Pressure-induced Charge Transfer Research Articles

Modeling of pressure-induced charge transfer character in piezoluminescent pyridylvinylanthracene crystals

External pressure alters the nature of the emissive state of piezoluminescent pyridylvinylanthracene crystals.

Pressure-induced superconductivity and phase transitions in Bi2S3 under non-hydrostatic conditions

Despite extensive and systematic studies on pressure-induced phase transitions and superconductivity in sesqui-chalcogenide A2B3 (A=Sb, Bi; BS, Se, and Te) compounds, the detailed transporting properties of Bi2S3 have rarely been reported. Here, we discovered an unambiguous correlation between superconductivity and phase transitions in Bi2S3 under non-hydrostatic conditions. The rapid decrease in resistance below 10 GPa and the occurrence of superconductivity at approximately 25.5 GPa should be caused by the two unusual pressure-induced isostructural transitions. Subsequently, the superconducting transition temperature (TC) increased to around 7.3 K, which was accompanied by the amorphization of Bi2S3 above 43 GPa. Furthermore, divergence between the theoretical calculations and experimental measurements showed that the above phase transitions and pressure-induced charge transfer had a critical effect on the electronic properties of this compound. The results of this study indicated that high pressure could help to improve thermoelectric properties.

Phase transition, mechanical and electronic properties of Ti3B4 under high pressure

We explore the crystalline structures for Ti3B4 in the 0–200 GPa pressure range via first-principles calculations based on density functional theory and advanced crystal structures prediction method using an evolutionary algorithm. The low-pressure-stable Immm phase of Ti3B4 gives way to a newly predicted orthorhombic phase (Pmmm) at 136 GPa. The B six-membered rings in the Immm phase transform to linked B zigzag chains in the Pmmm phase. Based on the electronic band structures, both Immm and Pmmm are metallic from 1 atm to 200 GPa; pressure-induced charge transfer promotes this phase transition. The simulated Vickers hardness values of Immm and Pmmm at ambient pressure are 41.5 and 34.9 GPa, respectively. The hardness of the Immm phase reduces with pressure while the opposite trend is observed for the Pmmm phase. These findings on the pressure-dependent properties of Ti3B4 yield insights into the intriguing high-pressure behaviors of the titanium-boron system.

Metallization of hydrogen by intercalating ammonium ions in metal fcc lattices at lower pressure

Metallic hydrogen is capable of showing room temperature superconductivity, but its experimental syntheses are extremely hard. Therefore, it is desirable to reduce the synthesis pressure of metallic hydrogen by adding other chemical elements. However, for most hydrides, the metallization of hydrogen by “chemical precompression” to achieve high-temperature superconductivity still requires relatively high pressure, making experimental synthesis difficult. How to achieve high-temperature superconductivity in the lower-pressure range (≤50 GPa) is a key issue for realizing practical applications of superconducting materials. Toward this end, this work proposes a strategy for inserting ammonium ions in the fcc crystal of metals. High-throughput calculations of the periodic table reveal 12 elements that can form kinetically stable and superconducting hydrides at lower pressures, where the highest superconducting transition temperatures of AlN2H8, MgN2H8, and GaN2H8 can reach up to 118, 105, and 104 K. Pressure-induced bond length changes and charge transfer reveal the physical mechanism of high-temperature superconductivity, where the H atom continuously gains electrons leading to the transition of H+ ions to atomic H, facilitating metallization of hydrogen under less extreme high pressure. Our results also reveal two strong linear scalar relationships: one is the H-atom charge vs superconducting transition temperature, and the other is the first ionization energy vs the highest superconducting transition temperature. In addition, ZnN2H8, CdN2H8, and HgN2H8 were found to be superconductors at ambient pressure, and the presence of interstitial electrons suggests that they are also electrides, whose relatively low work functions (3.03, 2.78, and 3.05 eV) imply that they can be used as catalysts for nitrogen reduction reactions as well.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Investigations of Structural, Electronic and Magnetic Properties of MnSe under High Pressure.

Properties of pressurized MnSe were investigated based on the first-principles methods using exchange–correlation functionals of the local density approximation (generalized gradient approximation) with and without the Hubbard U correction. Our results show that the Hubbard U (U = 4 eV) correction is necessary to correctly describe the phase transition behaviors of MnSe. We found that at the static condition, phase transitions from the low-temperature phase with a NiAs-type structure (P63/mmc) to the P4/nmm phase at 50.5 GPa and further to the Pnma phase at 81 GPa are observed. However, if the transition starts from the room-temperature phase with a NaCl-type structure (Fm-3m), the transition-sequences and -pressures will be different, indicating that temperature can strongly affect the phase transition behaviors of MnSe. Furthermore, we found that pressure-induced negative charge transfer will promote spin crossover. The calculated superconducting properties of the Pnma phase indicate that it may be an unconventional superconductor.

Pressure-Induced Donor–Acceptor Charge Transfer in a Fullerene Molecular Complex with Ferrocene

The pressure-induced charge transfer in a molecular complex of fullerene with ferrocene, C60·{Fe2+(η5 -C5H5)2•}2, was studied by ab initio calculations, single-crystal X-ray diffraction analysis, and Raman spectroscopy at pressures up to 5.3 GPa. The calculations based on high-pressure crystal structure data, performed by the fragment molecular orbital method and hybrid density functional level chemistry, show a pressure-dependent partial ionization of the C60 and Fe2+(η5 -C5H5)2• molecules. At pressures up to 2 GPa, the calculated charges of the two molecules nearest to the C60 ferrocenes grow equally, while at P > 2 GPa, the transferred charges are clearly different. The unequal charge transfer is due to the different orientations of the two neighboring ferrocene molecules with respect to a fullerene molecule cage. The Raman spectra of the complex show a peculiarity near 2 GPa in the pressure dependence of the frequency and bandwidth of the ferrocene intramolecular breathing mode associated with unequal charge transfer.

Pressure-induced charge transfer of Prussian blue analogues Cu0.64Mn0.86[Fe(CN)6]⋅nH2O

The Prussian blue analogue Cu0.64Mn0.86[Fe(CN)6]⋅nH2O was synthesized by coprecipitation method and pressurized to 0 MPa, 1 MPa, 20 MPa, 30 MPa, 40 MPa, respectively. The effect of pressure on its properties was investigated by X-ray diffraction and Mössbauer spectroscopy. According to XRD results, the sample has a face-centered cubic (Fm-3m) structure. As the pressure increases, the lattice parameters become slightly smaller, but no new high-pressure phase is generated. The results of the Mössbauer spectrum show that as the pressure increases, the singlet of Cu0.64Mn0.86[Fe(CN)6]⋅nH2O increases, and the doublet decreases. Its indicate that the pressure-induced charge is transferred from CuII/MnII to FeIII and forms a mixed-valence ferricyanide system.

Self-Doping Effect in FeSe Superconductor by Pressure-Induced Charge Transfer

Several unambiguous experimental observations clearly showed that the critical temperature of FeSe superconductor depends significantly on the microstructure. Experiments also showed that the critical temperature can be greatly enhanced by the application of external pressure. The present paper deals with the effect of pressure on charge transfer and self-doping properties of the superconducting compound FeSe, based on the investigation of the pressure dependence of the Fermi surface by means of first-principles methods. From the numerically evaluated electronic and crystalline properties, the pressure-induced modifications of the FeSe Fermi surface’s topology are determined. The Luttinger theorem was also used to evaluate the carrier concentration on the Fe atomic sites from the evolution of the Fermi surface. We have found that the electronic density at the Fe sites increases with the increase in external pressure, following the distortion of Fermi surface. Our simulations reveal that the pressure-induced charge transfer from Se atoms to Fe atoms in the FeSe superconductor can be interpreted as a direct correlation between the electron carrier concentration and the applied pressure. The pressure dependence of superconducting properties of FeSe can be reasonably ascribed to a self-doping effect. The predictions about the electronic density on the Fe sites (reaching a value of 0.1, where the corresponding pressure is about 8.6 GPa) are in good agreement with experimental data available in literature. The interpretation of the pressure-induced Tc enhancement in FeSe caused by the electron transfer and self-doping effect is carried out, which can be the correct approach to explain the Tc behavior under a wide type of mechanical loading.

High-pressure behavior of the crystal structure of the fullerene molecular complex with ferrocene C60·{Fe(C5H5)2}2

The crystal structure of the molecular complex C60·{Fe(C5H5)2}2 was studied by single crystal X-ray diffraction (XRD) analysis at pressures up to 5 GPa using the diamond anvil cell (DAC) technique. The XRD data and subsequent structural analysis clearly show that there is no pressure-induced polymerization in fullerene layers in pressure range studied. The reciprocal unit cell volume V0/V is a smooth and monotonous function of pressure and fits well to the Murnaghan equation of state (V0/V) B'= {1 + P·(B'/B0)}, where V0 is the volume at ambient pressure, B0=8.7 GPa and B'=10.5 are the bulk modulus and its derivative, respectively. Pressure dependence of the shortest distance between the C5H5 ring of the ferrocene molecule and the center of the nearest fullerene molecule is linear, while the slope changes at 2.2 GPa indicating on the intermolecular interactions crossover. Other relevant parameter, the Fe-C bond lengths of ferrocene, sensitive to iron charge state, gradually increases. This peculiarity can a sign of pressure-induced partial charge transfer between the donor ferrocene and acceptor fullerene molecules.

Self-Doping Effect in FeSe Superconductor by Pressure-Induced Charge Transfer

Self-Doping Effect in FeSe Superconductor by Pressure-Induced Charge Transfer

Mechanical and chemical pressure effects on the AeFe2As2 (Ae = Ba, Sr, Ca) compounds: Density functional theory

We have studied the pressure-induced structural, magnetic and electronic properties of AeFe2As2 (Ae = Ba, Sr, Ca) compounds in the framework of density functional theory within the GGA-PBE method. The effects of chemical pressure generated by Sr and Ca substitutions in BaFe2As2 have been investigated. We have found a magnetic transition at the same primitive unit cell volume, around 81 Å3 for the (Ba⧹Ca)Fe2As2 compounds, which predicts a magnetic transition pressure of 12 GPa for SrFe2As2. The structural parameters of FeAs4 tetrahedra are obtained after ionic relaxation and compared with the existing experimental results. The change of these internal parameters is ascribed to pressure-induced charge transfer, i.e., as the As-Ae bonds absorb electrons, the Fe-As bonds are depleted, and thus the number of holes in the Fe-As layers increases. The changes of the density of states, the band structure and the hole- and electron- pockets of Fermi surface by pressure have been discussed.

Pressure-induced isostructural phase transition and charge transfer in superconducting FeSe

We present extensive investigations of the crystallographic phase diagram and electronic properties of the Fe-based superconductor FeSe under extreme conditions (high pressure (HP) and low temperature (LT)) by synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). An isostructural phase transition (Tetragonal (T) →high-pressure Tetragonal (Tʹ)) is discovered in FeSe at ∼2.8 GPa based on the axial ratio c/a with finer pressure step as observed in Fe-As-based superconductor such as EuFe2As2. We also find a pressure-induced Tʹ → MnP-type phase transition at 7.6 GPa in FeSe, which is consistent with the documented pressure-induced high-spin → low-spin transition (∼6–7 GPa). These results reveal the pressure-induced structural phase transition sequence in FeSe at room temperature to be T → Tʹ → Tʹ+MnP-type at pressures of 0–10.6 GPa, enriching the crystallographic phase diagram. The HPLT XRD data also indicate that a sluggish structural phase transition (Cmma → Pnma) begins at 7.5 GPa, and these two phases coexist up to 26.5 GPa. The HP X-ray absorption near-edge spectroscopy (XANES) measurement shows that Eo of Se experiences a pressure-induced shift to high energy, evidencing strongly charge transfer between Fe and Se under high pressure. Our results shed lights on the correlation between crystallographic/electronic structure and superconductivity in this material.

Effects of pressure on the structural and electronic properties of linear carbon chains encapsulated in double wall carbon nanotubes

High-pressure resonance Raman experiments were performed on linear carbon chains Cn confined in double-wall carbon nanotubes (Cn@DWCNT). Our results indicate that the frequency of the chain mode undergoes a non-linear decreasing as pressure increases, in agreement with previous studies of the behavior of carbon chains into multi-wall carbon nanotubes. After the pressure cycle, the Cn modes present an irreversible frequency downshift together with an irreversible upshift (downshift) of the RBM modes (G+ band) of the inner tube. These spectral changes are interpreted as evidence of cross-linking between the carbon chain and the wall of the inner tube. Density functional theory (DFT) calculations were performed in order to shed light on the interpretation of the experimental data. The theoretical calculations show a pressure-induced charge transfer from the nanotube to the carbon chain, thus resulting in an increasing of the C≡C bond length, and consequently, a softening frequency of the Cn Raman mode. Furthermore, an irreversible cross-linking between the tube and Cn is observed when the maximum applied pressure is higher than the pressure of radial tube collapse is predicted by modelling, thus supporting the experimental results.

New superconducting materials discovered via high pressures technology

In this study, we primarily select the author′s research to briefly introduce the various superconducting materials that are synthesized using high pressures. Superconducting systems included are ranging from cuprates, iron-based materials, to topological compounds. They are fabricated using: ″self-high-pressure oxidization″, which is produced using the chemical composition of the synthesized material itself rather than using traditional alien oxidizers; ″inner pressure effects″, which are generated using the atomic size changes; and a ″pure doping mechanism″ that is induced using a pressure-induced charge transfer, respectively. We focus on three categories of superconductors in our investigation. (1) Apical oxygen ordered cuprate superconductors that reach the highest T c for the monolayered cuprate superconductors, Cu12( n −1) n homologues series cuprate superconductors having a T c of up to 123 K, and a Cl2( n −1) n homologues series cuprate superconductor that is the first halogen-containing superconducting system having a T c higher than liquid nitrogen: These cuprate superconductors comprise only copper and alkaline earth metal oxides, which are the simplest reagents that are necessary to form high T c cuprates. However, those high pressure synthesized materials exhibit very high T c that usually can only be realized in cuprates superconductors with complex compositions if prepared at ambient condition such as YBCO123, Bi22( n −1) n , or Hg12( n −1) n . The new superconductors exhibit excellent critical electrical current density as a function of temperature, especially at high temperatures. They show superior properties than the Bi/Hg cuprate homologues superconductors and are comparable to the classical Y123. (2) The ″111″ iron-based superconducting system: This system features neutral-charged cleaved fresh surfaces that are perfect for experimental studies based on angle resolved photoemission electron spectroscopy (ARPES) or scanning transmission microscopy (STM). Compared with other important iron-based superconductors, this unique feature facilitates the unambiguous detection of intrinsic properties without surface reconstruction. Studies on LiFeAs, which are representative members of the ″111″ system, reveal the non-Fermi surface nesting characteristics that are in sharp contrast to the observations in previous iron-based superconducting systems. This study indicates that Fermi surface nesting is not necessary for the generation of superconductivity in iron-based materials. This is a critical step to understand the mechanisms of iron-based superconductors. (3) Pressure-induced superconductivity in the topological compound Bi2Te3: this is the first superconductor in a topological compound generated by applying pressure and without using chemical doping.

Pressure induced self-doping and dependence of critical temperature in stoichiometry YBa2Cu3O6.95 predicted by first-principle and BVS calculations

This paper deals with the pressure effect on self-doping and critical temperature in optimum oxygen stoichiometry YBa2Cu3O6.95 of high temperature superconductor (HTS) based on a numerical study combined the first-principle with bond valence sum (BVS) calculations. The microscopic electronic properties and equilibrium ionic position configurations in the superconductor under external pressure are firstly calculated using the first-principle method. The results show that the apex oxygen in the cuprate superconductor shifts towards CuO2 plane due to pressure effect, and the minimum buckling angle of CuO2 plane is correlated with the maximum critical temperature. A BVS formalism is then utilized for evaluating the valences of all ions in the superconductor on the basis of the electronic and ionic properties and the hole concentration in both CuO2 plane and Cu-O chain are deduced. It demonstrates that the pressure-induced charge redistribution leads to a self-doping process of the hole-transfer into CuO2 plane from both Cu-O chain and Y site in the cuprate superconductor, which is the dominant mechanism of pressure effect on the superconductive properties. In order to quantitatively predict critical temperature profile of YBa2Cu3O6.95 under pressure, a modified formula describing pressure-induced charge transfer taking into account pressure dependence of the optimum hole concentration is developed. The predicted results exhibit good agreements with the experimental data in the literature, and the model parameters on the critical characteristics of the superconductor are discussed in details.

An Electrically Conductive Single-Component Donor–Acceptor–Donor Aggregate with Hydrogen-Bonding Lattice

An electrically conductive D-A-D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex 1H2). The crystal structure of complex 1H2 has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex 1H2 is semiconducting (Ea = 0.29 eV, ρrt = 9.1 × 104 Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (Ea = 0.13 eV, ρrt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π-π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D-A-D aggregate.

Pressure‐Induced Charge Transfer Doping of Monolayer Graphene/MoS2 Heterostructure

A unique way of achieving controllable, pressure-induced charge transfer doping in the graphene/MoS2 heterostructure is proposed. The charge transfer causes an upward shift in the Dirac point with respect to Fermi level at a rate of 15.7 meV GPa(-1) as a function of applied hydrostatic pressure, leading to heavy p-type doping in graphene. The doping was confirmed by I2D /IG measurements.

First-principles study on the electronic structure, phonons and optical properties of LaB6 under high-pressure

The electronic structure, phonons and optical properties of LaB6 compound under different pressure have been studied by first-principles calculation. The electronic structure calculation shows that the d band along the M-Γ direction of the Brillouin zone moves up with increasing pressure and the band minimum is above the Fermi level at 45 GPa. The pressure-induced charge transfer from La to B atoms is reflected in the upshift of d band along the M-Γ direction with pressure. The calculated phonon dispersion curve at zero pressure is in good agreement with the experimental results. However, the phonon dispersion under high pressure does not show any information about the phase transition at 10 GPa, which was reported previously. The acoustic and optical phonon modes harden all the way with increasing pressure. In addition, the dielectric function is in accordance with the Drude model in the pressure range of 0 GPa–35 GPa and follows the Lorentz model at 45 GPa. The LaB6 compound exhibits better visible light transmittance performance with the increasing pressure in the range of 0 GPa–35 GPa and visible light transmittance peak would be shifted towards ultraviolet region.

Predicted Formation of H3(+) in Solid Halogen Polyhydrides at High Pressures.

The structures of compressed halogen polyhydrides HnX (X = F, Cl and n > 1) and their evolution under pressure are studied using ab initio calculation based on density functional theory. HnF (n > 1) are metastable up to 300 GPa, whereas for HnCl (n > 1), four new stoichiometries (H2Cl, H3Cl, H5Cl, and H7Cl) are predicted to be stable at high pressures. Interestingly, triangular H3(+) species are unexpectedly found in stoichiometries H2F with [H3](+)[HF2](-), H3F with [H3](+)[F](-), H5F with [H3](+)[HF2](-)[H2]3, and H5Cl with [H3](+)[Cl](-)[H2] above 100 GPa. Importantly, formation processes of H3(+) species are clearly seen on the basis of comparing bond lengths, bond overlap populations, electron localization functions, and Bader charges as a functions of pressure. Further analysis reveals that the formation of H3(+) species is attributed to the pressure-induced charge transfer from hydrogen atoms to halogen atoms.