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Carbon and nitrogen are considered as candidate light elements present in planetary cores. However, there is limited understanding regarding the structure and physical properties of Fe‐C‐N alloys under extreme conditions. Here diamond anvil cell experiments were conducted, revealing the stability of hexagonal‐structured Fe7(N0.75C0.25)3 up to 120 GPa and 2100 K, without undergoing any structural transformation or dissociation. Notably, the thermal expansion coefficient and Grüneisen parameter of the alloy exhibit a collapse at 55–70 GPa. First‐principles calculations suggest that such anomaly is associated with the spin transition of iron within Fe7(N0.75C0.25)3. Our modeling indicates that the presence of ∼1.0 wt% carbon and nitrogen in liquid iron contributes to 9–12% of the density deficit of the Earth's outer core. The thermoelastic anomaly of the Fe‐C‐N alloy across the spin transition is likely to affect the density and seismic velocity profiles of (C,N)‐rich planetary cores, thereby influencing the dynamics of such cores. Plain Language Summary: A significant amount of light elements are believed to be present in the cores of Earth and other planets to explain the density difference between iron‐nickel alloys and geophysical observations. This study used experiments at high‐pressure and high‐temperature conditions and theoretical simulations to investigate a specific candidate phase of the core called Fe7(N0.75C0.25)3, which has a hexagonal structure, at high pressures similar to those in planetary cores. This phase did not undergo any major structural changes under the conditions investigated. However, its properties related to the thermoelastic behaviors showed significant changes between 55 and 70 GPa. Theoretical calculations indicate that this anomalous behavior is linked to the magnetic transition within h‐type Fe7(N0.75C0.25)3. These findings suggest that the characteristics of a planetary core would be altered in the transition region, leading to a more complex thermal evolution than previously believed. This study provides insights into the behavior of light elements in planetary cores and its implications for planetary dynamics. Key Points: The h‐type Fe7(N0.75C0.25)3 is stable to 120 GPa and 2100 K, but undergoes a pressure‐induced spin transitionSpin transition causes a significant reduction in γ0 and α0, and the thermal equation of state of h‐type non‐magnetic Fe7(C,N)3 is determinedThe density profile and seismic features of a planetary core could be altered across the spin transition, which may affect core dynamics [ABSTRACT FROM AUTHOR]

Barocaloric cooling, that is, lowering the temperature of a material under pressure action, is an attractive solid‐state effect that can potentially compete with volatile gas‐based cooling. To observe the barocaloric effect (BCE), it is necessary for materials to have high‐entropy, low‐hysteretic phase transitions with a large volume change between phases. Here details on a new FeII complex [Fe(L)(NCS)2], L = N1,N3‐bis((1‐propyl‐1H‐1,2,3‐triazol‐4‐yl)methylene)‐2,2‐dimethylpropane‐1,3‐diamine) possessing spin crossover (SCO) behavior near room temperature with large entropy and volume change are reported, which provides high sensitivity to external pressure. The observed BCE effect, characterized using variable pressure calorimetry, powder X‐ray diffraction, UV–vis, IR, and Raman spectroscopy, shows a colossal isothermal entropy change of >100 J kg−1 K−1 and a reversible adiabatic temperature change of ≈16 K at a pressure of 1 kbar, demonstrating a high refrigerant efficiency compared to other solid‐state materials. These results stimulate further investigations of SCO materials as barocaloric refrigerants, which depend on the proper design of their constituent organic ligands. [ABSTRACT FROM AUTHOR]

Physical properties of mantle minerals are essential for comprehensive geodynamic modeling. High-pressure experiments allow measurements of physical properties but fundamental insights into their evolution with pressure are often experimentally inaccessible. Here we report the first in situ experimental determination of the optical refractive index, its wavelength-dispersion, and optical absorption coefficient of ferropericlase up to ~140 GPa at room temperature. All these properties change gradually in dominantly high-spin (below ~50 GPa) and low-spin (above ~80 GPa) ferropericlase. However, in the mixed-spin state (i.e., significant presence of both high- and low-spin iron), the index dispersion and the absorption coefficient decrease by a factor of three and ~30%, respectively. These anomalies suggest that charge transport by small polaron is reduced in mixed-spin ferropericlase, providing fundamental insights into the factor-of-three lower electrical conductivity of ferropericlase at ~50–70 GPa. [ABSTRACT FROM AUTHOR]

In this study, we investigated reversible intermolecular proton shifting (IPS) coupled with spin transition (ST) in a novel FeII complex. The host FeII complex and the guest carboxylic acid anion were connected by intermolecular hydrogen bonds (IHBs). We extended the intramolecular proton transfer coupled ST phenomenon to the intermolecular system. The dynamic phenomenon was confirmed by variable‐temperature single‐crystal X‐ray diffraction, neutron crystallography, and infrared spectroscopy. The mechanism of IPS was further validated using density functional theory calculations. The discovery of IPS‐coupled ST in crystalline molecular materials provides good insights into fundamental processes and promotes the design of novel multifunctional materials with tunable properties for various applications, such as optoelectronics, information storage, and molecular devices. [ABSTRACT FROM AUTHOR]

We have conducted electrical conductivity measurements of FeCO3 siderite under high pressure up to 63 GPa in order to understand the nature and effect of iron spin transition and its influence on the geophysical properties of siderite, which is an end-member of major carbonate minerals. The results from Raman and Mössbauer spectroscopic measurements show that the high- to low-spin transition of iron occurs at around 50 GPa in agreement with previous studies. A sharp decrease of the electrical conductivity was also observed at around 50 GP, which is associated with the spin transition in iron. Although the stability of FeCO3 siderite may be limited under high-temperature conditions along with the mantle geotherm, solid solutions in the MgCO3-FeCO3 system, Mg1-xFexCO3, could be stable up to the pressure-temperature condition of the lowermost mantle. The pressure-temperature range of the spin transition in Mg1-xFexCO3 is narrower than those of the major lower mantle minerals, ferropericlase and bridgmanite, and thus the drop of the electrical conductivity induced by the spin transition could be clearer under lower mantle conditions. Therefore, the existence of Mg1-xFexCO3 may affect the observed heterogeneity of electrical conductivity in the mid-lower mantle. [ABSTRACT FROM AUTHOR]

Iron hydroxide FeO2Hx (x ≤ 1) and ferrous iron chloride FeCl2 can adopt the HP-PdF2-type (space group: , Z = 4) structure in the lowermost mantle, potentially contributing to the geochemical cycles of hydrogen and chlorine within Earth’s deep interior, respectively. Here we investigate the high-pressure behavior of HP-PdF2-type FeCl2 by X-ray diffraction (XRD) and Raman measurements in laser-heated diamond anvil cells. Our results show that HP-PdF2-type FeCl2 can be formed at 60‒67 GPa and 1650‒1850 K. Upon cold decompression, the diffraction peaks at pressures above 10 GPa can be indexed to the HP-PdF2-type structure. Intriguingly, the calculated cell volumes reveal a remarkable decrease of ΔV / V = ∼ 14% between 36 and 40 GPa, which is possibly caused by a pressure-induced spin transition of Fe2+ (HS: high-spin → LS: low-spin). We also observe distinct changes in Raman spectra at 33‒35 GPa, practically coinciding with the onset pressures of isostructural phase transition in XRD results. Our observations combined with previous studies conducted at megabar pressures suggest that HP-PdF2-type FeCl2, with a wide pressure stability range, if present in subducting slabs, could facilitate the transport of chlorine from the middle lower mantle to the outer core. [ABSTRACT FROM AUTHOR]

Iron hydroxide FeO2Hx (x ≤ 1) and ferrous iron chloride FeCl2 can adopt the HP-PdF2-type (space group: , Z = 4) structure in the lowermost mantle, potentially contributing to the geochemical cycles of hydrogen and chlorine within Earth’s deep interior, respectively. Here we investigate the high-pressure behavior of HP-PdF2-type FeCl2 by X-ray diffraction (XRD) and Raman measurements in laser-heated diamond anvil cells. Our results show that HP-PdF2-type FeCl2 can be formed at 60‒67 GPa and 1650‒1850 K. Upon cold decompression, the diffraction peaks at pressures above 10 GPa can be indexed to the HP-PdF2-type structure. Intriguingly, the calculated cell volumes reveal a remarkable decrease of ΔV / V = ∼ 14% between 36 and 40 GPa, which is possibly caused by a pressure-induced spin transition of Fe2+ (HS: high-spin → LS: low-spin). We also observe distinct changes in Raman spectra at 33‒35 GPa, practically coinciding with the onset pressures of isostructural phase transition in XRD results. Our observations combined with previous studies conducted at megabar pressures suggest that HP-PdF2-type FeCl2, with a wide pressure stability range, if present in subducting slabs, could facilitate the transport of chlorine from the middle lower mantle to the outer core. [ABSTRACT FROM AUTHOR]

In this article, we study the optimized structures, spin crossover, and coherent quantum transport properties of a series of cyclopentadienyl metalloporphyrin (PTMCp, TM = transition metal) complexes using the density functional theory combined with the non-equilibrium Green's function method. The structure of the complexes can be classified into the sandwich type and the biplanar type. Energetic analyses of spin states reveal that the IS(intermediate-spin)-HS(high-spin) spin transition may appear in PMnCp and PFeCp, and the LS(low-spin)-IS(intermediate-spin)-HS(high-spin) two-step spin transition may appear in PCoCp under external stimulus. We predict that giant magnetoresistance may be observed in the ground-state sandwich PTMCp depending on the contact between the electrodes and the molecule in measurements. These results indicate that PTMCp complexes could be promising materials for spintronics. [ABSTRACT FROM AUTHOR]

In dinitrogen (N2) fixation chemistry, bimetallic end‐on bridging N2 complexes M(μ‐η1 : η1‐N2)M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2(μ‐N2)}0/1+/2+ (1–3) are incapable to cleave N2, in contrast to their Mo congeners. Remarkably, cross this series the N−N bond length of the N2 ligand and the N−N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand‐mediated two‐center spin transitions, distinct from discrete dinuclear spin crossovers. In‐depth analyses using wave function based ab initio calculations reveal that the Cr‐N2‐Cr bonding in complexes 1–3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1–3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ‐η1 : η1‐N2)M complexes compared to their 4d and 5d analogs. [ABSTRACT FROM AUTHOR]

In dinitrogen (N2) fixation chemistry, bimetallic end‐on bridging N2 complexes M(μ‐η1 : η1‐N2)M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2(μ‐N2)}0/1+/2+ (1–3) are incapable to cleave N2, in contrast to their Mo congeners. Remarkably, cross this series the N−N bond length of the N2 ligand and the N−N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand‐mediated two‐center spin transitions, distinct from discrete dinuclear spin crossovers. In‐depth analyses using wave function based ab initio calculations reveal that the Cr‐N2‐Cr bonding in complexes 1–3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1–3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ‐η1 : η1‐N2)M complexes compared to their 4d and 5d analogs. [ABSTRACT FROM AUTHOR]

In dinitrogen (N2) fixation chemistry, bimetallic end‐on bridging N2 complexes M(μ‐η1 : η1‐N2)M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2(μ‐N2)}0/1+/2+ (1–3) are incapable to cleave N2, in contrast to their Mo congeners. Remarkably, cross this series the N−N bond length of the N2 ligand and the N−N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand‐mediated two‐center spin transitions, distinct from discrete dinuclear spin crossovers. In‐depth analyses using wave function based ab initio calculations reveal that the Cr‐N2‐Cr bonding in complexes 1–3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1–3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ‐η1 : η1‐N2)M complexes compared to their 4d and 5d analogs. [ABSTRACT FROM AUTHOR]

In dinitrogen (N2) fixation chemistry, bimetallic end‐on bridging N2 complexes M(μ‐η1 : η1‐N2)M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2(μ‐N2)}0/1+/2+ (1–3) are incapable to cleave N2, in contrast to their Mo congeners. Remarkably, cross this series the N−N bond length of the N2 ligand and the N−N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand‐mediated two‐center spin transitions, distinct from discrete dinuclear spin crossovers. In‐depth analyses using wave function based ab initio calculations reveal that the Cr‐N2‐Cr bonding in complexes 1–3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1–3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ‐η1 : η1‐N2)M complexes compared to their 4d and 5d analogs. [ABSTRACT FROM AUTHOR]

Abstract Carbon and nitrogen are considered as candidate light elements present in planetary cores. However, there is limited understanding regarding the structure and physical properties of Fe‐C‐N alloys under extreme conditions. Here diamond anvil cell experiments were conducted, revealing the stability of hexagonal‐structured Fe7(N0.75C0.25)3 up to 120 GPa and 2100 K, without undergoing any structural transformation or dissociation. Notably, the thermal expansion coefficient and Grüneisen parameter of the alloy exhibit a collapse at 55–70 GPa. First‐principles calculations suggest that such anomaly is associated with the spin transition of iron within Fe7(N0.75C0.25)3. Our modeling indicates that the presence of ∼1.0 wt% carbon and nitrogen in liquid iron contributes to 9–12% of the density deficit of the Earth's outer core. The thermoelastic anomaly of the Fe‐C‐N alloy across the spin transition is likely to affect the density and seismic velocity profiles of (C,N)‐rich planetary cores, thereby influencing the dynamics of such cores.

The spin state of the cobalt(II) phenylborate hexa-n-butylsulfide clathrochelate in solutions is studied by paramagnetic NMR spectroscopy. This cage complex is shown to undergo the temperature-induced spin transition in solvents of different nature (acetonitrile, chloroform, dichloromethane, and benzene). The previously developed method for an analysis of paramagnetic shifts in NMR spectra allows the determination of the thermodynamic parameters (enthalpy and entropy) of the spin equilibrium in the solutions. In spite of the conformational rigidity of the macrobicyclic tris(dioximate) molecules, substantial changes in the electronic structures and spin equilibrium parameters are observed depending on the polarity of the solvent used. This provides opportunities for fine tuning of spin switch characteristics by changing this parameter of the medium. [ABSTRACT FROM AUTHOR]

The photoexcitation dynamics of N3−‐bound ferric myoglobin (MbN3) were investigated after exciting MbN3 in D2O at 283 K with a 575 nm pulse by probing the anti‐symmetric stretching mode of the azide. Global fitting of the overall time‐resolved spectra revealed that thermal relaxation of two stretching bands proceeded with a time constant of 6 ps, and that a new absorption band formed and decayed with time constants of 0.6 and 23 ps, respectively. The new absorption near 2040 cm−1 was attributed to the high‐spin species 2.4 kJ/mol above the low‐spin species, as the excited low‐spin relaxes thermally via the high‐spin species. However, this absorption could also arise from deligated N3̄ remaining within the protein. The decay of this absorption can be interpreted as either spin transition of the high‐spin species into the low‐spin species or geminate recombination of the dissociated N3̄. The implications of both interpretations are discussed. [ABSTRACT FROM AUTHOR]

This work is devoted to the study of the impurities effect in 2D spin crossover nanosystem in frame work of an Ising-like model. Results were obtained by means of Monte Carlo modeling technique, based on the heat bath algorithm. It is shown, that the impurities change the system’s thermal hysteresis width and shift the spin transition curves (HS → LS). In this manner, impurities can act as an additional influence parameter on the transition curves. The considered effect is not significant, but nevertheless should be taken into account in the developing process of nanoscale devices based on spin crossover compounds.

In this article, we study the optimized structures, spin crossover, and coherent quantum transport properties of a series of cyclopentadienyl metalloporphyrin (PTMCp, TM = transition metal) complexes using the density functional theory combined with the non-equilibrium Green’s function method. The structure of the complexes can be classified into the sandwich type and the biplanar type. Energetic analyses of spin states reveal that the IS(intermediate-spin)-HS(high-spin) spin transition may appear in PMnCp and PFeCp, and the LS(low-spin)-IS(intermediate-spin)-HS(high-spin) two-step spin transition may appear in PCoCp under external stimulus. We predict that giant magnetoresistance may be observed in the ground-state sandwich PTMCp depending on the contact between the electrodes and the molecule in measurements. These results indicate that PTMCp complexes could be promising materials for spintronics.

Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth’s lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe3+-rich phases, FeMg0.5Si0.5O3 and Fe0.5Mg0.5Al0.5Si0.5O3, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg0.5Si0.5O3 phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe0.5Mg0.5Al0.5Si0.5O3 phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe3+ in a (Fe0.7,Mg0.3)O6 octahedral site of the FeMg0.5Si0.5O3 phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe0.5Mg0.5Al0.5Si0.5O3 phase as all Fe3+ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al0.5,Si0.5)O6.

As a result of the high symmetry of the Aea2 structure, the asymmetric unit of the title compound, [FeII(C18H15N6)2]·2MeOH, consists of half of a charge-neutral complex molecule and a discrete methanol molecule. The planar anionic tridentate ligand 2-[5-(3,4-dimethylphenyl)-4H-1,2,4-triazol-3-ato]-6-(1H-pyrazol-1-yl)pyridine coordinates the FeII ion meridionally through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octahedral coordination sphere of the central ion. The average Fe—N bond distance is 1.955 Å, indicating a low-spin state of the FeII ion. Neighbouring cone-shaped molecules, nested into each other, are linked through double weak C—H(pz)...π(ph') interactions into mono-periodic columns, which are further linked through weak C—H...N′/C′ interactions into di-periodic layers. No interactions shorter than the sum of the van der Waals radii of the neighbouring layers are observed. Energy framework analysis at the B3LYP/6–31 G(d,p) theory level, performed to quantify the intermolecular interaction energies, reproduces the weak interlayer interactions in contrast to the strong interaction within the layers. Intermolecular contacts were quantified using Hirshfeld surface analysis and two-dimensional fingerprint plots, showing the relative contributions of the contacts to the crystal packing to be H...H 48.5%, H...C/C...H 28.9%, H...N/N...H 16.2% and C...C 2.4%.

The unit cell of the title compound, [FeII(C17H12BrN6O)2]·2MeOH, consists of a charge-neutral complex molecule and two independent molecules of methanol. In the complex molecule, the two tridentate ligand molecules 2-[5-(3-bromo-4-methoxyphenyl)-4H-1,2,4-triazol-3-yl]-6-(1H-pyrazol-1-yl)pyridine coordinate to the FeII ion through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octahedral coordination sphere around the central ion. In the crystal, neighbouring asymmetric molecules are linked through weak C—H(pz)...π(ph) interactions into chains, which are then linked into layers by weak C–H...N/C interactions. Finally, the layers stack into a three-dimensional network linked by weak interlayer C—H...π interactions between the methoxy groups and the phenyl rings. The intermolecular contacts were quantified using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H...H 34.2%, H...C/C...H 25.2%, H...Br/Br...H 13.2%, H...N/N...H 12.2% and H...O/O...H 4.0%. The average Fe—N bond distance is 1.949 Å, indicating the low-spin state of the FeII ion. Energy framework analysis at the HF/3–21 G theory level was performed to quantify the interaction energies in the crystal structure.

The electrical conductivity and elasticity of deep hydrous phases are essential to constraining water distribution, as well as deciphering the origins of conductivity anomalies in the lower mantle. To uncover the impact of iron-bearing δ-AlOOH on the geophysical properties of the lower mantle, we carried out synchrotron X‑ray diffraction and electrical conductivity measurements on δ-(Al0.52Fe0.48)OOH and (Al0.95Fe0.05)OOH in diamond-anvil cells at pressures up to 75 GPa at room temperature. A sharp volume reduction of ~6.5% was observed in δ-(Al0.52Fe0.48)OOH across the spin transition at 40.8–43.3 GPa, where its electrical conductivity increases steadily without abrupt changes. The electrical conductiv‑ ity of δ-(Al0.52Fe0.48)OOH is greater than that of pure δ-AlOOH at high pressure, suggesting that both small polaron and proton conduction mechanisms dominate in iron-bearing δ-AlOOH. Furthermore, the high-pressure electrical conductivity profiles are comparable between δ-(Al0.95Fe0.05)OOH and δ-(Al0.52Fe0.48)OOH, indicating that high-iron content only marginally influences the conductivity of iron-bearing δ-AlOOH. Notably, the electrical conductivity of iron-bearing δ-AlOOH along the North Philippine geotherm is greater than the average 1D electrical conductivity profile in the mantle (Ohta et al. 2010a). This result suggests that δ-(Al,Fe)OOH is a promising candidate to account for high conductivity in some subducting slabs. [ABSTRACT FROM AUTHOR]

Discrete spin crossover (SCO) heteronuclear cages are a rare class of materials which have potential use in next‐generation molecular transport and catalysis. Previous investigations of cubic cage [Fe8Pd6L8]28+ constructed using semi‐rigid metalloligands, found that FeII centers of the cage did not undergo spin transition. In this work, substitution of the secondary metal center at the face of the cage resulted in SCO behavior, evidenced by magnetic susceptibility, Mössbauer spectroscopy and single crystal X‐ray diffraction. Structural comparisons of these two cages shed light on the possible interplay of inter‐ and intramolecular interactions associated with SCO in the NiII analogue, 1 ([Fe8Ni6L8(CH3CN)12]28+). The distorted octahedral coordination environment, as well as the occupation of the CH3CN in the NiII axial positions of 1, prevented close packing of cages observed in the PdII analogue. This led to offset, distant packing arrangements whereby important areas within the cage underwent dramatic structural changes with the exhibition of SCO. [ABSTRACT FROM AUTHOR]

Nitrogen is an essential element for life, one of the most abundant volatiles in the atmosphere, and an important component in the Earth's interior, where iron nitride is an essential host of deep nitrogen. Here, we investigate the pressure-induced electronic spin-pairing transition of iron in siderazot (Fe3N1.2) at pressures up to 45.8 GPa at room temperature, using diamond-anvil cell techniques coupled with synchrotron X-ray emission spectroscopy. The integrated intensity of the satellite emission peak (K′β) decreases upon compression but remains unchanged at pressures greater than 30.5 GPa. In other words, the high-spin to low-spin transition of iron in Fe3N1.2 starts immediately at very low pressures and completes at ~30.5 GPa. The iron spin transition completion pressures increase with the nitrogen concentration of hexagonal close-packed iron nitrides (i.e., Fe3N1.2, Fe7N3, and Fe2N). Moreover, the identity and concentration of light elements in binary iron-rich compounds such as Fe3N, Fe3C, Fe3P, Fe3S, Fe7C3, and Fe7N3, together with their crystal structure, could affect the iron spin transition pressures. The spin transition of iron-rich alloys could alter the bonding nature and the physical properties, including the thermal and electrical conductivity, thereby influencing the thermal state and evolution of planetary interiors. [ABSTRACT FROM AUTHOR]

Hydrogen is the most abundant element in the solar system and has been considered one of the main light elements in the Earth's core. The hydrogen content in the Earth's core is determined normally by matching the volume expansion caused by the incorporation of hydrogen into FeHx to the Earth's core density deficit. The magnitude of this volume expansion at the pressure (P) and temperature (T) conditions of the Earth's core is still unknown, and the effect of spin transition in FeHx at high pressure is usually ignored. In this study, we simulate the Fe spin transition, equation of state, and hydrogen-induced volume expansion in Fe-H binaries at high P-T conditions using density functional theory (DFT) calculations. Our results indicate that hydrogen could stabilize the magnetic properties of fcc Fe from ~10 to ~40 GPa. A volume expansion induced by hydrogen is linear with pressure except at the Fe spin transition pressure, where it collapses significantly (~30%). The fcc FeH lattice is predicted to expand at an average rate of ~1.38 and 1.07 Å3 per hydrogen atom under the Earth's outer and inner core P-T conditions, where the hydrogen content is estimated to be ~0.54–1.10 wt% and ~0.10–0.22 wt%, respectively. These results suggest that the Earth's core may be a potentially large reservoir of water, with up to ~98 times as much as oceans of water being brought to the Earth's interior during its formation. Based on our predicted hydrogen content in the Earth's core, we propose that the presence of hydrogen would induce a relatively lower core temperature, ~300–500 K colder than it has been previously speculated. [ABSTRACT FROM AUTHOR]

The phase and spin transitions in single-crystal monoclinic ferrosilite, FeSiO3, were investigated using X-ray diffraction and Mössbauer spectroscopy up to lower-mantle pressures and room temperature in a helium pressure medium. Using single-crystal X-ray diffraction, we measured the equation of state of ferrosilite up to about 43 GPa. We observed a P21/c-to-C2/c phase transition between 1.5 and 1.7 GPa and a phase transition from C2/c to a distinct P21/c structure between 30 and 34 GPa. With time-domain Mössbauer spectroscopy, we determined the hyperfine parameters of ferrous iron up to 95 GPa. The phase transitions were correlated with discontinuities in Mössbauer spectral features. We observed the onset of high-spin-to-low-spin transitions in the M1 and M2 sites at ∼37 GPa and ∼74 GPa, respectively. Understanding the electronic structure of iron in a well-characterized single crystal of ferrosilite may help interpret the behavior of iron in complex dense silicate phases.

Compounds of nitrilotrimethylenephosphonic acid (NTP) with transition metals are often used as corrosion inhibitors for steels. An FeNTP anticorrosion coating is formed on the steel surface. Doping with certain metals (for example, Zn or Cd) greatly improves anticorrosion properties. Despite the fact that FeNTP, FeZnNTP, and FeCdNTP are completely isostructural compounds, substantial changes in their properties occur upon doping: (i) according to the X-ray electron spectra, Fe atoms in FeNTP are in a high-spin (HS) state, while FeZnNTP and FeCdNTP contain Fe atoms with zero spin (LS); (ii) the difference in the quadrupole splitting of the Mössbauer spectra is specific to the ratio between the HS and LS states. Quantum mechanical calculations of the FeNTP system showed two solutions with the properties coincideing with the experimentally found LS and HS states for these systems. In this study, we tested the hypothesis about the coexistence of two states. To study possible thermal redistribution between two magnetic states, the Mössbauer spectra of FeNTP, FeZnNTP, and FeCdNTP were recorded at various temperatures (77, 300, and 373 K). The Mössbauer data indicate that, with an increase in the temperature, a second component occurs in FeNTP (the ground state is HS; LS occurs already at room temperature and its fraction increases with an increase in the temperature) and FeZnNTP (the ground state is LS; HS occurs at 373 K). At all investigated temperatures, FeCdNTP has only one LS component. [ABSTRACT FROM AUTHOR]

[Display omitted] • An abrupt spin-crossover appeared at T 1/2 ↑ = 255 K and T 1/2 ↓ = 234 K. • The wider hysteresis (up to ca. 39 K) with the slower scan. • A mechanism of interplaying spin crossover and structural phase transition is described. • A tripod ligand py 3 COH would have an advantage for hysteretic SCO. A new compound (Me 4 P)[Fe(py 3 COH)(NCS) 3 ]•MeOH was prepared and structurally characterized, where py 3 COH stands for tris(2-pyridyl)methanol. After annealed at 400 K, desolvated (Me 4 P)[Fe(py 3 COH)(NCS) 3 ] displayed an abrupt spin-crossover (SCO) at T 1/2 ↑ = 255 K and T 1/2 ↓ = 234 K. Namely, 21 K-wide hysteresis was recorded at a temperature scan rate of 5 K min−1. The χ m T vs T profile was temperature-scan rate dependent; the wider hysteresis with the slower scan. The secondary SCO with T 1/2 ↑ = 275 K emerged, and a new concomitant hysteresis loop overlapped the primary one. The total hysteresis width became 39 K at a slow scan with 0.1 K min−1. A mechanism of interplaying spin crossover and structural phase transition is described. Combining a similar result on the corresponding Me 4 N+ analog, we supposed that the tripod py 3 COH ligand would have an advantage for hysteretic SCO and possibly for double SCO behavior. [ABSTRACT FROM AUTHOR]

Spin-crossover solids have been studied for many years for their promising applications as optical switches and reversible high-density memories for information storage. This study reports the effect of random metal dilution on the thermal and structural properties of a spin-crossover single crystal. The analysis is performed on a 2D rectangular lattice using an electro-elastic model. The lattice is made of sites that can switch thermally between the low-spin and high-spin states, accompanied by local volume changes. The model is solved by Monte Carlo simulations, running on the spin states and atomic positions of this compressible 2D lattice. A detailed analysis of metal dilution on the magneto-structural properties allows us to address the following issues: (i) at low dilution rates, the transition is of the first order; (ii) increasing the concentration of dopant results in a decrease in cooperativity and leads to gradual transformations above a threshold concentration, while incomplete spin transitions are obtained for big dopant sizes. The effects of the metal dilution on the spatiotemporal aspects of the spin transition along the thermal transition and on the low-temperature relaxation of the photo-induced metastable high-spin states are also studied. Significant changes in the organization of the spin states are observed for the thermal transition, where the single-domain nucleation caused by the long-range elastic interactions is replaced by a multi-droplet nucleation. As to the issue of the relaxation curves: their shape transforms from a sigmoidal shape, characteristic of strong cooperative systems, into stretched exponentials for high dilution rates, which is the signature of a disordered system. [ABSTRACT FROM AUTHOR]

The unit cell of the title compound, [FeII(C17H12BrN6O)2].2MeOH, consists of a charge-neutral complex molecule and two independent molecules of methanol. In the complex molecule, the two tridentate ligand molecules 2-[5-(3-bromo-4-methoxyphenyl)-4H-1,2,4-triazol-3-yl]-6-(1H-pyrazol-1-yl)pyridine coordinate to the FeII ion through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octahedral coordination sphere around the central ion. In the crystal, neighbouring asymmetric molecules are linked through weak C--H(pz)...π(ph) interactions into chains, which are then linked into layers by weak C-H···N/C interactions. Finally, the layers stack into a three-dimensional network linked by weak interlayer C--H...π interactions between the methoxy groups and the phenyl rings. The intermolecular contacts were quantified using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H...H 34.2%, H...C/C...H 25.2%, H...Br/Br...H 13.2%, H....N/N....H 12.2% and H·O/O...H 4.0%. The average Fe--N bond distance is 1.949 A °, indicating the lowspin state of the FeII ion. Energy framework analysis at the HF/3-21 G theory level was performed to quantify the interaction energies in the crystal structure. [ABSTRACT FROM AUTHOR]

As a result of the high symmetry of the Aea2 structure, the asymmetric unit of the title compound, [FeII(C18H15N6)2]·2MeOH, consists of half of a chargeneutral complex molecule and a discrete methanol molecule. The planar anionic tridentate ligand 2-[5-(3,4-dimethylphenyl)-4H-1,2,4-triazol-3-ato]-6-(1Hpyrazol-1-yl)pyridine coordinates the FeII ion meridionally through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudooctahedral coordination sphere of the central ion. The average Fe--N bond distance is 1.955 A °, indicating a low-spin state of the FeII ion. Neighbouring cone-shaped molecules, nested into each other, are linked through double weak C--H(pz)····(ph') interactions into mono-periodic columns, which are further linked through weak C--H···N0/C0 interactions into di-periodic layers. No interactions shorter than the sum of the van der Waals radii of the neighbouring layers are observed. Energy framework analysis at the B3LYP/6-31 G(d,p) theory level, performed to quantify the intermolecular interaction energies, reproduces the weak interlayer interactions in contrast to the strong interaction within the layers. Intermolecular contacts were quantified using Hirshfeld surface analysis and two-dimensional fingerprint plots, showing the relative contributions of the contacts to the crystal packing to be H···H 48.5%, H···C/C···H 28.9%, H···N/N···H 16.2% and C···C 2.4%. [ABSTRACT FROM AUTHOR]

Using light as a local heat source to induce a temporary pyroelectric current is widely recognized as an effective way to control the polarization of crystalline materials. In contrast, harnessing light directly to modulate the polarization of a crystal via excitation of the electronic bands remains less explored. In this study, we report an FeII spin crossover crystal that exhibits photoinduced macroscopic polarization change upon excitation by green light. When the excited crystal relaxes to the ground state, the corresponding pyroelectric current can be detected. An analysis of the structures, magnetic properties and the Mössbauer and infrared spectra of the complex, supported by calculations, revealed that the polarization change is dictated by the directional relative movement of ions during the spin transition process. The spin transition and polarization change occur simultaneously in response to light stimulus, which demonstrates the enormous potential of polar spin crossover systems in the field of optoelectronic materials. [ABSTRACT FROM AUTHOR]

The cyanidocobaltate of formula fac‐PPh4[CoIII(Me2Tp)(CN)3] ⋅ CH3CN (1) has been used as a metalloligand to prepare polynuclear magnetic complexes (Me2Tp=hydrotris(3,5‐dimethylpyrazol‐1‐yl)borate). The association of 1 with in situ prepared [FeII(bik)2(MeCN)2](OTf)2 (bik=bis(1‐methylimidazol‐2‐yl)ketone) leads to a molecular square of formula {[CoIII{(Me2Tp)}(CN)3]2[FeII(bik)2]2}(OTf)2 ⋅ 4MeCN ⋅ 2H2O (2), whereas the self‐assembly of 1 with preformed cluster [CoII2(OH2)(piv)4(Hpiv)4] in MeCN leads to the two‐dimensional network of formula {[CoII2(piv)3]2[CoIII(Me2Tp)(CN)3]2 ⋅ 2CH3CN}∞ (3). These compounds were structurally characterized via single crystal X‐ray analysis and their spectroscopic (FTIR, UV‐Vis and 59Co NMR) properties and magnetic behaviours were also investigated. Bulk magnetic susceptibility measurements reveal that 1 is diamagnetic and 3 is paramagnetic throughout the explored temperature range, whereas 2 exhibits sharp spin transition centered at ca. 292 K. Compound 2 also exhibits photomagnetic effects at low temperature, selective light irradiations allowing to promote reversibly and repeatedly low‐spin⇔high‐spin conversion. Besides, the diamagnetic nature of the Co(III) building block allows us studying these compounds by means of 59Co NMR spectroscopy. Herein, a 59Co chemical shift has been used as a magnetic probe to corroborate experimental magnetic data obtained from bulk magnetic susceptibility measurements. An influence of the magnetic state of the neighbouring atoms is observed on the 59Co NMR signals. Moreover, for the very first time, 59Co NMR technique has been successfully introduced to investigate molecular materials with distinct magnetic properties. [ABSTRACT FROM AUTHOR]

The spin state of the magnetic center for a spin-variable cation with nitrate [FeIII(3-OMe-Sal2trien)]NO3⋅H2O(I) and iodide [FeIII(3-OMe-Sal2trien)]I (II) anions is studied by Mössbauer spectroscopy on 57Fe nuclei (3-OMe-Sal2trien is the condensation product of triethylenetetramine with 3-methoxy salicylaldehyde) in the temperature ranges of 5 to 305 and 85 to 296 K, respectively. It was shown that, for both complexes, only one low-spin (S = 1/2) doublet from iron(III) ions appears in the spectra, despite significant differences (up to ~10%) in the length of the iron(III)–ligand bonds in the N4O2 coordination octahedron of the salt II compared with I, according to X-ray diffraction data at 293 K. The Debye temperatures were determined for both compounds, Θ = 157 and 153 K. [ABSTRACT FROM AUTHOR]

Magnetostructural relationships between the spin state of the FeIII ion, conformations of the ethylene bridges (-CH2-CH2-) in the five-membered chelates of the ligand, and Fe-N/O bonds were constructed due to an analysis of the literature data for 29 structures based on the [N,N′-3,6-diazaoctane-1,8-diylbis(salicylidenaldiminato)]iron(iii) [FeIII(Sal2trien)]+ (H2Sal2trien is the hexadentate ((Nam)2(Nim)2O2) Schiffbase, condensation product of triethylenetetramine and salicylaldehyde). According to the analysis results for the dependences of the Fe-N bond lengths on the fraction of the high-spin state (γHS, S = 5/2), the Fe-Nam and Fe-Nim bonds depend most strongly on the change in γHS, since the donor N atoms are characterized by high σ-overlapping with the d-orbitals of the metal ion. The conformational switch of the ethylene bridges of the ligand occurs in the range 40% < γHS < 85% at the relative elongations of the Fe-Nam and Fe-Nim bonds equal to 4.0–8.5% and 3.4–8.6%, respectively. Relationships were revealed between the γHS values and ratios of high- and low-spin conformers of the complex cation in the unit cell. The scatter of bond length values was found in the polyhedron FeN4O2 to 4% at γHS = 0% and to 5% γHS = 100%, which is caused by the effect of the crystalline environment of the complex cation. The determined features of the structural rearrangement of the hexadentate macrocyclic ligand of the Saltrien type can be used as a criterion for the estimation of γHS and development of molecular design of FeIII compounds with controlled spin transition. [ABSTRACT FROM AUTHOR]

The research progress on 3D Prussian blue analogues (PBAs) and their successful downsizing using capping ligands to achieve 2D, 1D, and even discrete molecular analogues has greatly enhanced our understanding of the switchable phenomena such as spin crossover (SCO) and electron transfer coupled spin transition (ETCST) in these systems. These molecules exhibit diverse optical and magnetic properties, making them highly attractive for next-generation compact smart devices. However, better control over switchability is necessary to fulfil specific purposes. While substantial work over the past three decades has elucidated the rationale behind ETCST occurrence in some complexes but not in others, parallel investigations into the peculiar ETCST behaviour in response to subtle changes in ligand field, reaction conditions, and guest molecule size have provided improved control over parameters like H-bonding and π⋅⋅⋅π interactions. Efforts to increase the operational temperature range for chemical, thermal, piezo, and photo-responsive switchability of these molecules to practical levels and preserve their pristine properties after integration into devices require further exploration to unlock the optimisation possibilities. This review discusses factors affecting various stimuli-responsive ETCST behaviour in both solid (ligand field strength, lattice solvents, counter-ions and crystal packing) and solution phases (solvent polarity and protonation/deprotonation) with representative examples. We envisage that a better understanding of the factors controlling ETCST properties will enable precise design and targeted synthesis of these molecules. [Display omitted] • Magnetic switchability in [Fe-Fe] and [Fe-Co] PBAs. • Electron transfer sensitivity to primary and secondary coordination environments. • Chemical modification of organic ligands for thermal and optical bistability. • Secondary interactions in tuning the redox potential of metal ions. • Understanding of magnetic and electronic properties to integrate in smart devices. [ABSTRACT FROM AUTHOR]

An isolated cyanide-bridged {FeIII 2 FeII} n one-dimensional double-zigzag chain {[Fe(Tp)(CN) 3 ] 2 Fe(btb)} n (Tp = hydrotris(pyrazolyl)borate, btb = 1,4-(1H-1,2,4-triazole)butane) has been synthesized via hydrothermal reaction. Single-crystal X-ray data, magnetic measurement, and infrared (IR) absorption spectra indicate that this complex exhibits thermal and light induced spin transition behavior in the Fe(II) sites. [Display omitted] • A cyanide-bridged {FeIII 2 FeII} n 1D double-zigzag chain has been synthesised. • This complex exhibits thermal induced spin transition in the Fe(II) sites. • Light-induced excited spin-state trapping of this complex was also observed at low temperature. A cyanide-bridged {FeIII 2 FeII} n one-dimensional chain {[Fe(Tp)(CN) 3 ] 2 Fe(btb)} n (1) (Tp = hydrotris(pyrazolyl)borate, btb = 1,4-(1H-1,2,4-triazole)butane) has been synthesized via the hydrothermal reaction of Li[Fe(Tp)(CN) 3 ], btb, and FeII(ClO 4) 2 ·6H 2 O at 120 °C. Single-crystal X-ray analysis reveals that compound 1 is an isolated neutral mixed-valence {FeIII 2 FeII} n double-zigzag chain with each [FeIII(Tp)(CN) 3 ]− unit connecting with two Fe(II) centres via two of its three CN− groups in the cis -positions, and two triazole groups from btb coordinated with Fe(II) ion in its axial positions. Single-crystal data obtained at different temperature together with magnetic data indicate that complex 1 exhibits thermal induced spin transition in the Fe(II) sites with T 1/2 ≈ 165 K. Additionally, light-induced excited spin-state trapping (LIESST) of 1 was also observed at low temperature. [ABSTRACT FROM AUTHOR]

The theoretical principles of a spin transition phenomenon in pseudo-octahedral iron(II) complexes are summarized. The intrinsic transfer of electrons between d-orbitals of a central ion under the action of external factors and the corresponding changes in the structural, spectral, and magnetic characteristics of matter are considered. Cooperative interactions that lead to the appearance of hysteresis of physical properties have been studied in detail. Perspective areas of practical applications of compounds with spin transitions have been provided. [ABSTRACT FROM AUTHOR]

There is growing interest in material candidates with properties that can be engineered beyond traditional design limits. Compositionally complex oxides (CCO), often called high entropy oxides, are excellent candidates, wherein a lattice site shares more than four cations, forming single-phase solid solutions with unique properties. However, the nature of compositional complexity in dictating properties remains unclear, with characteristics that are difficult to calculate from first principles. Here, compositional complexity is demonstrated as a tunable parameter in a spin-transition oxide semiconductor La 1- x (Nd, Sm, Gd, Y) x/4 CoO 3 , by varying the population x of rare earth cations over 0.00≤ x≤ 0.80. Across the series, increasing complexity is revealed to systematically improve crystallinity, increase the amount of electron versus hole carriers, and tune the spin transition temperature and on-off ratio. At high a population (x = 0.8), Seebeck measurements indicate a crossover from hole-majority to electron-majority conduction without the introduction of conventional electron donors, and tunable complexity is proposed as new method to dope semiconductors. First principles calculations combined with angle resolved photoemission reveal an unconventional doping mechanism of lattice distortions leading to asymmetric hole localization over electrons. Thus, tunable complexity is demonstrated as a facile knob to improve crystallinity, tune electronic transitions, and to dope semiconductors beyond traditional means., (© 2024 Wiley‐VCH GmbH.)

In this study, we investigated reversible intermolecular proton shifting (IPS) coupled with spin transition (ST) in a novel Fe II complex. The host Fe II complex and the guest carboxylic acid anion were connected by intermolecular hydrogen bonds (IHBs). We extended the intramolecular proton transfer coupled ST phenomenon to the intermolecular system. The dynamic phenomenon was confirmed by variable-temperature single-crystal X-ray diffraction, neutron crystallography, and infrared spectroscopy. The mechanism of IPS was further validated using density functional theory calculations. The discovery of IPS-coupled ST in crystalline molecular materials provides good insights into fundamental processes and promotes the design of novel multifunctional materials with tunable properties for various applications, such as optoelectronics, information storage, and molecular devices., (© 2024 Wiley-VCH GmbH.)