermal cycling. Fractioning was studied using both SQUID and EPR methods. Magnetic properties of the LS and HS phases were analyzed in the framework of cooperative interactions with anionic sublattice: Anion radical layers Ni(dmit)2 (1), and H-bonded chains with NO3 and I (2,3). LS phase of (1) exhibited unusual quasi-two-dimensional conductivity related to the Arrhenius mechanism in the anion radical layers, ρ||c = 2 × 105 Ohm·cm and ρ⊥c = 7 × 102 Ohm·cm at 293 K. Ground spin state of the insulating HS phase was distinctive by ferromagnetically coupled spin pairs of HS Fe3+, S = 5/2, and metal-dithiolate radicals, S = 1/2.

Three Jahn-Teller States of Matter in Spin-Crossover System Mn(taa).

Three high-spin phases recently discovered in the spin-crossover system Mn(taa) are identified through analysis by a combination of first-principles calculations and MonteCarlo simulation as a low-temperature Jahn-Teller ordered (solid) phase, an intermediate-temperature dynamically correlated (liquid) phase, and an uncorrelated (gas) phase. In particular, the Jahn-Teller liquid phase arises from competition between mixing with low-spin impurities, which drive the disorder, and intermolecular strain interactions. The latter are a key factor in both the spin-crossover phase transition and the magnetoelectric coupling. Jahn-Teller liquids may exist in other spin-crossover materials and materials that have multiple equivalent Jahn-Teller axes.

Experimental evidence for the elastic long-range character of the spin crossover transition in cooperative single crystals

We evidenced by means of optical microscopy the long-range character of the interactions acting in spin crossover (SCO) single crystals through a study of the optical density (OD) variation during the spin transition between high-spin (HS) and low-spin (LS) states. The time evolution of the local OD, measured in four single crystals, revealed the existence of precursor phenomena, which are manifest through the appearance of significant changes in the OD of the probed area well before the arrival of the front interface in the case of HS to LS transition and after the passage of the interface in the case of LS to HS transition. These two effects are attributed to the manifestation of long-range interactions that develop during the propagation of the HS/LS interface, which is accompanied by a volume change, generating elastic stresses in the material that deploy far from the sources. Interestingly, the experimental investigations indicate that the precursor and the after effect phenomena occur mostly in the HS phase, which is softer than the LS phase. These original experimental results are well reproduced using an electroelastic modeling in which we considered, in addition to the lattice misfit between HS and LS states, the existence of different elastic constants in the HS and LS phases.

Theoretical investigations on the pressure effects in spin-crossover materials: Reentrant phase transitions and other behavior

Pressure effects have been widely studied in spin-crossover (SCO) solids due to their immediate influence on the thermal dependence of the high-spin (HS) fraction. In most of the cooperative SCO materials, the applied pressure shifts the transition temperatures upward and decreases the thermal hysteresis widths to such an extent that it vanishes at some critical pressure. However, several other unexpected experimental features were found in the literature, showing that the applied pressure may (i) induce an increase of the thermal hysteresis width or even (ii) lead to a reentrant behavior on the thermal hysteresis whose width first increases for low applied pressures and then decreases at high-pressure values. These nonstandard behaviors have been classified as anomalous, even though the transition temperature always increases under pressure. In this theoretical contribution, we rationalize all these behaviors by describing the spin-crossover system under pressure with an elastic description accounting for the difference of lattice parameters between the low-spin (LS) and HS phases including the pressure effects. The analytical study of this elastic model in the homogeneous mechanical system demonstrated its isomorphism with an Ising-like model with infinitely long-range interactions, in which the pressure acts linearly on the ligand field and nonlinearly on the strength of the interactions. The resolution of this model brings to light the existence of a pressure-induced interplay between these two contributions allowing one to recover a wide range of normal and abnormal observed experimental thermal dependences of the spin transition under pressure.

Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3

Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt3 with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin phases. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these phase transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which for θ = 90° (B||c-axis) and vicinity abruptly become very strong in the 24.8–30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T for θ = 0° but moves to higher fields as 1/cosθ. This merge, suggesting a tricritical behavior, occurs at θ ≈ 51° from the ab-plane. This is an unique off-symmetry angle where the length change along the c-axis is precisely zero due to the anisotropic nature of MS in UPt3 for all magnetic field values.

Lattice phonon modes of the spin crossover crystal [Fe(phen)2(NCS)2] studied by THz, IR, Raman spectroscopies and DFT calculations

[Fe(phen)2(NCS)2] is a prototype transition metal complex material, which undergoes a phase transition between low-spin (LS) and high-spin (HS) phases, induced by temperature, pressure or light. Vibrational modes play a key role for spin-state switching both in thermal and photo-induced cases, by contributing to vibrational entropy for thermal equilibrium transitions or driving the fast structural trapping of the photoinduced high spin state. Here we study the crystal phonon modes of [Fe(phen)2(NCS)2], by combining THz, IR, and Raman spectroscopies sensitive to modes in different frequency ranges and different symmetries. We compare the experimental results to DFT calculations performed in a periodic 3D crystal for understanding the phonon modes in the crystal, compared to molecular vibrations. Indeed, each vibrational mode of the isolated molecule combines into several modes of different symmetry and frequency in the crystal, as the unit cell contains four molecules. We focus our attention on the HS symmetric and anti-symmetric breathing modes in the crystal as well as on the N-CS stretching modes.

Interplay between a crystal's shape and spatiotemporal dynamics in a spin transition material.

We investigated by means of optical microscopy (OM) the spatiotemporal features of the thermo-induced spin transition of [Fe(2-pytrz)2{Pd(CN)4}]·3H2O (1) (2-pytrz = 4-(2-pyridyl)-1,2,4,4H-triazole) single crystals having two different shapes (triangle and rectangle). While magnetic and calorimetric measurements, performed on a polycrystalline material, showed the respective average heating and cooling transition temperatures of (Tdown1/2 ∼ 152 K, Tup1/2 ∼ 154 K) and (Tdown1/2 ∼ 160.0 K, Tup1/2 ∼ 163.5 K), OM studies performed on a unique single crystal revealed significantly different switching temperatures (Tdown1/2 ∼ 152 K and Tup1/2 ∼ 162 K). OM investigations showed an interface spreading over all crystals during the spin transition. Thanks to the color contrast between the low-spin (LS) and the high-spin (HS) states, we have been able to follow the real time dynamics of the spin transition between these two spin states, as well as access the thermal hysteresis loop of each single crystal. After image processing, the HS-LS interface's velocity was carefully estimated in the ranges [4.4-8.5] μm s-1 and [2.5-5.5] μm s-1 on cooling and heating, respectively. In addition, we found that the velocity of the interface is shape-dependent, and accelerates nearby the crystal's borders. Interestingly, we observed that during the propagation process, the interface optimizes its shape so as to minimize the excess of elastic energy arising from the lattice parameter misfit between the LS and HS phases. All of these original experimental results are well reproduced using a spatiotemporal model based on the description of the spin-crossover problem as a reaction diffusion phenomenon.

Pressure-induced two-step spin crossover in a double-layered elastic model

We study the two-step spin crossover in a double-layered elastic model based on transition metal complexes each taking high spin (HS) and low spin (LS) states. Here, only the simplest elastic interactions between adjacent molecules are considered and the system is exposed to the external pressure within the framework of $NPT$-Monte Carlo method. As a certain amount of pressure is applied, the first order thermal transition between uniform HS and LS phases transforms to a two-step transition with an emergent intermediate spin (IS) phase, where the HS and LS molecules are paired face to face between layers and form diagonally striped clusters within the layer. The difference in the size of HS and LS molecules is reflected both in the elastic interactions and in the enthalpy, and the IS phase could gain the latter over the loss of the former by significantly reducing its volume. The present pressure effect is interpreted to the chemical one in double-layered transition metal materials, which actually reveals a variety of multistep spin crossover transitions relevant to our numerical result.

First Step Towards a Devil's Staircase in Spin-Crossover Materials.

The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)3 )2 ](4,4'-bipy)2 }n exhibits a thermal high-spin (HS)↔low-spin (LS) staircase-like conversion characterized by a multi-step dependence of the HS molar fraction γHS . Between the fully HS (γHS =1) and LS (γHS =0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the γHS ≈0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the γHS ≈0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS-LS sequence incommensurate with the molecular lattice. The formation of these different long-range spatially ordered structures of LS and HS states during the multi-step spin-crossover is discussed within the framework of "Devil's staircase"-type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.

First Step Towards a Devil's Staircase in Spin‐Crossover Materials

AbstractThe unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)3)2](4,4′‐bipy)2}n exhibits a thermal high‐spin (HS)↔low‐spin (LS) staircase‐like conversion characterized by a multi‐step dependence of the HS molar fraction γHS. Between the fully HS (γHS=1) and LS (γHS=0) phases, two steps associated with different ordering appear in terms of spin‐state concentration waves (SSCW). On the γHS≈0.5 step, a periodic SSCW forms with a HS‐LS‐HS‐LS sequence. On the γHS≈0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS‐LS sequence incommensurate with the molecular lattice. The formation of these different long‐range spatially ordered structures of LS and HS states during the multi‐step spin‐crossover is discussed within the framework of “Devil's staircase”‐type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.

Spin Crossover in Fe(II)-M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2-Substituted Pyrazines.

Discovery of spin-crossover (SCO) behavior in the family of Fe(II)-based Hofmann clathrates has led to a "new rush" in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe(2+), square planar tetracyanometalates M(II)(CN)4(2-) (M(II) = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe(II) ion has a pseudo-octahedral coordination environment supported by four μ4-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M(II) = Ni (1), Pd (2), Pt (3); X = Me and M(II) = Ni (4), Pd (5)) or one 2X-pyrazine and one water molecule (X = I and M(II) = Ni (7), Pd (8), Pt (9)), or, alternatively, two distinct Fe(II) positions with either two pyrazines or two water molecules (X = Me and M(II) = Pt (6)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN6 units (1-6) display cooperative spin transition, while Fe(II) ions in N5O or N4O2 surrounding are high spin (HS). Structural changes in the nearest Fe(II) environment upon low-spin (LS) to HS transition, which include ca. 10% Fe-N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of 1-9 (see abstract figure). Effects of a pyrazine substituent and M(II) nature on the hyperfine parameters in both spin states are established.

Nuclear inelastic scattering study of a dinuclear iron(II) complex showing a direct spin transition

The results of the nuclear inelastic scattering (NIS)/nuclear resonance vibrational spectroscopy (NRVS) for the powder spectra of dimeric [Fe 2 L 5(NCS) 4] (L = N-salicylidene-4-amino-1,2,4-triazole) complex are presented. This system is spin crossover (SCO) material tagged with a fluorophore that can sense or “feel” the SCO signal ripping through the molecular network and thereby providing an opportunity to register the SCO transition. The spectra have been measured for the low-spin and high-spin phases of the complex. The high-spin isomer reveals one broad band above 200 cm −1, while the low-spin one displays two intense bands in the range from 390 to 430 cm −1, accompanied by a number of weaker bands below this area and one at ca. 490 cm −1. A normal coordinate analysis based on density functional calculations yields the assignment of the spin marker bands to particular molecular modes. In addition the vibrational contribution to the spin transition has been estimated

Temperature dependence of iron local magnetic moment in phase-separated superconducting chalcogenide

We have studied local magnetic moment and electronic phase separation in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ by x-ray emission and absorption spectroscopy. Detailed temperature dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature (T$_c$) and the coexisting phases exchange spectral weights with the low spin state gaining intensity at the expense of the higher spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at T$_c$ as well as superconductivity disappear. The results clearly underline that the coexistence of the low spin and high spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of electronically inhomogeneous K$_{x}$Fe$_{2-y}$Se$_2$ system.

Unprecedented bistability domain and interplay between spin crossover and polymorphism in a mononuclear iron(ii) complex

A mononuclear complex, [FeL2](BF4)2·xH2O (1LS∙xH2O), where L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine, can be converted into several phases showing different spin crossover regimes. In the first heating 1LS∙xH2O loses water molecules and converts into a mixture of two high spin phases, 1A/HS and 1C/HS. Further cycling produces the low spin phase 1A/LS. The transition 1A/LS↔1A/HS is accompanied by a 130 K wide hysteresis loop (Tc↑ = 490 K, Tc↓ = 360 K). Annealing the complex 1LS∙xH2O at 500 K yields a high spin phase 1C/HS. The phase 1C/HS undergoes spin conversion to the corresponding low spin phase 1C/LS on cooling, T1/2 ≈ 320 K. Dehydration of 1LS∙xH2O at 370 or 400 K yields a certain low spin phase, 1X/LS, which irreversibly transforms into a high spin phase 1B/HS, which, in turn, reversibly transforms to the low spin phase 1B/LS on cooling, T1/2 ≈ 320 K.