Pressure-induced Spin Transition Research Articles

Anomalous Pressure Response of Temperature-Induced Spin Transition and a Pressure-Induced Spin Transition in Two-Dimensional Hofmann Coordination Polymers.

Spin transition (ST) compounds have been extensively studied because of the changes in rich physicochemical properties accompanying the ST process. The study of ST mainly focuses on the temperature-induced spin transition (TIST). To further understand the ST, we explore the pressure response behavior of TIST and pressure-induced spin transition (PIST) of the 2D Hofmann-type ST compounds [Fe(Isoq)2M(CN)4] (Isoq-M) (M = Pt, Pd, Isoq = isoquinoline). The TISTs of both Isoq-Pt and Isoq-Pd compounds exhibit anomalous pressure response, where the transition temperature (T1/2) exhibits a nonlinear pressure dependence and the hysteresis width (ΔT1/2) exhibits a nonmonotonic behavior with pressure, by the synergistic influence of the intermolecular interaction and the distortion of the octahedral coordination environment. And the distortion of the octahedra under critical pressures may be the common behavior of 2D Hofmann-type ST compounds. Moreover, ΔT1/2 is increased compared with that before compression because of the partial irreversibility of structural distortion after decompression. At room temperature, both compounds exhibit completely reversible PIST. Because of the greater change in mechanical properties before and after ST, Isoq-Pt exhibits a more abrupt ST than Isoq-Pd. In addition, it is found that the hydrostatic properties of the pressure transfer medium (PTM) significantly affect the PIST due to their influence on spin-domain formation.

Pressure-Induced Mixed States Caused by Spin-Elastic Interactions during First-Order Spin Phase Transition in Spin Crossover Compounds.

Recently, the possibility of exploiting the phenomenon of spin transition (ST) has been intensively investigated; therefore, it is particularly important to study the behavior of ST under various stimuli. Here, the shape and content of the intermediate phase of ST in Hoffmann-like compounds [Fe(Fpz)2M(CN)4] (M = Pt, Pd) under external stimuli are studied. For this purpose, magnetic and Raman spectroscopy studies were carried out. In pressure-induced spin transition (PIST), a mixture of high-spin and low-spin states appears, while in temperature-induced spin transition (TIST), a homogeneous state occurs. The first-order ST induced by pressure has a hysteresis but is not abrupt. However, the temperature-induced spin transition at ambient pressure is hysteretic and abrupt. To investigate this difference, we discuss using a thermodynamic model that considers elastic interactions, showing that the slope of the hysteresis loop is related to the appearance of internal pressure, which is related to the difference in sample compressibility under high-spin and low-spin states.

Electron Density Changes across the Pressure-Induced Iron Spin Transition.

High-pressure single-crystal x-ray diffraction is used to experimentally map the electron-density distribution changes in (Fe,Mg)O as ferrous iron undergoes a pressure-induced transition from high- to low-spin states. As the bulk density and elasticity of magnesiowüstite-one of the dominant mineral phases of Earth's mantle-are affected by this electronic transition, our results have applications to geophysics as well as to validating first-principles calculations. The observed changes in diffraction intensities indicate a spin-transition-induced change in orbital occupancies of the Fe ion in general accord with crystal-field theory, illustrating the use of electron density measurements for characterizing high-pressure d-block chemistry and motivating further studies characterizing chemical bonding under pressure.

Pressure Tunable Electronic Bistability in Fe(II) Hofmann-like Two-Dimensional Coordination Polymer [Fe(Fpz)2Pt(CN)4]: A Comprehensive Experimental and Theoretical Study.

A comprehensive experimental and theoretical study of both thermal-induced spin transition (TIST) as a function of pressure and pressure-induced spin transition (PIST) at room temperature for the two-dimensional Hofmann-like SCO polymer [Fe(Fpz)2Pt(CN)4] is reported. The TIST studies at different fixed pressures have been carried out by magnetic susceptibility measurements, while PIST studies have been performed by means of powder X-ray diffraction, Raman, and visible spectroscopies. A combination of the theory of elastic interactions and numerical Monte Carlo simulations has been used for the analysis of the cooperative interactions in TIST and PIST studies. A complete (T, P) phase diagram for the compound [Fe(Fpz)2Pt(CN)4] has been constructed. The critical temperature of the spin transition follows a lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions. The analysis shows the exceptional role of the total entropy and phonon contribution in setting the temperature of the spin transition and the width of the hysteresis. The anomalous behavior of the thermal hysteresis width under pressure in [Fe(Fpz)2Pt(CN)4] is a direct consequence of a local distortion of the octahedral geometry of the Fe(II) centers for pressures higher than 0.4 GPa. Interestingly, there is not a coexistence of the high- and low-spin (HS and LS, respectively) phases in TIST experiments, while in PIST experiments, the coexistence of the HS and LS phases in the metastable region of the phase transition induced by pressure is observed for a first time in a first-order gradual spin transition with hysteresis.

Spin transition in SrFeO2 under pressure by x-ray spectroscopy

We have investigated the local Fe electronic changes in the square planar Fe(II) compound ${\mathrm{SrFeO}}_{2}$ by x-ray emission (XES) and x-ray absorption spectroscopy (XAS) through the pressure-induced spin transition up to 60 GPa. The analysis of Fe $K\ensuremath{\beta}$ XES confirms the transition under pressure from a high spin to intermediate spin state at ${P}_{c}=43$ GPa while XAS at the Fe K edge reveals the spectral signatures of the lattice compression under pressure. Simulations of the XAS spectra point to significant changes in Fe-$p$ states across ${P}_{c}$ and highlight the active role of the Sr-$p$ states at the transition.

Variable Cooperative Interactions in the Pressure and Thermally Induced Multistep Spin Transition in a Two-Dimensional Iron(II) Coordination Polymer.

Two types of experiments conducted to investigate the effect of pressure on the spin crossover (SCO) properties of the 2D Fe(II) coordination polymer formulated {Fe[bipy(ttr)2]}n are reported, namely, (1) magnetic measurements performed at variable temperature and at fixed pressure and (2) visible spectroscopy at variable pressure and fixed temperature. The magnetic experiments carried out under a hydrostatic pressure constraint of 0.04, 0.08, and 0.8 GPa reveal a two-step spin transition behavior. The characteristic critical temperatures of the spin transition are shifted upward in temperature as pressure increases. The slope of the straight-line of the Tc vs P plot, dTc/dP, is 775 K/GPa and 300 K/GPa, for the high temperature and the low temperature steps, respectively. These values are remarkably large and denote the extreme sensitivity of the material to the application of pressure. Indeed, the visible spectroscopic measurements performed at 293 K show that a complete spin transition is induced at pressures as low as 0.4 GPa. Moreover, the pressure-induced spin transition is reversible and shows an asymmetric hysteresis. An analysis of the cooperative interactions of the thermal- and pressure-induced spin transition in the framework of the model of elastic interactions reveals that the elastic energy of the lattice as well as the interaction parameter between the SCO centers change during the course of the spin transition. Consequently, the character of the spin transition varies from abrupt for the high temperature step to continuous for the low temperature step.

Pressure-induced spin transitions in garnets

Pressure-induced spin transitions in garnets

Pressure-induced spin transition and site-selective metallization in CoCl2

The interplay between spin states and metallization in compressed CoCl2 is investigated by combining diffraction, resistivity and spectroscopy techniques under high-pressure conditions and ab-initio calculations. A pressure-induced metallization along with a Co2+ high-spin (S = 3/2) to low-spin (S = 1/2) crossover transition is observed at high pressure near 70 GPa. This metallization process, which is associated with the p-d charge-transfer band gap closure, maintains the localization of 3d electrons around Co2+, demonstrating that metallization and localized Co2+ -3d low-spin magnetism can coexist prior to the full 3d-electron delocalization (Mott-Hubbard d-d breakdown) at pressures greater than 180 GPa.

Electrical Voltage Control of the Pressure-Induced Spin Transition at Room Temperature in the Microporous 3D Polymer [Fe(pz)Pt(CN)4

Fine control and direct monitoring of the spin crossover properties driven by pressure at room temperature are reported for the porous three-dimensional coordination polymer {Fe(pz)[Pt(CN)4]} by using a homemade pressure cell that transforms a DC voltage into pressure. The pressure induced spin state switching is steadily driven through a piezoelectric ceramic element, which transforms the voltage 1−4 kV in pressures in the 0.001−0.035 GPa range. At the same time, the spin state is easily monitored through changes in the optical spectra of the title compound. The results demonstrate that {Fe(pz)[Pt(CN)4]} responds to as small pressure variations as 0.001 GPa (10 atm), thereby proving its efficacy to work as an effective pressure sensor.

Effect of temperature on the pressure-induced spin transition in siderite and iron-bearing magnesite: a Raman spectroscopy study

High-pressure and high-temperature Raman spectra of synthetic FeCO 3 siderite and Mg 0.76 Fe 0.24 CO 3 magnesite were measured across the spin transition up to nearly 60 GPa and 700 K. In pure siderite the spin transition is sharp and observed between 44 and 46 GPa, with no discernible temperature dependence up to 700 K. The spin transition in Fe-bearing magnesite (“ferromagnesite”) is also sharp and takes place between 45 and 47 GPa at ambient temperature, whereas the transition pressure range broadens significantly at about 600 K (45–52 GPa). Our results suggest that the onset pressure of the spin transition in the siderite–magnesite solid solution series is independent of temperature and composition up to 700 K, whereas the broadening of the spin transition range at higher temperature is driven by the Mg content of the sample. Finally, comparison of the (Mg,Fe)CO 3 and the (Mg,Fe)O systems indicates that the onset pressure of the spin transition is temperature-independent in both cases, which is rationalized in terms of the FeO 6 octahedral compression.

Effect of Fe-site Substitution on Pressure-induced Spin Transition in SrFeO2

The effect of Fe-site substitution on structural and physical properties of the infinite layer iron oxide SrFeO2 was investigated under high pressure by 57Fe Mossbauer spectroscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray magnetic circular dichroism, and electrical resistance measurements using a diamond-anvil cell. Both 20% Mn- and Co-substituted samples exhibit spin transitions from a high-spin (S = 2) to an intermediate-spin (S = 1) state at Pc ∼ 32 GPa, which is much the same pressure 33 GPa observed in SrFeO2. This result indicates that the spin transition pressure is insensitive to the d-orbital electron counts [Mn2+ (d5), Fe2+ (d6), Co2+ (d7)], but is governed by the local structure around the Fe site.

Pressure-induced structural and spin transitions of Fe3S4

Greigite (Fe3S4), isostructural with Fe3O4 has recently attracted great scientific interests from material science to geology due to its complicated structure and electronic and magnetic configurations. Here, an investigation into the structural, magnetic and electronic properties of Fe3S4 under high pressure has been conducted by first-principle calculations based on density functional theory. The results show that a first-order phase transition of Fe3S4 would occur from the inverse spinel (SP) structure to the Cr3S4-type (CS) structure at 3.4 GPa, accompanied by a collapse of 9.7% in the volume, a redistribution of iron cations, and a half-metal to metal transition. In the CS-Fe3S4, Fe2+ located at octahedral environment firstly undergoes a transition from high-spin (HS) state to low-spin (LS) state at 8.5 GPa and Fe3+ subsequently does at 17 GPa. The Equation of State for different phases of Fe3S4 are also determined. Our results not only give some clues to explore novel materials by utilizing Fe3S4 but also shed light on the fundamental information of Fe3O4, as well as those of other SP-AB2X4 compounds.

Reduced radiative conductivity of low spin FeO6-octahedra in FeCO3 at high pressure and temperature

The ability of Earth's mantle to conduct heat by radiation is determined by optical properties of mantle phases. Optical properties of mantle minerals at high pressure are accessible through diamond anvil cell experiments, but because of the intense thermal radiation at T>1000 K such studies are limited to lower temperatures. Accordingly, radiative thermal conductivity at mantle conditions has been evaluated with the assumption of the temperature-independent optical properties. Particularly uncertain is the temperature-dependence of optical properties of lower mantle minerals across the spin transition, as the spin state itself is a strong function of temperature. Here we use laser-heated diamond anvil cells combined with a pulsed ultra-bright supercontinuum laser probe and a synchronized time-gated detector to examine optical properties of high and low spin ferrous iron at 45–73 GPa up to 1600 K in an octahedral crystallographic unit (FeO6), one of the most abundant building blocks in the mantle. Siderite (FeCO3) is used as a model for FeO6-octahedra as it contains no ferric iron and exhibits a sharp optically apparent pressure-induced spin transition at 44 GPa, simplifying data interpretation. We find that the optical absorbance of low spin FeO6 increases with temperature due to the partially lifted Laporte selection rule. The temperature-induced low-to-high spin transition, however, results in a dramatic drop in absorbance of the FeO6 unit in siderite. The absorption edge (Fe–O charge transfer) red-shifts (∼1 cm−1/K) with increasing temperature and at T>1600 K and P>70 GPa becomes the dominant absorption mechanism in the visible range, suggesting its superior role in reducing the ability of mantle minerals to conduct heat by radiation. This implies that the radiative thermal conductivity of analogous FeO6-bearing minerals such as ferropericlase, the second most abundant mineral in the Earth's lower mantle, is substantially reduced approaching the core–mantle boundary conditions.

Predicted pressure-induced spin and electronic transition in double perovskite R2CoMnO6 (R = rare-earth ion)

Specific first-principles calculations are performed to predict structural, magnetic and electronic properties of seven double perovskite R2CoMnO6 materials, with R being a rare-earth ion, under hydrostatic pressure. All these compounds are found to undergo a first-order transition from a high spin (HS) to low spin (LS) state at a critical pressure (whose value is dependent on the R ion). Such transition not only results in a significant volume collapse but also yields a dramatic change in electronic structure. More precisely, the HS-to-LS transition is accompanied by a transition from an insulator to a half-metallic state in the R2CoMnO6 compounds having the largest rare-earth ionic radius (i.e., Nd, Sm, Gd and Tb) while it induces a change from an insulator to a semiconductor having a narrow band gap for the smallest rare-earth ions (i.e., R = Dy, Ho and Er). Experiments are called for to confirm these predictions.

Optical properties of (Mg0.97, Fe0.03)O ferropericlase under the pressure of the Earth’s lower mantle

The optical-absorption and refractive-index properties of (Mg0.97, Fe0.03)O ferropericlase crystals without and with Mg and O ionic divacancy point-defect under the pressure of the Earth’s lower mantle are investigated using the first-principles calculations. Optical-absorption data show that the perfect-crystal results are similar to the predictions from the crystal-field theory:the pressure-induced spin transition of iron in ferropericlase causes a large blue-shift in its optical-absorption spectrum, leaving the near-infrared region transparent. However, when there are point defects in ferropericlase, the calculated optical-absorption results are completely inconsistent with predictions from the crystal-field theory, the spin transition causes the enhancement in the optical absorption in the near-infrared region. Refractive-index data of defect crystal indicate that the effects of pressure, wavenumber, and spin-transition on the high-pressure refractive-index of (Mg0.97, Fe0.03)O ferropericlase are obvious, but perfect-crystal results show that those effects should be relatively weak. The ～15%-20% iron-bearing ferropericlase is currently considered as an important mineral in the Earth’s lower mantle. Due to similar characteristics of the observed high-pressure optical-absorption spectrum in ferropericlase with different iron content, we suggest that:(1) the above-mentioned calculated results is conducive to the understanding of high-pressure optical properties of lower-mantle ferropericlase and the exploring of the origin of discrepancies in its high-pressure optical-absorption spectrum between experiment and crystal-field theory; (2) the high-pressure optical-absorption spectrum measurements may be a good approach for probing iron spin state.

Pressure-induced spin transition and evolution of the electronic excitations of FeBO3: Resonant inelastic x-ray scattering results

A high-pressure resonant inelastic x-ray scattering (RIXS) of at the Fe K pre-edge has been carried out to study the evolution of electronic excitations through the pressure-induced spin transition. Systematic peak shifts with insignificant peak width change are observed with increasing pressure in the high-spin state. An electronic transition occurs in tandem with the high-spin to low-spin transition, observed as the emergence of multiple new low-energy peaks in the spectra. The energy gap is reduced due to these low-energy peaks, not a peak width broadening. The observed electronic excitations are associated with dd excitations, which are calculated using a full-multiplet theory. We consider changes in crystal-field splitting and covalency to explain the observed peak shifts in the high-spin state. The new peaks that emerge upon the high-to low-spin transition are compared with dd excitations for the low-spin configuration.

Pressure and temperature induced high spin–low spin phase transition: Macroscopic and microscopic consideration

The behavior under pressure of the high spin–low spin phase transition in the coordination compounds containing 3d ions is analyzed using thermodynamic and microscopic approaches. For thermodynamic approach the mean field model with interactions between spin-crossover molecules is considered. Microscopic model takes into account the interaction of d electrons of the transition metal ions with full symmetric distortions of the ligands. The relationship of the thermodynamic interaction parameters with microscopic ones is installed and shown how the quantum–mechanical interactions form the cooperativity of the system. Within the microscopic model the temperature and pressure dependences of the high spin fraction in 2-D compounds {Fe(3-Fpy)2[M(CN)4]} (M=Pd, Pt) are simulated and microscopic parameters are evaluated. It is concluded that different experimental behaviors of the temperature and pressure induced spin transitions are determined by different variations of the inelastic and elastic energies under pressure, and vibrational component of the free energy drives the ST equally with electronic part.

Electronic Structures and Magnetism of SrFeO2 under Pressure: A First-Principles Study

We have studied the electronic structures and magnetism of SrFeO2 under pressure by first-principles calculations in the framework of density functional theory (DFT) with GGA+U and HSE06 hybrid functionals, respectively. The pressure-induced spin transition from S = 2 to S = 1 and the antiferromagnetic-ferromagnetic (AFM-FM) transition observed in experiment are well reproduced by taking the site repulsion U and its pressure dependence into account. The electronic structure and its change with the pressure can be qualitatively understood in an ionic model together with the hybridization effects between the Fe 3d and O 2p states. It is found that the pressure leads to a change in Fe 3d electronic configuration from (d(z(2)))(2)(d(xz)d(yz))(2)(d(xy))(1)(d(x(2)-y(2)))(1) under ambient conditions to (d(z(2)))(2)(d(xz)d(yz))(3)(d(xy))(1)(d(x(2)-y(2)))(0) at high pressure. As a result, the spin state transits from S = 2 to S = 1 and both the antiferromagnetic intralayer Fe-O-Fe superexchange interaction and the interlayer Fe-Fe direction exchange coupling at ambient pressure become ferromagnetic at high pressure according to the Goodenough-Kanamori (G-K) rules. Additionally, our calculations predict another spin transition from S = 1 to S = 0 at pressures of about 220 GPa.

Thermodynamic Interpretation of Pressure-Induced Spin Transitions in Thiosemicarbazonates of Iron(III)

The thermodynamic interpretation of the spin transition in iron(II) complexes taking into consideration cooperative domains is given in the literature. We use a similar interpretation for thiosemicarbazonates of aromatic aldehydes of iron(III) compounds. We extend the spin transition interpretation to pressure-induced spin transitions. The relationship between a domain size n and the interaction between a high spin and a low spin states Γ given by Gutlich et al. allows us to deduce a modi ed equation of the spin transition pressure pc (at the temperature T = Tc). On the basis of this equation, the content of the fraction of the high spin state γ and of the fraction of the low-spin state, 1− γ, as a function of pressure can be theoretically estimated. The derived equation enables us to predict the width of hysteresis which refers to the high spin and low spin states formed under the high pressure as a result of the spin transition.

On the optical properties and low pressure-induced spin transition in magnetite from ab initio calculations

The electrical anomaly observed in the high-temperature phase of magnetite under pressure is studied in the framework of DFT/LSDA+U formalism. We show that the phenomenon cannot be accounted for by a coordination crossover nor by a Fe2+ high-spin (HS) to an intermediate-spin (IS) transition. We suggest the transition may be related to the formation of an inhomogeneous mixed-spin state system HS→HS/LS with cations in octahedral sites.