High-spin Sites Research Articles

Imaging Single Prussian Blue Nanoparticles with Extraordinary Low-Spin Iron Capacity.

In attempts to obtain high-capacity Prussian blue nanomaterials, current efforts are predominantly focused on the particle-ensemble-level understanding of their structure-activity relationships. Complementarily, it would be insightful to screen out extraordinary individuals from the nanoparticle population. Using a simple and efficient technique of bright-field microscopy, this work enables, for the first time, quantitative characterization of the overall two-redox-center electrochemistry of single Prussian blue nanoparticles many at a time. Quantitative optical voltammograms with little interference from solvent breakdown and non-Faradaic electrode charging/discharging are extracted for each single nanoparticle, revealing clear heterogeneity among them. On this basis, the microscopic method allows a detailed comparative analysis between the two redox-active sites. It is found that while the synthesized nanoparticles show a similar specific capacity of the high-spin (HS-Fe) sites with STD/mean = 30%, most individual nanoparticles exhibit monodispersedly small capacities of the low-spin iron (LS-Fe) sites, only about of the HS-Fe capacity. Most importantly, it is discovered that there is always a small fraction (∼8%) of the single nanoparticles showing an impressively tripled LS-Fe capacity. Facilitated by optical imaging, the discovery of this easily overlooked extraordinary subpopulation confers alternative opportunities for targeted efforts for material chemists to improve synthesis and material design based on these unusual individuals, which in turn implies the general significance of nanoparticle screening.

Enhancing cyclic stability of Mn-based Prussian blue cathode for Potassium Ion storage by High-spin Fe substitution strategy

Aqueous K-ion batteries (AKIBs) are being considered as promising candidates, although they still confront numerous problems. Here we propose a high-spin Fe substitution method to enhance the long-term cyclic stability of Mn-based Prussian blue analogue (Mn-PBA) cathode. The prepared K1.86Mn[Fe(CN)6]0.96·0·19H2O (KMF) are electrochemically activated for 20-cycle CV scan in the hybrid electrolyte containing KNO3+Fe(NO3)3. The modified cathode (KMF-100) achieves a high capacity of 185.8 mAh g−1 and high-capacity retention of 89.8 % at 2 A g−1 after 1500 cycles (based on maximum capacity). The high-spin Fe substitution strategy not only activates the transition metal at high spin sites to contribute extra capacity, but also promotes the partial dissolved Mn2+ to return to the PBA in the hybrid electrolyte with Fe3+, relieving the Jahn-Teller effect of Mn(Ⅲ). In contrast, the KMF in the pure KNO3 electrolyte does not exhibit this behavior. The DFT calculation further demonstrate the reconstituted Prussian blue structure can increase the electrical conductivity and improve ion diffusion kinetics of the cathode material. Notably, it suppresses the change in C-Fe bond lengths during K-ion extraction, maintaining outstanding structural stability throughout cycle. This work provides a novel way to prepare high-stability manganese-based PBA cathode.

Bimetallic Fe, Ni-PBA on hollow graphite tube for capacitive deionization with exceptional stability

Faradic materials become more and more attractive for capacitive deionization (CDI) due to their high ion adsorption capacity. However, it is still a great challenge to design a stable and high charge-transfer rate material for high-performance desalination. Herein, we report a bimetallic Fe, Ni-doped Prussian blue analogue (PBA)-packed hollow graphite tube (FeNiHCF@HGT) electrode for boosting the CDI performance. The high conductivity of HGT and the tight adhesion of PBA crystals on HGT endow the FeNiHCF@HGT electrode with fast charge-transfer, excellent redox activity and high stability through buffering the crystal deformation of PBAs in the ion-intercalation/deintercalation process. In addition, the bimetallic Fe, Ni-doping in PBA at the high-spin (HS) sites enhances the redox activity of FeIIC6/ FeIIIC6 in FeNiHCF with Ni substitution for partial HS Fe. Under the optimal Ni-doping condition, the obtained Fe1Ni1HCF@HGT delivers a high specific capacity and superior rate performance (364F g−1 at 1 A g−1, 88.2% retention at 20 A g−1), with an ultra-high surface capacitance contribution (92.3% at 20 mV s−1). The CDI results demonstrate its outstanding salt adsorption capacity (60.2 mg g−1 in 100 mM NaCl) and exceptional cycle stability (106.6% retention after 100 cycles). This new strategy with the synergy of conductive tube matrix and Ni substitution at PBA lattice sites fully achieves the high-performance CDI.

Electron-Transfer Activity in a Cyanide-Bridged Fe42 Nanomagnet.

The ability to switch a molecule between different magnetic states is of considerable importance for the development of new molecular electronic devices. Desirable properties for such applications include a large-spin ground state with an electronic structure that can be controlled via external stimuli. Fe42 is a cyanide-bridged stellated cuboctahedron of mixed-valence Fe ions that exhibits an extraordinarily large S = 45 spin ground state. We have found that the spin ground state of Fe42 can be altered by controlling the humidity and temperature. Dehydration results in a 15 μB reduction of the saturation magnetization that can be partially recovered upon rehydration. The complementary use of UV-vis, IR, L2,3-edge X-ray absorption spectroscopy and X-ray magnetic circular dichroism is applied to uncover the mechanism for the observed dynamic behavior. It is identified that dehydration is concurrent with metal-to-metal electron transfer between Fe pairs via a cyanide π hybridization. Upon dehydration, electron transfer occurs from low-spin {FeII(Tp)(CN)3} sites to high-spin FeIII centers. The observed reduction in magnetization upon dehydration of Fe42 is inconsistent with a ferrimagnetic ground state and is proposed to originate from a change in zero-field splitting at electron-reduced high-spin sites.

FeCr2O4spinel to near megabar pressures: Orbital moment collapse and site-inversion facilitated spin crossover

The interplay between lattice, orbital, and spin degrees of freedom in iron chromite (spinel $[\mathrm{Fe}]{\mathrm{C}{\mathrm{r}}_{2}}{\mathrm{O}}_{4})$ has been investigated to near megabar pressures. The cubic-to-tetragonal transition, from static Jahn-Teller distortions at the Fe locality, rises from ${T}_{\mathrm{J}\text{\ensuremath{-}}\mathrm{T}}\ensuremath{\sim}135\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure to 300 K by $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The tetragonal distortion progressively increases and orbital moment quenching is triggered beyond $\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, as monitored by the magnetic hyperfine field ${H}_{hf}$ at Fe sites. In the range 30--60 GPa, original $\mathrm{F}{\mathrm{e}}^{2+}$ tetrahedral sites with an unquenched orbital moment $({H}_{hf}\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ coexist with newly evolved $\mathrm{F}{\mathrm{e}}^{2+}$ at tetrahedral sites having the orbital moment quenched and a resultant large ${H}_{hf}\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Additionally, new $\mathrm{F}{\mathrm{e}}^{2+}$ sites having distinguishable orbital moment quenching signatures $({H}_{hf}\ensuremath{\sim}42\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ are discerned. Those sites also have other Fe nuclear hyperfine interaction parameter values typical of $\mathrm{F}{\mathrm{e}}^{2+}$ in octahedral coordination. There is a concurrent change to a steeper decrease of unit-cell volume as pressure rises above $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. These electronic and lattice responses are interpreted as signatures of progressive partial spinel inversion from high-spin $\mathrm{Fe}\ensuremath{\leftrightarrow}\mathrm{Cr}$ tetrahedral/octahedral site exchange, triggered near $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Beyond 60 GPa a new diamagnetic low-spin M\ossbauer spectral component emerges. This is preceded by an inflection and discontinuity in the pressure dependence of the resistance and tetragonal unit-cell volume, respectively. By $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, half of the iron is low spin in octahedral sites from evolved tetragonal-spinel inversion processes. The remainder reside in tetrahedral high-spin sites with ${H}_{hf}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The charge gap from electron correlations does not close, despite an anticipated appreciable band broadening from a $\ensuremath{\sim}30%$ unit-cell volume reduction upon pressurization to $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. This is attributable to an increase in effective on-site repulsion (Hubbard ${U}_{\mathrm{eff}}$) with increasing pressure or at spin crossover, specific to the $\mathrm{C}{\mathrm{r}}^{3+}({d}^{3})$ and $\mathrm{F}{\mathrm{e}}^{2+}({d}^{6})$ electronic configurations, respectively. Thus a $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ ``Mott'' gap prevails in the mixed spin-state of the partially inverted tetragonal phase $[\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{r}}_{x}]{\mathrm{F}{\mathrm{e}}_{x}\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}}{\mathrm{O}}_{4}\phantom{\rule{0.28em}{0ex}}(x\ensuremath{\sim}0.5)$ up to near a megabar (100 GPa) densification.

Determination of electronic ground state properties of a dinuclear iron(II) spin crossover complex

The dinuclear complex [(Fe(L-N4Me2))2(BiBzIm)](ClO4)2⋅2EtCN (1) has been investigated by Mossbauer spectroscopy carried out in the temperature range from 5 to 150 K with externally applied magnetic fields of up to B = 5 T. By means of a consistent simulation of all experimental data sets within the Spin Hamiltonian formalism, the zero-field splitting D and the rhombicity parameter E/D of the ferrous high-spin (HS) site in this complex was determined to be D = −15.0 ± 1.0 cm−1 and E/D = 0.33 respectively. The sign of the quadrupole splitting of the HS site is positive which indicates that this iron site of the dinuclear complex 1 has an electronic ground state with the dxy orbital being twofold occupied.

Crystal structure and spin state of mixed-crystals of iron with NCS and NCBH3 for the assembled complexes bridged by 1,3-bis(4-pyridyl)propanes

New mixed crystals, Fe(NCS) x (NCBH3)(2−x)(bpp)2, (bpp = 1,3–bis(4–pyridyl)propane) were synthesized. The formation of mixed crystals was confirmed by powder X-ray diffraction patterns and single crystal X-ray structural analysis. Elemental analyses showed that all Fe(NCS) x (NCBH3)(2−x)(bpp)2 samples had more Fe(NCS)2 unit than the preparation fraction. 57Fe Mossbauer spectroscopy revealed that all the spectra consist of only Fe(NCS)2 unit and Fe(NCBH3)2 unit. And the fraction of low-spin state in the Fe(NCBH3)2 unit changed with the change of x. The results suggested that the high spin site of Fe(NCS)2 unit affects the spin state of Fe(NCBH3)2 unit.

Spectroscopic and Theoretical Investigation of a Complex with an [O═FeIV–O–FeIV═O] Core Related to Methane Monooxygenase Intermediate Q

Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=Fe(IV)-O-Fe(IV)-OH core that has two ferromagnetically coupled Sloc = 1 sites. Addition of base to 2 at -85 °C elicits its conjugate base 6 with a novel O═Fe(IV)-O-Fe(IV)═O core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (Sloc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral O═Fe-Fe═O angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled Sloc = 1 and Sloc = 2 sites to give total spin St = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O═Fe-Fe═O angle of 6b is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear Fe(IV)═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.

High-field Mössbauer spectroscopy on dinuclear iron(II) spin crossover complexes

The dinuclear complex ({Fe(L-N4Me2)}2(BzImCOO))(ClO4)2*0.5(CH3)2CO has been investigated by Mossbauer spectroscopy carried out in the temperature range from T = 5Ku p to T= 190 K with externally applied magnetic fields of up to B = 5 T. By means of a consistent simulation of all experimental data sets within the Spin Hamiltonian formalism, the zero-field splitting and the rhombicity parameter of the ferrous high-spin site could be determined to be D = 7.2 ± 0.5 cm −1 and E/D = 0.1 ± 0.02. The sign of the quadrupole splitting is positive which indicates that the ferrous high-spin site of the dinuclear complex ({Fe(L-N4Me2)}2(BzImCOO))- (ClO4)2*0.5(CH3)2CO has an electronic ground state with the dxy orbital twofold occupied.

Ligand‐Induced Distortions and Magneto‐Structural Correlations in a Family of Dinuclear Spin Crossover Compounds with Bipyridyl‐Like Bridging Ligands

AbstractThe synthesis and characterization of a new series of dinuclear compounds [{Fe(dpia)(NCBH3)2}2(A)], where dpia = di(2‐picolyl)amine, A = 4,4′‐bipyridine (1), 1,2‐di(4‐pyridyl)ethyne (bpac) (2), or 1,2‐di(4‐pyridyl)ethane (bpen) (3), are reported. Variable‐temperature magnetic susceptibility measurements of the three compounds show that they have different magnetic properties. Compound 1 exhibits a relatively cooperative two‐step spin crossover (SCO) behavior. A short plateau at 210 K separates two steps and conforms to about 50 % of the complexes undergoing a thermal spin conversion. Compounds 2 and 3 present gradual full one‐step spin transitions. The thermodynamic parameters of 1–3, obtained through the fitting of the magnetic data with the regular solution model, are compared with those for similar dinuclear SCO compounds. The single‐crystal X‐ray structure of 1 was solved at three temperatures, 290 K, 211 K, and 110 K, corresponding to three spin‐state isomers, namely [HS–HS], [HS–LS], and [LS–LS]. At the plateau temperature, both HS (high spin) and LS (low spin) sites in the mixed pair are unresolved and only an average Fe–N bond length was obtained. The structural characteristics and magnetic behavior of the new two‐step SCO compound 1 are in full agreement with magneto–structural correlations for previously reported related dinuclear SCO compounds. This analysis confirms the validity of a recently proposed mechanism responsible for three types of SCO in dinuclear compounds (one‐step, two‐step, and partial 50 % transition).

Study on the spin crossover transition and glass transition for Fe(II) complex film, [Fe(II)(H-triazole) 3 ]@Nafion, by means of Mössbauer spectroscopy

[Fe(II)(H-trz)3]@Nafion (trz = triazole) is a transparent spin crossover complex film, where the spin crossover transition between the low-spin (S = 0) and the high-spin (S = 2) states takes place between 225 K and 300 K. In this film, two doublets corresponding to the low-spin and high-spin states were observed in the 57Fe Mossbauer spectra, reflecting the spin crossover transition. From the analysis of 57Fe Mossbauer spectra, the Debye temperatures of the low-spin and high-spin sites were estimated at 185 K and 176 K, respectively, in the temperature range between 10 K and 150 K. In this film, the total intensity of the Mossbauer spectra corresponding to the low-spin and high-spin sites drastically decreases above 200 K, reflecting the glass transition of Nafion, where the lattice vibration of [Fe(H-trz)3]\(_{\rm n}^{\,\,\rm 2n+}\) is softened just as in solution due to micro-Brown motion of the segment of Nafion polymer membrane.

Metastable state of the photomagnetic Prussian blue analog K0.3Co[Fe(CN)6]0.77·3.6H2O investigated by various techniques

The thermodynamic properties of the photomagnetic Prussian blue analog K0.3Co[Fe(CN)6]0.77 · 3.6H2O, which exhibits a charge-transfer-induced spin transition (CTIST), are reported. According to the thermal history of the sample, different low-temperature states are obtained: a quenched high-spin state (Q), a low-spin state (LT) achieved through the decay of the Q state, and an intermediate state (IM) combining low-spin (LS) and high-spin (HS) local states. The IM state is obtained by slow cooling and displays a slight kinetic dependence. The long lifetime of the IM state, which persists up to the vicinity of the CTIST temperature, is unusual and differs from all metastable states reported so far for photomagnetic Prussian blue analogs. The properties of the different states and of their photoexcited counterparts are investigated by magnetic and photomagnetic measurements, calorimetry, diffuse reflectance, and x-ray diffraction. A key feature of the IM state revealed by x-ray diffraction is the onset of phase separation between a LT-type fraction and a mixed HS-LS fraction (approximately 50 : 50). X-ray patterns of the IM state obtained during successive cooling and heating stages document irreversible transformations. The formation and properties of the IM state suggest a self-organization process between lowand high-spin sites in response to internal structural stresses, and this effect is hindered by irradiation with visible light. The relaxation kinetics of the Q and photoexcited states are analyzed by using a two-variable model, which accounts for the onset of correlations due to short-range elastic interactions. A quantitative comparison to the analogous sodium compound Na0.32Co[Fe(CN)6]0.74 · 3.4H2O confirms the nonstandard behavior of the title potassium compound and supports the structural origin of the self-organization processes.

Lattice-Mediated Propagation of Photoinduced Phase Transition in Co–Fe Cyanide

Spin-crossover compounds show many photoinduced phase transition (PIPT) phenomena, reflecting the strong coupling between the electron, spin, and lattice degrees of freedoms. In these compounds, the photoinduced electronic excitation triggers the intersystem of crossing in Fe2þ or Co3þ with a high probability, and alters the spin state of the transition metals (light-induced excited spin state trapping: LIESST). The spin state transition further causes local lattice deformation as a result of the variation in the ionic radii of Fe2þ or Co3þ.14) The resultant local lattice distortion modifies the electronic structure of the surrounding sites, causing a nonlinear response of the system. Ogawa et al. reported the threshold and incubation behaviors in the photoinduced phase transition of [Fe(2-pic)3]Cl2 EtOH (2-pic = 2-aminomethyl-pyridine) at 2.2K. Liu et al. reported a dynamical phase transition in Nafion–[Fe(Htrz)3] (Htrz = 1,2,4-4H-triazole) and an optical hysteresis in [Fe(pyz)6](BF4)2 (ptz = 1-propyltetrazole) at 77K. Several researchers theoretically investigated the photoinduced dynamics beyond the mean-field approximation, even though the approach had been successful in several situations. In this Note, we reported the mesoscale lattice effect on PIPT of a Co–Fe cyanide, Na0:27Co[Fe(CN)6]0:713.8H2O, at 200K by a microscopic spectroscopy. The compound shows a spin-crossover-type PIPT from the low-spin (LS) phase with LS Co3þ and LS Fe2þ to the high-spin (HS) phase with HS Co2þ and LS Fe3þ, accompanying a significant lattice expansion of 3%. We carefully investigated the PIPT dynamics at each position around the photoexcitation spot, which is 75 m in diameter. We found that the PIPT at each position takes place simultaneously within 0.2 s, even though the density (nHS) of the HS sites is widely distributed in the range of 0–0.6. The distribution of the linear expansion coefficient ( L=L) within the HS region suggests that the propagation of the PIPT region is mediated by a coherent lattice expansion. A thin film of Na0:27Co[Fe(CN)6]0:713.8H2O, 1310 nm in thickness, was electrochemically synthesized on an indium–tin oxide (ITO) transparent electrode. The chemical composition was determined by inductively coupled plasma (ICP) and CHN organic elementary analyses. The Xray diffraction pattern revealed that the compound is facecentered cubic with a lattice constant of a 1⁄4 10:34 A. The surface and cross-sectional scanning electron microscopy (SEM) images revealed that the film consists of rodlike crystals, 600 nm in length and 130 nm in area. The film shows a thermally induced LS–HS phase transition at 230K (1⁄4 Tc) in the warming run, and the film changes from violet (LS phase) to red (HS phase). The photoinduced dynamics was recorded with a laboratory-built microscopy system equipped with a charge-coupled device (CCD) camera. We used the second harmonics (532 nm) of the yttrium aluminum garnet (YAG) laser as the excitation light source. The spot size of the excitation light was 75 m, and the excitation density was 6.6 photon s 1 site . A notch filter was inserted to eliminate the scattering from the excitation light. The magnitudes of nHS at each time and position were determined by the chromaticity in the microscopy moving image. The spatial resolution of the system was 5 m. To increase the signal-to-noise ratio, the chromaticity was averaged within the 10 10 pixels around each position. Figure 1 shows examples of the microscopy images of the Na0:27Co[Fe(CN)6]0:713.8H2O film under photoirradiation at 200K at various times. The bright region corresponds to the HS region. No macroscopic PIPT is observed at (b) 41.7 s. The PIPT starts around the central region of the excitation spot at (c) 43.3 s, and then expands outside at (d) 43.5 s. A similar incubation behavior of the PIPT was observed in [Fe(2-pic)3]Cl2 EtOH. One may notice that the PIPT region at (d) 43.5 s extends beyond the excitation spot [broken-line circle in Fig. 1(a)]. The inset of Fig. 2(b) shows a prototypical example of the time dependence of nHS. (a) 0 .0 s

Light-induced spin-crossover magnet

The light-induced phase transition between the low-spin (LS) and high-spin (HS) states of some transition-metal ions has been extensively studied in the fields of chemistry and materials science. In a crystalline extended system, magnetically ordering the HS sites of such transition-metal ions by irradiation should lead to spontaneous magnetization. Previous examples of light-induced ordering have typically occurred by means of an intermetallic charge transfer mechanism, inducing a change of valence of the metal centres. Here, we describe the long-range magnetic ordering of the extended Fe(II)(HS) sites in a metal-organic framework caused instead by a light-induced excited spin-state trapping effect. The Fe-Nb-based material behaves as a spin-crossover magnet, in which a strong superexchange interaction (magnetic coupling through non-magnetic elements) between photo-produced Fe(II)(HS) and neighbouring Nb(IV) atoms operates through CN bridges. The magnetic phase transition is observed at 20 K with a coercive field of 240 Oe.

Crystal structure and magnetic property of spin crossover complex Fe II(3-phenylpyridine) 2[Au I(CN) 2] 2

A novel two-dimensional network bimetallic Fe Au spin crossover coordination polymer based on 3-phenylpyridine-coordinated iron centers and linear gold cyanide bridges {Fe(3-phenylpyridine) 2[Au(CN) 2] 2} n ( 1), has been synthesized. The compound is characterized by elemental analysis, IR, single-crystal X-ray analysis at 300 and 90 K and magnetic measurements. The Fe II ions in 1 have octahedral Fe IIN 6 coordination geometries, which are linked by [Au(CN) 2] − units at the equatorial plane to form a polymeric 2D sheet architecture. The two pyridine rings coordinate in axial position. Variable-temperature (2–300 K) magnetic susceptibility measurements of 1 were performed to determine the spin transition behavior. SQUID data show that high and low spin states exist in a 1:1 ratio at 90 K. However, only one kind of Fe II atom is apparent crystallographically at 90 K, indicating that the high and low spin sites are disordered in the polymeric 2D framework.

Crystallization Process of Photoexcited High-Spin Sites in Co–Fe Cyanide Film

The dynamics of the photoinduced phase transition (PIPT) was systematically investigated in Co–Fe cyanide film, Na 0.16 Co[Fe(CN) 6 ] 0.71 5.8H 2 O, at 160 and 190 K above the melting temperature ( T m = 120 K) of the photoinduced phase. Below T m , the film shows a permanent photoinduced phase transition from a low-spin (LS) phase to a static high-spin (HS) phase. We found that the relaxation curve from the photo-excited state changes from the exponential type to Avrami's type as the excitation photon density ( n ) increases. We interpreted this behavior in terms of crystallization process of the photo-created HS sites.

Magnetic and Transport Properties of Ba2Co9O14 and Ba1.9A0.1Co9O14 (A=La or Na)

The valence and spin–state distributions of Co ions and the complex structure of antiferromagnetic Ba 2 Co 9 O 14 have led to the suggestion that doped Ba 2 Co 9 O 14 compounds may be good thermoelectric materials. We have checked this suggestion by measuring the magnetic properties as well as the transport properties of nominal Ba 1.9 A 0.1 Co 9 O 14 ( A =La or Na). We show that although all compounds are indicated to be single phase by powder X-ray diffraction analysis, they are all p -type polaronic conductors with low mobile-hole concentrations. Magnetic-susceptibility data of the parent and La-doped compounds give evidence of a second magnetic phase with ferromagnetic order setting in below 215 K; but this second phase is not seen in the Na-doped sample. We conclude that the structure is stabilized by oxidation and that cation exolution from the Ba 2 Co 9 O 14 structure creates cation vacancies that oxidize the high-spin (HS) Co(II) to the intermediate-spin Co(III) where there is no hole doping; the low-spin (LS) Co(III) of the parent structure is not reduced by the substitution of La for Ba. Disordering of the HS and LS sites does not occur, so the doped Ba 2 Co 9 O 14 compositions are not candidates for thermoelectric materials.

A Novel Heme a Insertion Factor Gene Cotranscribes with the Thermus thermophilus Cytochrome ba 3 Oxidase Locus

Studying the biogenesis of the Thermus thermophilus cytochrome ba(3) oxidase, we analyze heme a cofactor insertion into this membrane protein complex. Only three proteins linked to oxidase maturation have been described for this extreme thermophile, and in particular, no evidence for a canonical Surf1 homologue, required for heme a insertion, is available from genome sequence data. Here, we characterize the product of an open reading frame, cbaX, in the operon encoding subunits of the ba(3)-type cytochrome c oxidase. CbaX shares no sequence identity with any known oxidase biogenesis factor, and CbaX homologues are found only in the Thermaceae group. In a series of cbaX deletion and complementation experiments, we demonstrate that the resulting ba(3) oxidase complexes, affinity purified via an internally inserted His tag located in subunit I, are severely affected in their enzymatic activities and heme compositions in both the low- and high-spin sites. Thus, CbaX displays typical features of a generic Surf1 factor essential for binding and positioning the heme a moiety for correct assembly into the protein scaffold of oxidase subunit I.

Spin states and hyperfine interactions of iron in (Mg,Fe)SiO 3 perovskite under pressure

With the guidance of first-principles phonon calculations, we have searched and found several metastable equilibrium sites for substitutional ferrous iron in MgSiO 3 perovskite. In the relevant energy range, there are two distinct sites for high-spin, one for low-spin, and one for intermediate-spin iron. Because of variable d-orbital occupancy across these sites, the two competing high-spin sites have different iron quadrupole splittings (QS). At low pressure, the high-spin iron with QS of 2.3–2.5 mm/s is more stable, while the high-spin iron with QS of 3.3–3.6 mm/s is more favorable at higher pressure. The crossover occurs between 4 and 24 GPa, depending on the choice of exchange-correlation functional and the inclusion of on-site Coulomb interaction (Hubbard U). Our calculation supports the notion that the transition observed in recent Mössbauer spectra corresponds to an atomic-site change rather than a spin-state crossover. Our result also helps to explain the lack of anomaly in the compression curve of iron-bearing silicate perovskite in the presence of a large change of quadrupole splitting, and provides important guidance for future studies of thermodynamic properties of this phase.

Understanding the Two‐Step Spin‐Transition Phenomenon in Iron(II) 1D Chain Materials

Three analogous one dimensional (1D) polymeric iron(II) spin crossover (SCO) materials containing the new ligand 4,6-bis(2',2''-pyridyl)pyrazine (bdpp) have been comprehensively characterised magnetically (thermal and light-induced) and structurally. Within this series are two polymorphs of the formula [Fe(NCS)(2)(bdpp)], 1 and 2 a, which differ magnetically in that phase 1 undergoes a full two-step SCO (T(1/2(1))=135 K and T(1/2(2))=90 K) whereas phase 2 a remains high spin (HS) over all temperatures. The central distinction between these two materials lies in the presence of intermolecular pi-pi interactions generated by the crystal packing in 1, which are absent in 2 a. The isostructural selenocyanate analogue of 2 a, [Fe(NCSe)(2)(bdpp)], 2 b, undergoes a full two-step SCO (T(1/2(1))=200 K and T(1/2(2))=125 K). Structural analyses of 1 and 2 b at a range of temperatures provide deep insight into their two-step SCO nature. Structural analysis of 1 at 25 K (1(LS-LS)), 123 K (1(LS-HS)) and 250 K (1(HS-HS)) reveals two distinct iron(II) centres at each temperature, with ordered, alternating HS and LS (low spin) sites at the intermediate plateau (IP) temperatures. In contrast, structural analysis of 2 b at 90 K (2 b(LS)), 150 K (2 b(LS/HS)) and 250 K (2 b(HS)) reveals one unique iron(II) centre at each temperature with an "averaged" LS/HS character at the IP temperature. Weak planes of diffuse scattering in the single-crystal X-ray diffraction patterns were observed for this phase at 90 and 150 K, indicating that 1D long range ordering of alternating HS/LS iron(II) centres occurs along the 1D coordination chains, but that there is no correlation between chains. The lack of observable diffuse scattering at 250 K suggests that the onset of the 1D structural ordering in the chain direction corresponds to the first step of the SCO and that this structural transition is electronically driven. The photomagnetic properties of both 1 and 2 b have been investigated and show approximately 62 and 53 % photo-excitation of a HS metastable state at low temperatures and T(LIESST) values of 55 and 49 K, respectively. Relaxation studies on the HS fraction in 2 b fitted well to a stretched exponential model with kinetic parameters indicative of weak cooperativity.