Pressure-induced phase transitions of spin-crossover materials were simulated by a Monte Carlo simulation in the constant pressure ensemble for the first time. Here, as the origin of the cooperative interaction, we adopt elastic interaction among the distortions of the lattice due to the difference of the molecular sizes in different spin states, i.e., the high spin (HS) state and the low spin (LS) state. We studied how the temperature dependence of the ordering process changes with the pressure, and we obtained a standard sequence of temperature dependences that has been found in changing other parameters such as strength of the ligand field (S. Miyashita et al., Prog. Theor. Phys. \textbf{114}, 719 (2005)). Various effects of pressure on the spin-crossover ordering process are examined from a unified point of view.
Based on the Phase Hamiltonian, two types of solitons are found to exist in the crossover region between band insulator and Mott insulator in one-dimension. Both of these solitons have fractional charges but with different spins, zero and 1/2, respectively. The results are in accord with the experimental results by Kanoda et al. for TTF-Chloranil under pressure.
We study the two-step spin crossover in a double-layered elastic model based on transition metal complexes each taking high spin (HS) and low spin (LS) states. Here, only the simplest elastic interactions between adjacent molecules are considered and the system is exposed to the external pressure within the framework of $NPT$-Monte Carlo method. As a certain amount of pressure is applied, the first order thermal transition between uniform HS and LS phases transforms to a two-step transition with an emergent intermediate spin (IS) phase, where the HS and LS molecules are paired face to face bet…
We derived simple rules for the sign of superexchange interaction based on the multielectron calculations of the superexchange interaction in the transition metal oxides that are valid both below and above spin crossover under high pressure. The superexchange interaction between two cations in d$^n$ configurations is given by a sum of individual contributions related to the electron-hole virtual excitations to the different states of the d$^{n + 1}$ and d$^{n - 1}$ configurations. Using these rules, we have analyzed the sign of the superexchange interaction of a number of oxides with magnetic …
The recently discovered kagome superconductors AV3Sb5 exhibit tantalizing high-pressure phase diagrams, in which a new dome-like superconducting phase emerges under moderate pressure. However, its origin is as yet unknown. Here, we carried out the high-pressure electrical measurements up to 150 GPa, together with the high-pressure X-ray diffraction measurements and first-principles calculations on CsV3Sb5. We find the new superconducting phase to be rather robust and inherently linked to the interlayer Sb2-Sb2 interactions. The formation of Sb2-Sb2 bonds at high pressure tunes the system from …
The laws of quantum mechanics are often tested against the behaviour of the lightest element in the periodic table, hydrogen. One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H2 molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To…
Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of the Earth's lower mantle. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to non-magnetic spin state. We present a finite-temperature first-principles phase diagram of this spin crossover, finding a broad pressure range with coexisting magnetic and non-magnetic ions due to favorable enthalpy of mixing of the two. Furthermore, we find the electrical conductivity of the mineral to reach semi-metallic values inside the Earth.
The localized-itinerant nature of Ce-4$f$ valence electrons in heavy fermion compound CeIn$_{3}$ under pressure is studied thoroughly by means of the combination of density functional theory and single-site dynamical mean-field theory. The detailed evolutions of electronic structures of CeIn$_{3}$, including total and partial density of states, momentum-resolved spectral functions, and valence state histograms etc., are calculated in a wide pressure range where the corresponding volume compression $V/V_0 \in [0.6,1.0]$ (here $V_0$ is the experimental crystal volume) at $T \cong 116$ K. Upon in…
We present a first evaluation of the potential for spin crossover (SCO) compounds to be considered as a new class of giant mechanocaloric effect material. From literature data on the variation of the spin crossover temperature with pressure, we estimate the maximum available adiabatic temperature change for several compounds and the relatively low pressures that may be required to observe these effects.
Layered magnetic transition-metal thiophosphate NiPS3 has unique two-dimensional (2D) magnetic properties and electronic behavior. The electronic band structure and corresponding magnetic state are expected to sensitive to the interlayer interaction, which can be tuned by external pressure. Here, we report an insulator-metal transition accompanied with magnetism collapse during the 2D-3D crossover in structure induced by hydrostatic pressure. A two-stage phase transition from monoclinic (C2=m) to trigonal (P-31m) lattice is identified by ab initio simulation and confirmed by high-pressure XRD …
Crystal structures of $\textrm{CLi}_4$ compounds are explored through \emph{ab} \emph{initio} evolutionary methodology. Phase transition from metal to semimetal and semiconductor, and eventually to insulator with increasing pressure are revealed under pressure. Pressure-induced evolution of anti-metallization has been described quantitatively by Fermi Surface Filling Ratio and electron energy band gap using \emph{ab} \emph{initio} \emph{GW} calculations. Anti-metallization is attributed to the hybrid valence electrons and their repulsion by core electrons into the lattice interstices. Very wea…
Structural, magnetic and electrical-transport properties of α-LiFeO2, crystallizing in the rock salt structure with random distribution of Li and Fe ions, have been studied by synchrotron X-ray diffraction, 57Fe Mössbauer spectroscopy and electrical resistance measurements at pressures up to 100 GPa using diamond anvil cells. It was found that the crystal structure is stable at least to 82 GPa, though a significant change in compressibility has been observed above 50 GPa. The changes in the structural properties are found to be on a par with a sluggish Fe3+ high- to low-spin (HS-LS) transition…
Recent research suggests that the supercritical state consists of liquidlike and gaslike states where particle dynamics and key system properties are qualitatively different. We report experimental evidence of the structural crossover in supercritical neon at pressure and temperature conditions significantly exceeding the critical point values: 250$\,P_c$ and 6.6$\,T_c$. The experimental results show a crossover of the medium-range order structure evidenced by the change of the structure factor with pressure. We also observe the crossover of the short-range order structure indicated by changes…
Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo s…
X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl$_{2}$O$_{4}$ transforms to a tetragonal phase at ~18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ~45 GPa, after which a transformation to a Cmcm post-spinel phase is onset. This is attributable to pressure driven Fe-Al site inversion at room temperature, corroborated by signatures in the 57Fe Mössbauer spectroscopy pressure data. At the tetragonal to post-spinel transition, onset in the range 45-50 GPa, there is a concurrent emergence of a non-magnetic…
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 measurements 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…
MgGeO$_3$-perovskite is known to be a low-pressure analog of MgSiO$_3$-perovskite in many respects, but especially in regard to the post-perovskite transition. As such, investigation of spin state changes in Fe-bearing MgGeO$_3$ might help to clarify some aspects of this type of state change in Fe-bearing MgSiO$_3$. Using DFT+U calculations, we have investigated pressure induced spin state changes in Fe$^{2+}$ and Fe$^{3+}$ in MgGeO$_3$ perovskite and post-perovskite. Owing to the relatively larger atomic size of germanium compared to silicon, germanate phases have larger unit cell volume and …
The spin crossover of iron in Fe$^{3+}$-bearing bridgmanite, the most abundant mineral of the Earth's lower mantle, is by now a well-established phenomenon, though several aspects of this crossover remain unclear. Here we investigate effects of disorder, iron concentration, and temperature on this crossover using ab initio LDA + U$_{sc}$ calculations. The effect of concentration and disorder are addressed using complete statistical samplings of coupled substituted configurations in super-cells containing up to 80 atoms. Vibrational/thermal effects on the crossover are addressed within the quas…
High-pressure properties of MnPS$_3$ are investigated by using the hybrid functional, we report a spin-crossover pressure of 35 GPa consisting with experimental observation (30 GPa), less than half of existing report (63 GPa) using the Hubbard U correction. Interestingly, a spin-crossover induced antiferromagnetism-to-ferromagnetism transition combined with stacking-order change from monoclinic to rhombohedral are founded, and the ferromagnetism origins from the partially occupied $t_{2g}$ orbitals. Different from previous understanding, the Mott metal-insulator transition of MnPS$_3$ does not…
X-ray diffraction studies to ~90 GPa at room temperature show that Fe$_{2}$TiO$_{4}$ ferrous inverse spinel undergoes the following sequence of structural transitions : cubic (Fd3m) to tetragonal (I41/amd)to orthorhombic(Cmcm) to orthorhombic(Pmma),at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting ~25% of all Fe locations. This is manifest as a steeper volume decrease of dV/V0 ~ 3.5% beyond ~40 GPa and an emergent diamagnetic component discerned in 57Fe Mössbauer spectro…
(MgFe)O is a solid solution with ferrous iron undergoing the high to low spin state (HS-LS) crossover under high pressure. The exact state of the material in the region of the crossover is still a mystery, as domains with different spin states may coexist over a wide pressure range without changing the crystal structure neither from the symmetry nor from the atomic positions point of view. At the conditions of the crossover, (MgFe)O is a special type of microscopic disorder system. We explore the influences of (a) stress-strain relations in a diamond anvil cell, (b) time relaxation processes, …
Materials with spin-crossover (SCO) properties hold great potentials in information storage and therefore have received a lot of concerns in the recent decades. The hysteresis phenomena accompanying SCO is attributed to the intermolecular cooperativity whose underlying mechanism may have a vibronic origin. In this work, a new vibronic Ising-like model in which the elastic coupling between SCO centers is included by considering harmonic stretching and bending (SAB) interactions is proposed and solved by Monte Carlo simulations. The key parameters in the new model, $k_1$ and $k_2$, corresponding…
We report the pressure dependence of perovskite distortions in rare-earth (R) orthochromites (RCrO3) probed using Raman scattering in order to investigate the origin of structural transition from orthorhombic Pnma to rhombohedral R-3C phase in LaCrO3. The pressure induced changes in octahedral tilt modes demonstrates that tilt distortions are suppressed in LaCrO3 and are enhanced in the remaining members of RCrO3 family. This crossover between the two opposite pressure behaviors occurs at a critical R-ion radius of 1.20 Å. We attempted to establish the relation between this unusual crossover a…
Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Université, Muséum National d’Histoire Naturelle（矿物学、材料物理与宇宙化学研究所，索邦大学，国家自然历史博物馆）；ISTerre, Université Grenoble Alpes（ISTerre实验室，格勒诺布尔阿尔卑斯大学）；Laboratoire pour l’Utilisation des Lasers Intenses (LULI), Ecole Polytechnique（高强度激光利用实验室（LULI），巴黎综合理工学院）；Earth and Planetary Sciences, Stanford University（地球与行星科学系，斯坦福大学）；School of Earth and Space Exploration, Arizona State University（地球与空间探索学院，亚利桑那州立大学）；European Synchrotron Radiation Facility（欧洲同步辐射装置）
Ferropericlase (Fp), (Mg$_\mathrm{1-x}$Fe$_\mathrm{x}$)O, is the second most abundant phase in the Earths lower mantle. At relevant pressure-temperature conditions, iron in Fp undergoes a high spin (HS), S=2, to low spin (LS), S=0, state change. The nature of this phenomenon is quite well understood now, but there are still basic questions regarding the structural stability and the existence of soft phonon modes during this iron state change. General theories exist to explain the volume reduction, the significant thermo-elastic anomalies, and the broad nature of this HS-LS crossover. These the…
Pressure-Induced Low-Spin State Destabilization and Piezo-Chromic Effect in an Iron(II) Spin Crossover Complex with Pyrazol-Pyridine-Triazolate Coordination Core
Ferromagnesite (Mg,Fe)CO3, also referred to as magnesiosiderite at high iron concentration, is a solid solution of magnesite (MgCO3) and siderite (FeCO3). Ferromagnesite is believed to enter the Earth's lower mantle via subduction and is considered a major carbon carrier in the Earth's lower mantle, playing a key role in the Earth's deep carbon cycle. Experiments have shown that ferromagnesite undergoes a pressure-induced spin crossover, accompanied by volume and elastic anomalies, in the lower-mantle pressure range. In this work, we investigate thermal properties of (Mg,Fe)CO3 using first-pri…
We report first principle results for nuclear structure and optical responses of high pressure liquid hydrogen along two isotherms in the region of molecular dissociation. We employ Density Functional Theory with the vdW-DF approximation (vdW) and we benchmark the results against existing predictions from Coupling Electron-Ion Monte Carlo (CEIMC). At fixed density and temperature, we find that pressure from vdW is higher than pressure from CEIMC by about 10 GPa in the molecular insulating phase and about 20 GPa in the dissociated metallic phase. Molecules are found to be overstabilized using v…
Three high-spin phases recently discovered in the spin-crossover system Mn(taa) are identified through analysis by a combination of first-principles calculations and Monte Carlo simulation as a low-temperature Jahn-Teller ordered (solid) phase, an intermediate-temperature dynamically correlated (liquid) phase, and an uncorrelated (gas) phase. In particular, the Jahn-Teller liquid phase arises from competition between mixing with low-spin impurities, which drive the disorder, and inter-molecular strain interactions. The latter are a key factor in both the spin-crossover phase transition and the…
Supercritical fluids play a significant role in elucidating fundamental aspects of liquid matter under extreme conditions. They have been extensively studied at pressures and temperatures relevant to various industrial applications. However, much less is known about the structural behaviour of supercritical fluids and no structural crossovers have been observed in static compression experiments in any temperature and pressure ranges beyond the critical point. The structure of supercritical state is currently perceived to be uniform everywhere on the pressure-temperature phase diagram, and to c…
We use the diffusion quantum Monte Carlo to revisit the enthalpy-pressure phase diagram of the various products from the different proposed decompositions of H$_2$S at pressures above 150~GPa. Our results entails a revision of the ground-state enthalpy-pressure phase diagram. Specifically, we find that the C2/c HS$_2$ structure is persistent up to 440~GPa before undergoing a phase transition into the C2/m phase. Contrary to density functional theory, our calculations suggest that the C2/m phase of HS is more stable than the I4$_1$/amd HS structure over the whole pressure range from 150 to 400 …
We analyze in detail the electronic properties of high pressure hydrogen around the liquid-liquid phase transition based on Coupled Electron-Ion Monte Carlo calculations. Computing the off-diagonal single particle density matrix and the momentum distribution we discuss localization properties of the electrons. The abrupt changes of these distributions indicate a metal to insulator transition occurring together with the structural transition from the atomic to molecular fluid. We further discuss the electron-proton and electron-electron pair correlation functions, which also change abruptly at …
We study the low-pressure (0 to 10 GPa) phase diagram of crystalline benzene using quantum Monte Carlo (QMC) and density functional theory (DFT) methods. We consider the $Pbca$, $P4_32_12$, and $P2_1/c$ structures as the best candidates for phase I and phase II. We perform diffusion quantum Monte Carlo (DMC) calculations to obtain accurate static phase diagrams as benchmarks for modern van der Waals density functionals. We use density functional perturbation theory to compute phonon contribution in the free-energy calculations. Our DFT enthalpy-pressure phase diagram indicates that the $Pbca$ …
