High-spin Phases Research Articles

Fe(nsal2trien)]SCN, a New Two-Step Iron(III) Spin Crossover Compound, with Symmetry Breaking Spin-State Transition and an Intermediate Ordered State

We report the synthesis of the iron(III) complex of the hexadentate Schiff base ligand nsal2trien obtained from the condensation of triethylenetetramine and 2 equiv. of 2-hydroxy-1-naphthaldehyde. The study of the salt [Fe(nsal2trien)]SCN (1) by magnetic susceptibility measurements and Mössbauer spectroscopy reveals a rather unique behavior that displays thermally induced spin crossover (SCO) with two well-separated steps at 250 (gradual transition) and 142 K (steep transition). Single crystal X-ray structures were obtained at 294, 150, and 50 K, for the high spin (HS), intermediate (Int), and low spin (LS) phases. The HS and LS phases are isostructural, and based on a single Fe(III) site (either HS or LS) an unusual symmetry break occurs in the transition to the Int ordered phase, where the unit cell includes two distinct Fe(III) sites and is based on a repetition of the [HS-LS] motif. The two-step SCO behavior of 1 must result from the existence of structural constraints preventing the full conversion HS ↔ LS in a single step.

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

We investigated domain growth in switching processes between the low-spin and high-spin phases in thermally induced hysteresis loops of spin-crossover (SC) solids. Elastic interactions among the molecules with different volumes induce effective long-range interactions, and thus the boundary condition plays a significant role in the dynamics. In contrast to SC systems with periodic boundary conditions, where uniform configurations are maintained during the switching process, we found that domain structures appear with open boundary conditions. Unlike Ising-type models with short-range interactions, domains always grow from the corners of the system. The present clustering mechanism is universal for materials with the volume change in the unit molecule, and it provides an insight into the switching dynamics of their nanoscale systems.

Synchrotron powder-diffraction study of the spin transition compound [Fe(bpp) 2](NCS) 2·2H 2O and soft X-ray-induced structural phase conversion

We have studied the high spin and low spin structural phases of the two-step spin transition compound [Fe(bpp) 2](NCS) 2·2H 2O (bpp: 2,6-bis(pyrazol-3-yl)pyridine) of different sample history with X-ray synchrotron ( λ ∼ 0.32 Å) powder diffraction at different temperatures as well as with usual X-ray ( λ ∼ 1.54 Å) powder diffraction at room temperature by performing rapid repeated scans at regular intervals of time. A structural phase transition occurs only during the first step of the spin transition. Time dependent interconversions of the two crystallographic phases have been observed at room temperature for this material. The results indicate that a soft X-ray (Cu-K α)-induced structural change occurs in this material at room temperature. Possible mechanisms for this unusual observation are discussed.

Study of Neutral Fe(III) Complexes of Pyridoxal-N-Substituted Thiosemicarbazone with Desolvation-Induced Spin-State Transformation above Room Temperature

The preparation and characterization of two new neutral ferric complexes with desolvation-induced discontinuous spin-state transformation above room temperature are reported. The compounds, [Fe(Hthpy)(thpy)].CH3OH.3H2O (1) and [Fe(Hmthpy)(mthpy)].2H2O (2), are low-spin (LS) at room temperature and below, whereas their nonsolvated forms are high-spin (HS), exhibiting zero-field splitting. In these complexes, Hthpy, Hmthpy, and thpy, mthpy are the deprotonated forms of pyridoxal thiosemicarbazone and pyridoxal methylthiosemicarbazone, respectively; each is an O,N,S-tridentate ligand. The molecular structures have been determined at 100(1) K using single-crystal X-ray diffraction techniques and resulted in a triclinic system (space group P1) and monoclinic unit cell (space group P21/c) for 1 and 2, respectively. Structures were refined to the final error indices, where RF = 0.0560 for 1 and RF = 0.0522 for 2. The chemical inequivalence of the ligands was clearly established, for the "extra" hydrogen atom on the monodeprotonated ligands (Hthpy, Hmthpy) was found to be bound to the nitrogen of the pyridine ring. The ligands are all of the thiol form; the doubly deprotonated chelates (thpy, mthpy) have C-S bond lengths slightly longer than those of the singly deprotonated forms. There is a three-dimensional network of hydrogen bonds in both compounds. The discontinuous spin-state transformation is accompanied with liberation of solvate molecules. This is evidenced also from DSC analysis. Heat capacity data for the LS and HS phases are tabulated at selected temperatures, the values of the enthalpy and entropy changes connected with the change of spin state were reckoned at DeltaH = 12.5 +/- 0.3 kJ mol-1 and DeltaS = 33.3 +/- 0.8 J mol-1 K-1, respectively, for 1 and DeltaH = 6.5 +/- 0.3 kJ mol-1 and DeltaS = 17.6 +/- 0.8 J mol-1 K-1, respectively, for 2.

Quantum chemical calculations of the isomer shift in iron(II) complexes of 1,2,4-triazoles with a 1 A 1 ⇔ 5 T 2 spin transition

DFT calculations have been performed to determine the isomer shift for a series of iron(II) clusters with nitrogen-containing ligands which serve as models of coordination units in Fe(II) complexes with 1,2,4-triazoles possessing a 1 A 1 ⇔ 5 T 2 spin transition. Good agreement has been found between the theoretical and experimental values of the isomer shift for both low-and high-spin phases. Our calculations confirmed the hypothesis about relationship between the experimentally observed differences in the isomer shift for the low-spin phases of the complexes and variations of the Fe-N mean bond length.

Spin- and phase transition in the spin crossover complex [Fe(ptz) 6](BF 4) 2 studied by nuclear inelastic scattering of synchrotron radiation and by DFT calculations

Nuclear inelastic scattering (NIS) spectra of [Fe(ptz) 6](BF 4) 2 (ptz = 1- n-propyl-tetrazole) have been measured for five phases differing in spin state and crystallographic structure. Different spectral patterns have been found for the low-spin and high-spin phases and are described in terms of normal coordinate analysis of the complex molecule. For both low-spin and high-spin phases the conversion from ordered to disordered phase results in splitting of the observed NIS bands. Packing becomes visible in the NIS spectra via coupling of the Fe–N stretching vibrations with those of the terminal n-propyl groups. The DFT-based normal coordinate analysis also reveals the character of Raman markers.

Structural Investigation of the Photoinduced Spin Transition in the [Fe(PM-BiA)<sub>2</sub>(NCS)<sub>2</sub>] Compound

We investigated the thermo- and photo-induced phase transitions between low spin (LS) and high spin (HS) states of the molecular crystal of [Fe(PM-BiA)2(NCS)2] in the orthorhombic form, by using X-ray diffraction. The structure of the photoinduced HS state, generated from the LS state at low temperature, is compared to the structures of the HS and LS phases at thermal equilibrium and to the thermally trapped HS state. The preliminary results presented here show that the structural reorganization is similar in the different HS states.

Nature and mechanism of the photoinduced spin transition in[Fe(PM−BiA)2(NCS)2]

We report the investigation of a prototype cooperative photoinduced spin transition in $[\mathrm{Fe}(\mathrm{PM}\text{\ensuremath{-}}\mathrm{BiA}{)}_{2}(\mathrm{NCS}{)}_{2}]$ by x-ray diffraction. The structural rearrangement accompanying the change of an electronic state between the low spin and the photoinduced out-of-equilibrium high spin phases is similar to the one occurring around the phase transition at thermal equilibrium. The cooperative behavior of the light-induced optical hysteresis and light-induced thermal hysteresis phenomena investigated here show a strong similarity with thermal hysteresis around first-order phase transitions, since they are driven by a domain nucleation process.

Synchrotron radiation study of photo-induced spin-crossover transitions: Microscopic origin of nonlinear phase transition

In situ X-ray absorption spectroscopy (XAS) using synchrotron radiation was used to probe the local structure of photo-induced phase transition of Fe (II) spin-crossover complex. We demonstrate that the local structure of spin-converted metastable phase can be probed by XAS using a novel pixel array detector using less than 1 mg of specimen. We find that the arrangement of nearest neighbors, i.e., the FeN 6 cluster does not change its symmetry upon the diamagnetic ( S = 0 ) ↔paramagnetic ( S = 2 ) transformation under laser light (532 nm) excitation below 50 K. The intermediate-range structure (next-nearest neighbor correlation) shows that ligand molecules in the photo-induced high spin phase is distorted. The results suggest that the essential difference between the photo-induced and thermally induced high spin phases is the absence of intermediate-range order in the former. We propose that quenched disorder (inhomogeneity) in the intermediate-range structure is the microscopic origin of nonlinear nature of photo-induced phase transition (crippled cooperativity). It is demonstrated that in situ XAS using synchrotron radiation and state-of-the-art X-ray detector can provide detailed information on the local structure of metastable states trapped at low temperature.

NRIS study on the [FeN6] core in photo-induced high-spin state of [Fe(2-pic)3]Cl2·EtOH

A comparison is reported for thermally and photo-induced high-spin phases of [Fe(2-pic)3]Cl2.EtOH (2-pic: 2-picolylamine) using the nuclear resonant inelastic scattering (NRIS) technique.

PHOTOINDUCED PHASE TRANSITION OF A SPIN-CROSSOVER COMPLEX STUDIED WITH THE COMBINATION OF SR AND LASER

We have performed a photoemission study on an organometal spin-crossover complex, [ Fe(2-pic) 3] Cl 2 EtOH , using the combination of synchrotron radiation and laser. The core-level and valence-band photoemission spectra were measured for high-spin, low-spin, and photoinduced phases. The N 1s and valence-band spectra showed remarkable changes due to the photoinduced phase transition, indicating that the electron charge was transferred from nitrogen to iron at the photoinduced phase transitions. It was also found that the valence-band structure of the photoinduced phase is very different from that of the high-spin phase caused by the thermally induced phase transition.

The spin-crossover complex [Fe(tpa)(NCS) 2

The temperature-induced spin crossover of iron(II) in the [ Fe ( tpa )( NCS ) 2 ] complex has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS), extended X-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy (MS) and by measurements of the magnetic susceptibility (SQUID). The various measurements consistently show that the transition is complete and abrupt and exhibits a hysteresis between 102 and 110 K. The dependence of the hyperfine parameters of the high-spin (HS) and of the low-spin (LS) phase on temperature is gradual while the effective thickness (determined by the Lamb-Mossbauer factor f LM ) shows a step at the transition temperature. This step could be identified clearly because the effective thickness is measured directly by NFS. The Lamb-Mossbauer factor, the Debye temperature and the mean-square displacement of iron(II) could be determined for the HS and for the LS phase. When comparing the NIS data with the results from density functional theory (DFT), the Fe-N stretching vibrations of both LS and HS phases could be unambiguously identified and the f LM could be factorized for both phases into a lattice and a molecular part. The structural information from EXAFS and DFT geometry optimization are in reasonable agreement.

Effect of powder dispersion on the1 A 1 ⇄5 T 2 spin transition in Fe(II) complexes with 4-amino-l,2,4-triazole

A series of Fe(II) complexes with 4-amino-1,2,4-triazole ground in an agate mortar for 10 min is studied by Mossbauer spectroscopy. Strong effects of powder dispersion both on the1A1 ⇄5T2 spin transition and on the structure dynamic characteristics of the complexes are found. Thus at 295 K the high-spin form of Fe(II) appears in the samples or its fraction increases; the ionicity of Fe-N bonds and the extent of distortion of the octahedral environment of iron atoms for the low-spin phases of the complexes also increase. It is established that powder dispersion markedly affects the probability of the Mossbauer effect and the vibrational spectrum of the lattice of coordination compounds. For both the low- and high-spin phases of the complexes, it is reported that the vibrational spectrum is “softened.” The main reason for these effects is supposed to be defectiveness rather than the size of the particles due to mechanical activation of the powder.

Excitation spectra ofLaCoO3

We study the excitation spectra of ${\mathrm{LaCoO}}_{3}$ using a multiorbital tight-binding model. Making the self-energy correction due to local three-body scattering to the Hartree-Fock (HF) approximation, we calculate the spectral density for the low-spin phase. It is shown that the band gap is strongly reduced from the HF value, and that the intensity is transferred from the lower part to the upper part of the valence band and to the satellite in the spectral density projected onto the Co 3d states. The results are in good agreement with the spectra of photoemission experiments. We also calculate the spectral densities for the intermediate-spin phase and the high-spin phase in relation to the nonmagnetic-to-paramagnetic transition. The peak intensities around the top of the valence band are smaller than that for the low-spin phase. In ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CoO}}_{3}$, the energies for the intermediate-spin and high-spin phases are found to become lower than for the low-spin phase with increasing x, indicating the instability of the low-spin phase. This is consistent with the susceptibility data and the photoemission spectra.

Magnetic Properties and Mössbauer Spectroscopy for Complex Fe(tpa)(NCS)2

The studies of Mössbauer spectroscopy and magnetic properties for complex Fe(tpa)(NCS)2 exhibit that low spin → high spin (HS) transition may be thermally induced. The impurities such as HS-Fe2+ residue and HS-Fe3+, that were caused by compound preparation, play a significant role in the investigation of spin transition curve. The Mössbauer spectroscopy is used to observe the impurities as separated forms from the spin transition phase directly, and the magnetic susceptibility measurement is used to determine the total paramagnetism from all HS phases. With these two useful tools, an exact spin transition curve was defined by removing the magnetic contribution of the impurities.

Simplified formula for the1 A 1⇄5 T 2 spin transition temperature in Fe(II) complexes

A formula relating the1A1⇄5T2 spin transition temperature (Tc) in Fe(II) complexes to characteristics of the compounds is derived. With certain assumptions, Tc is determined by the splitting parameter ΔLS of eg- and t2g-orbitals for the low-spin complexes and by the frequency ratio of normal vibrations of the low- and high-spin phases. For the group of compounds possessing spin transitions, the values of ΔLS are found and analyzed. Correlations between Tc and ΔLS are established; the values of the change in the probability of the Mossbauer effect are correlated with those of entropy of spin transition. The correlations are substantiated. It is concluded that for mononuclear Fe(II) complexes possessing sharp spin transitions, Tc may not be significantly higher than for Fe(Phy)2(BF4)2 (Tc=282 K).

The origin of cooperativity in high-spin—low-spin transitions in molecular crystals

A new model for the first-order transitions between the low-spin and high-spin phases of molecular crystals of some transition metal complexes is proposed. The abrupt spin transitions (or those exhibiting thermal hysteresis) are attributed to first-order transitions between the different crystal phases with the sublattice order. The necessary conditions for these transitions to take place are formulated in terms of intramolecular parameters of the spin transition and intermolecular interactions.

Magnetic x-ray dichroism in 2p absorption spectra of Fe/Cu(001)

Large magnetic circular dichroism using circularly polarized synchrotron radiation has been observed at the L2,3 absorption edges of thin (1–12 monolayers) fcc Fe films grown on Cu(001). Dramatic changes in the 2p branching ratio are observed when the orientation of photon helicity and sample magnetization are varied from parallel to antiparallel. The temperature and film thickness dependence of the perpendicular anisotropy in these films could be monitored by variations in the 2p branching ratio. Finally, our results are described in a simple theoretical framework that allows a determination between the predicted low- and high-spin phases for fcc Fe. Our data suggest a high-spin phase with a moment of 2.0–2.5 μB/atom. Interestingly, thicker films with remanant magnetic moments in the film plane present smaller branching ratio variations consistent with either a reduced moment or with domain closure for these films. The surface sensitivity and elemental specificity of this technique make it particularly attractive for the study of surface and thin film magnetism.

Magnetic instabilities in PtFe3 and in the fcc Ni-Fe system.

The self-consistent linear muffin-tin orbitals electronic structure calculations have been carried out for nonmagnetic and ferromagnetic phases of ${\mathrm{PtFe}}_{3}$ and for a few ordered phases of the Ni-Fe alloy. The fixed-spin-moment scheme has been employed. The results reveal the existence of high-spin (HS) and low-spin (LS) branches of the magnetization curve for both systems and allow for a direct calculation of the energy differences between the HS, LS, and nonmagnetic phases. For the Ni-Fe alloy, the total energy and the state equations are calculated for a number of concentrations. It is shown that a vanishing energy difference between the HS and the LS states and the magnetic instability close to equilibrium are correlated. For ${\mathrm{PtFe}}_{3}$ the pressure dependence of the total energy is discussed. It is shown that this information in conjunction with experimental values of the critical pressures at which the HS-LS transitions are observed allows for an independent estimation of the energy differences between these phases and that the estimate does not agree well with the results of local-density calculations. It is pointed out that this disagreement and the previously reported inability of the local-density approximation to account for observed high-field susceptibilities may be of common origin.

Magnetism of hexagonal 3dtransition metals

The magnetic and mechanical equations of state and the total energy as functions of both magnetic moment and atomic volume have been self-consistently calculated using the linear muffin-tin orbital method within the fixed-spin-moment scheme for all 3d transition metals from Sc to Ni in a hexagonal structure. At zero pressure, Co and Ni ferromagnetic high-spin phases are found to be stable. Cr is found to be ferromagnetic with a very small magnetic moment that increases slowly and gradually with atomic volume. Mn and Fe are predicted to be metamagnetic; however, for Fe a stable high-spin ferromagnetic phase at rather low negative internal pressure is found. The light 3d transition metals (Sc, Ti, V) become magnetic only at very large lattice expansions. The electronic structure, the densities of states, and the total energies are discussed for the phases that may probably be produced by molecular-beam epitaxy.