Studied Pressure Range Research Articles

Structural stability, anisotropic elasticities and electronic structure of η-MgZn2 under pressures: A first-principle investigation

The effects of pressure on structural stability, anisotropic elasticities and electronic properties of η-MgZn2 phase were systematically investigated. The smooth variations of lattice parameters a/a0, c/c0 and formation enthalpy ΔH with increasing pressure suggest no structure transition occurred within the studied pressure range, and the decreasing value of ΔH indicate the better thermodynamical stability at high pressure. It is found the elastic moduli are enhanced by increasing pressure, and the large values of both B/G and Poisson's ratio indicate the ductility of η phase. Besides, the elastic anisotropies that result from the distinct atomic arrangements along different orientations are pronounced when applied to compression. Moreover, as the pressure increases, not only the chemical bonds in η exhibit stronger covalency and ionicity, but also the bonding strength in different directions increase to vary extents. This can explain the higher stability and significant elastic anisotropies of η phase at high pressure.

Pressure dependent structural, elastic and mechanical properties with ground state electronic and optical properties of half-metallic Heusler compounds Cr2YAl (Y=Mn, Co): first-principles study

Hydrostatic pressure dependent structural, electronic, elastic and optical properties of Heusler compounds Cr2YAl (Y=Mn, Co) are investigated by the first-principles calculations. Present study shows good agreement between the calculated and experimental values of lattice parameters of these compounds. The gradual decrease of lattice parameter and unit cell volume with the increase of pressure eliminates the possibility of phase transition up to the considered pressure range 0–30 GPa. The absence of negative frequency in the phonon dispersion curves at various external pressures up to 30 GPa confirm the dynamical stability of these compounds. The values of Pugh's ratio, Poisson's ratio and Cauchy pressure indicate the ductility of Cr2CoAl while those criteria confirm the brittleness of Cr2MnAl. Pressure dependent behaviour of the elastic constants of these compounds shows the mechanical stability over the studied pressure range (0–30 GPa). Crystal stiffening of both the compounds is indicated by the increase of Debye temperature with increasing hydrostatic pressure. The pressure dependent band structure and density of states (DOS) calculations reveal the half- and the near half-metallic behaviour of Cr2MnAl and Cr2CoAl, respectively up to 20 GPa while those disappears at pressure around 30 GPa. The variations of optical constants with pressure is consistent with the results of the electronic structure calculations. High reflectivity (>45%) of both the compounds makes them attractive for optoelectronic device applications.

First-principles calculations of phase transition, elasticity, phonon spectra, and thermodynamic properties for CeO2 polymorphs

In this work, the structural transformation, elastic behavior, phonon spectra and thermodynamic properties of CeO2 under compressive and tensile loading were investigated by first-principles calculations based on density generalized function theory (DFT). Our calculated structural parameters and transition pressure under compressive condition are in good agreement with the experimental values. Under tensile condition, the phase transitions of CeO2 from fluorite-type (space group Fm3¯m) phase to cotunnite-type (space group Pnma), rutile-type (space group P 42/mnm) and marcasite-type (space group Pnnm) phases were predicted to occur at 23.2 ​GPa, −7.69 ​GPa, and −7.75 ​GPa respectively. The mechanical stability criteria are verified in terms of elastic constants and shows that the cotunnite and fluorite structures of CeO2 are mechanically stable in the studied pressure range, while the rutile and marcasite structures turn out to be mechanical unstable under high pressure. The calculated phonon spectra of CeO2 polymorphs demonstrate that all phases considered in present work are dynamically stable under ambient pressure. Furthermore, the calculated heat capacity, isothermal bulk modulus, Gibbs free energy, thermally expanding coefficient of CeO2 polymorphs as a function of temperature are discussed and compared with available experimental data.

Density functional prediction of the structural, elastic, electronic, and thermodynamic properties of the cubic and hexagonal (c, h)-Fe2Hf

The structural, elastic, electrical, and thermodynamic characteristics of Fe2Hf cubic and hexagonal phases with space group Fd-3m and P63/mmc are presented using the generalized gradient approximations. The k-points mesh density and plane-wave energy cut-off accomplish the energy convergence. The computed equilibrium parameters are closer to the theoretical data. The elastic tensor and crystal anisotropy of ultra-incompressible Fe2Hf are computed in a wide pressure range. The isothermal and adiabatic bulk modulus, as well as the heat capacity of Fe2Hf is successfully calculated utilizing the quasi-harmonic Debye Model. The Fd-3m and P63/mmc Fe2Hf structures are stable in the studied pressure range.

Adsorption behaviors of near-critical carbon dioxide on organic-rich shales: Modeling, multifractality, and kinetics

Studies on the near-critical carbon dioxide (NC-CO2) adsorption behavior of organic-rich shales are important for shallow shale gas exploration and hydrocarbon recovery enhancement of liquid CO2 fracturing, which further contributes to geological CO2 sequestration and global carbon neutrality. We performed CO2 isothermal adsorption and kinetic experiments on six organic-rich shale samples at a temperature of 303.15 K and pressures of up to 5 MPa to investigate the modeling, multifractality, and kinetics of NC-CO2 adsorption behavior on shales. The equilibrium data were fitted to the Langmuir, Freundlich, BET, modified BET (M−BET), DA, DR, and OK adsorption models; the multifractality of the isotherms was analyzed; and pseudo-first-order, pseudo-second-order, Elovich, and Avrami's fractional-order models were adopted to analyze the adsorption kinetics. Furthermore, an error analysis technique was employed to compare the accuracies of various models for fitting the experimental data. The results showed that various adsorption models presented different fitting effects; however, the M−BET, Freundlich, and DA adsorption models performed better over the entire studied pressure range. The fitting results of the M−BET model indicated that the number of adsorption layers ranged from 2.528 to 4.663 and had a multilayer adsorption mechanism. The multifractality of the adsorption behavior was described by the generalized fractal dimension and multifractal singularity spectra; we demonstrated that some multifractal parameters characterize the NC-CO2 adsorption behavior on shales well. Additionally, the TOC, clay, and quartz contents show close covariations with the isothermal adsorption and multifractal parameters. Moreover, the adsorption process was TOC-dependent and subject to Avrami's fractional-order model. Finally, we propose conceptual three-stage adsorption and geological storage models to understand the nature of the NC-CO2 adsorption behavior of shales.

Pressure-induced two magnetic collapses in the ferromagnetic L12-Fe3Pd alloy and related elasticity and lattice dynamics anomalies

Pressure effect on the magnetic, mechanical and lattice dynamical properties of the ferromagnetic L12-Fe3Pd crystalline alloy was investigated by first-principles calculations. The calculated properties under ambient pressure are in good agreement with available theoretical results. A two-step pressure-induced collapse of magnetic moment was observed, accompanied by the collapse of the volume. The first one is at about 34.2 GPa, and the other is at about 67 GPa. While the magnetic moment disappears around 84 GPa. Thelattice dynamics calculations under high pressure revealed that the dynamically stable and unstable cases appear alternately. The system is dynamically unstable at low pressures due to the spontaneous magnetization of the system, and then the system is dynamically stable in the pressure range of 33.5 to 66.9 GPa, and this pressure range is between the first collapse pressure and second collapse pressure. The system becomes dynamically unstable again at pressure ranging from 67 to 83.7GPa, and afterwards becomes stable. Elastic constants and their dependence on pressure and related mechanical properties were investigated. Stability criteria show that ferromagnetic L12-Fe3Pd crystalline alloy is mechanically stable over the entire studied pressure range. The bulk modulus exhibits pronounced softening with pressure, and present minimum values at magnetic collapse pressures. The mechanism of elastic softening was analyzed with the magnetoelastic force. From Pugh’s ratio and Poisson’s ratio, the system exhibits brittleness at the vicinity of magnetic collapse pressures. The linear thermal expansion coefficient was calculated based on the quasi-harmonic approximation. The results reveal that the alloy does notexhibit thermal Invar behavior under zero pressure and high pressure. Finally, a high-anisotropy tetragonal structurewith space group symmetry P4/mbm was obtained by soft-mode phase transition theory, and this structure is dynamically and mechanically stable at zero pressure. Theoretical prediction suggested that the P4/mbm structure is energetically favorable than the previously reported structures at the pressure below 32 GPa and also showed that above 32 GPa the hexagonal D019 structure becomes the energetically most stable structure.

Equations of state of new boron-rich selenides B6Se and B12Se

ABSTRACT Two novels boron-rich selenides, orthorhombic B6Se and rhombohedral B12Se, have been recently synthesized at high pressures and high temperatures. Room-temperature compressibilities of these phases were studied in a diamond anvil cell using synchrotron powder X-ray diffraction. A fit of experimental p–V data by the third-order Birch–Murnaghan equation of state yielded the bulk moduli of 155(2) GPa for B12Se and 144(3) GPa for B6Se. No pressure-induced phase transitions have been observed in the studied pressure range, that is, up to 35 GPa.

Nanodiamonds characterization and application as a burning rate modifier for solid propellants

A nanodiamond powder (n-D) synthesized by detonation method was characterized in detail, and the combustion of solid propellants contained 10 wt. % of n-D as a burning rate modifier was investigated. n-D characterization was implemented by XRD, SEM, DSC, and size distribution analyses. Specific surface area analysis was performed using BET-method. The size of primary nanodiamond particles has not changed after acid treatment and drying, but the particles' agglomeration decreased the value of a specific surface area. The effect of n-D modification on the burning rate of solid propellants with an active and inert binder was investigated. The addition of the n-D to propellants formulation instead of carbon black powder has significantly increased their burning rate (by at least 50 %) in the studied pressure range. The observed trend was associated with the high chemical reactivity of n-D and its extremely high value of specific surface area.

The investigation of OH radicals produced in a DC glow discharge by laser-induced fluorescence spectrometry

In this paper the OH radicals produced by a needle–plate negative DC discharge in water vapor, N2 + H2O mixture gas and He + H2O mixture gas are investigated by a laser-induced fluorescence (LIF) system. With a ballast resistor in the circuit, the discharge current is limited and the discharges remain in glow. The OH rotation temperature is obtained from fluorescence rotational branch fitting, and is about 350 K in pure water vapor. The effects of the discharge current and gas pressure on the production and quenching processes of OH radicals are investigated. The results show that in water vapor and He + H2O mixture gas the fluorescence intensity of OH stays nearly constant with increasing discharge current, and in N2 + H2O mixture gas the fluorescence intensity of OH increases with increasing discharge current. In water vapor and N2 + H2O mixture gas the fluorescence intensity of OH decreases with increasing gas pressure in the studied pressure range, and in He + H2O mixture gas the fluorescence intensity of OH shows a maximum value within the studied gas pressure range. The physicochemical reactions between electrons, radicals, ground and metastable molecules are discussed. The results in this work contribute to the optimization of plasma reactivity and the establishment of a molecule reaction dynamics model.

Phase Relations in the Ni–S System at High Pressures from ab Initio Computations

Based on the ab initio calculations within the density functional theory and crystal structure prediction algorithms, the structure and stability of compounds in the Ni–S system at pressures of 100–400 GPa were determined. As a result, a homologous series of discrete compounds (Ni and S) consisting of Ni14S-C2/m, Ni13S-R3̅, Ni12S-R3̅, Ni5S-C2/m, Ni4S-P1̅, and Ni3S-Cmcm is revealed. We also confirmed the absence of the stable Fe-bearing compounds between Fe and Fe2S in the studied pressure range. At the Earth’s core pressures, 4 wt % of sulfur can be dissolved in solid fcc-Ni without deformation of the structure. Significant deformations in the Ni structure occur at sulfur contents from 4 to 15 wt %. In contrast, up to 0.45 wt % of sulfur could be dissolved in hcp-Fe at 350 GPa and 0 K. For Ni3S, two phases with space groups I4̅ and Cmcm were predicted. Ni3S-I4̅ is stable at least from 100 GPa, whereas above 330 GPa, it transforms into Ni3S-Cmcm. The pressure of phase transition is almost independent of temperature. The Ni2S is stable in the entire pressure range and undergoes a single-phase transition from the Pnma- to P6̅2m-phase at 266 GPa and 0 K with a Clapeyron slope of 5 MPa/K. The S-rich sulfide NiS3 is characterized by Im3̅m symmetry and is thermodynamically stable from 100 to 318 GPa. Our new data on Ni sulfides might be important to constrain detailed thermodynamic models for Fe–Ni-bearing Earth and planetary cores.

Highly sensitive luminescent pressure sensor for vacuum measurement based on Pr3+:TeO2-ZnO-Na2O-La2O3 glasses

In this paper, the initial testing of a novel non-contact optical sensor is presented for vacuum measurement. Under the excitation of 445 nm diode laser, intense green and red emission of Pr3+ ion is observed. A drastic pressure dependence of the absolute luminescence intensity and the luminescence intensity ratio of the 3P0→3H4 and 3P0→3F2 transitions are achieved between 1.47 and 997 mbar pressure at room temperature. In the studied pressure range the absolute luminescence intensity depends linearly on pressure. The strong dependence of luminescence on pressure suggests that the title materials are potentially applicable as an optical sensor for vacuum measurement.

Spin Transitions and Compressibility of ε‐Fe7N3 and γ′‐Fe4N: Implications for Iron Alloys in Terrestrial Planet Cores

AbstractIron nitrides are possible constituents of the cores of Earth and other terrestrial planets. Pressure‐induced magnetic changes in iron nitrides and effects on compressibility remain poorly understood. Here we report synchrotron X‐ray emission spectroscopy (XES) and X‐ray diffraction (XRD) results for ε‐Fe7N3 and γ′‐Fe4N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe7N3 and γ′‐Fe4N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe7N3 by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe4N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yields V0 = 83.29 ± 0.03 (Å3), K0 = 232 ± 9 GPa, K0′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe7N3 above the spin transition completion pressure, and V0 = 54.82 ± 0.02 (Å3), K0 = 152 ± 2 GPa, K0′ = 4.0 ± 0.1 for γ′‐Fe4N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe3S, Fe3P, Fe7C3, and Fe3C based on previous XES and XRD measurements, we located the completion of high‐ to low‐spin transition at ~67, 38, 50, and 30 GPa at 300 K, respectively. The completion of spin transitions of Fe3S, Fe3P, and Fe3C induces elastic stiffening, whereas that of Fe7C3 induces elastic softening. Changes in compressibility at completion of spin transitions in iron‐light element alloys may influence the properties of Earth's and planetary cores.

Experimental equation of state of 11 lanthanide nitrides (NdN to LuN) and pressure induced phase transitions in NdN, SmN, EuN, and GdN

Through an extensive data analysis of powder X-ray diffraction data obtained at pressures up to at least 78 GPa, we report the experimental equations of state for all lanthanide nitrides between NdN and LuN, excluding the radioactive Pm. By fitting the obtained unit cell volumes as a function of pressure with the third order Birch–Murnaghan equation of state, we find that the bulk modulus increases with an increasing lanthanide number from K0 = 146(12) GPa for NdN to 182(7) GPa in EuN. Hereafter, the bulk modulus reaches a plateau for the rest of the series except for TmN which has a lower bulk modulus. We find that the first derivative of the bulk modulus is around 4 for all compounds except TbN, which displays a significantly different compression behavior. In addition, we find a B1 to B10 pressure-induced phase transition in NdN, SmN, EuN, and GdN at increasingly higher pressures. In fact, we observe that the onset pressure of the phase transition increases linearly with Ln atomic number. From TbN and onwards, we do not observe any sign of a B1 to B10 transition indicating that the transition pressure exceeds the studied pressure range. Therefore, we believe that, for the heavier lanthanides, the linear relationship between the onset pressure and the lanthanide number does not hold and even higher pressures are needed to observe the transition. This coherent study of the series of lanthanide nitrides offers a unique opportunity for benchmark studies of computational methods applied to compounds with 4f electrons.

Sensor Comparison for Corona Discharge Detection Under Low Pressure Conditions

Low pressure environments, situate insulation systems in a challenging position since partial discharges (PDs), corona and arc tracking are more likely to develop. Therefore, specific solutions are required to detect such harmful phenomena before major failure occurrence. This paper deals with three low-cost and small-size sensing methods, i.e., a single loop antenna, a visible-UV imaging sensor and the measurement of the leakage current to detect corona in the early stage, thus anticipating the appearance of severer effects such as arc tracking or disruptive breakdown. The three studied methods can be applied for an on-line monitoring of corona activity under low pressure environments, thus being compatible with predictive maintenance approaches. This on-line monitoring can be used to develop improved electrical protection devices able to detect such effects in an initial stage, thus improving current solutions which are unable to do so. All three studied sensors give consistent linear responses within the studied pressure range, i.e., 10-100 kPa, with almost no drift. The sensitivity of the visible-UV imaging sensor is slightly lower than that of the others, but it has the advantage of directly locating the discharge points. Results presented in this paper can be very useful for the more electrical aircraft (MEA), which is forcing electrical distribution systems to operate at higher voltage levels. Due to the little experience and scarcity of published data, the experimental results presented in this paper can be valuable for a better understanding of the combined action of high voltage and low pressure environments.

High-Pressure Structural and Electronic Properties of Potassium-Based Green Primary Explosives

Recently synthesized green primary explosives potassium 4,4′-bis(dinitromethyl)-3,3′-azofurazanate (K2BDAF) and potassium 1,1′-dinitramino-5,5′-bistetrazolate (K2DNABT) offer fast, powerful initiation capacity and high-performance detonation characteristics to replace the long-standing toxic primary explosives. In the present study, we report the structural and electronic properties of the emerging green primary explosives K2BDAF and K2DNABT under hydrostatic pressure up to 10 GPa. We observed that dispersion correction methods are important for capturing weak intermolecular interactions in order to accurately describe the geometries of the primary explosives. The computed ground state structural properties using optB86b-vdW method are in good agreement with the experimental results. The pressure-dependent lattice constants and bond parameters show anisotropic nature and also a sharp discontinuity around 4–5 GPa. The obtained equilibrium bulk modulus shows that K2BDAF is softer than K2DNABT. Bonding analysis revealed that the C-NO2 energetic functional groups are more sensitive than the stable ring structure under hydrostatic pressure. The calculated electronic structure of K2BDAF using the Tran–Blaha-modified Becke–Johnson (TB-mBJ) potential shows a direct-to-indirect band gap transition around 5 GPa, which is consistent with the aforementioned discontinuity of the structure, while K2DNABT is found to be a direct-band-gap insulator in the studied pressure range of 1–10 GPa. The abnormal trends in the structural and electronic properties suggest that K2BDAF may undergo a structural transition/distortion around 4–5 GPa of pressure.

Potential Interaction of Noble Gas Atoms and Anionic Electrons in Ca2N

Noble gas (NG) is often used as a hydrostatic pressure-transmitting medium in high-pressure experiments. However, the NG atoms under compression are found to be readily trapped in the voids of some ionic compounds. Electrides usually have large open frameworks and show strong hydrogen and oxygen affinities. To investigate the interaction of light NG with electrides under pressure, we perform the structural investigations of Ca2N in a diamond anvil cell under different NG (He, Ne, and Ar) conditions, assisted by density functional theory calculations. Experimental results find that in comparison with nonhydrostatic pressure, transition paths of Ca2N change with different pressure media and phase transition pressure is reduced significantly, and NG is chemically inert for electrides in the studied pressure range. Theoretical analysis indicates that the anionic electrons in Ca2N electride show strong repulsion with NG, preventing NG atoms from entering electrides. Such repulsion is due to the almost neutral character of noble atoms in electrides, which makes it difficult for them to interact with anionic electrons. Moreover, in the predicted metastable Ca2N–NG, the neutral features of NG atoms result in the reservation of the intrinsic electride character. Our results reveal that the NGs can be used as protective gases for electrides under ambient conditions or hydrostatic pressure-transmitting media for the studies of high-pressure electrides within 50 GPa.

Compressibility of hingganite-(Y): high-pressure single crystal X-ray diffraction study

Behaviour of hingganite-(Y), Y2□Be2Si2O8(OH)2, on compression to 47 GPa has been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction at room temperature in a diamond anvil cell. In the studied pressure range no obvious phase transitions have been observed. The compression of hingganite-(Y) crystal structure is anisotropic, with b axis showing the maximal compressibility. A fit of the experimental pressure–volume data by the Birch-Murnaghan third-order equation of state yielded the bulk modulus of 131(2) GPa and its pressure first derivative of 3.5(2). The difference between high-pressure behaviour of hingganite-(Y) and structurally related datolite is governed by the different chemical nature of interlayer cations.

Pressure Induced Stability Enhancement of Cubic Nanostructured CeO2 †.

Ceria (CeO2)-based materials are widely used in applications such as catalysis, fuel cells and oxygen sensors. Its cubic fluorite structure with a cell parameter similar to that of silicon makes it a candidate for implementation in electronic devices. This structure is stable in a wide temperature and pressure range, with a reported structural phase transition to an orthorhombic phase. In this work, we study the structure of CeO2 under hydrostatic pressures up to 110 GPa simultaneously for the nanometer- and micrometer-sized powders as well as for a single crystal, using He as the pressure-transmitting medium. The first-order transition is clearly present for the micrometer-sized and single-crystal samples, while, for the nanometer grain size powder, it is suppressed up to at least 110 GPa. We show that the stacking fault density increases by two orders of magnitude in the studied pressure range and could act as an internal constraint, avoiding the nucleation of the high-pressure phase.

Characterization of Aluminum Powders: III. Non‐Isothermal Oxidation and Combustion of Modern Aluminized Solid Propellants with Nanometals and Nanooxides

AbstractAluminum powders were comprehensively described as the prospective ingredients of the modern propellants. The paper also studied the influence of micro‐ and nanopowders of metals (μ‐Me and n‐Me) and metal oxides (μ‐MeO and n‐MeO) on the burning process of modern aluminized propellant with HMX, CL‐20, AP, and active binder. The following metal additives were used: Al, B, Zn, Ni, Co, and Mo. The effect of the following oxides CoO, V2O5, MnO2, and Fe2O3 was studied together with LiF. The combustion tests of modified propellant compositions were carried out in the Vielle bomb in a pressure range 2–10 MPa. n‐Me addition resulted in an increase in the burning rate by 10 % for n‐B, by 30–40 % for n‐Ni and n‐Mo in the studied pressure range. The introduction of n‐Cu caused a burning rate to increase fivefold. n‐Zn additive resulted in increasing of the propellant burning rate by 130 % and 260 % at 4 and 10 MPa, respectively. It was probably caused by the catalytic activity of those metals in the gaseous phase. The effect of complex additive was observed to be insignificant for additives with μ‐Co3O4, μ‐V2O5, μ‐Fe2O3 and n‐Fe2O3. The burning rate of propellant with n‐CuO additive value was higher by a factor of 4 in comparison with the basic formulation in the studied pressure range.

Interplay between local structure, vibrational and electronic properties on CuO under pressure.

The electronic and local structural properties of CuO under pressure have been investigated by means of X-ray absorption spectroscopy (XAS) at Cu K edge and ab initio calculations, up to 17 GPa. The crystal structure of CuO consists of Cu motifs within CuO4 square planar units and two elongated apical Cu-O bonds. The CuO4 square planar units are stable in the studied pressure range, with Cu-O distances that are approximately constant up to 5 GPa, and then decrease slightly up to 17 GPa. In contrast, the elongated Cu-O apical distances decrease continuously with pressure in the studied range. An anomalous increase of the mean square relative displacement (EXAFS Debye-Waller, σ2) of the elongated Cu-O path is observed from 5 GPa up to 13 GPa, when a drastic reduction takes place in σ2. This is interpreted in terms of local dynamic disorder along the apical Cu-O path. At higher pressures (P > 13 GPa), the local structure of Cu2+ changes from a 4-fold square planar to a 4+2 Jahn-Teller distorted octahedral ion. We interpret these results in terms of the tendency of the Cu2+ ion to form favorable interactions with the apical O atoms. Also, the decrease in Cu-O apical distance caused by compression softens the normal mode associated with the out-of-plane Cu movement. CuO is predicted to have an anomalous rise in permittivity with pressure as well as modest piezoelectricity in the 5-13 GPa pressure range. In addition, the near edge features in our XAS experiment show a discontinuity and a change of tendency at 5 GPa. For P < 5 GPa the evolution of the edge shoulder is ascribed to purely electronic effects which also affect the charge transfer integral. This is linked to a charge migration from the Cu to O, but also to an increase of the energy band gap, which show a change of tendency occurring also at 5 GPa.