Ionic Character Research Articles

Cr-doped Sb2Te materials promising for high performance phase-change random access memory

Transition metal doping is an effective strategy to improve the properties of phase-change random access memory (PCRAM), while the underlying mechanism remains to be sufficiently investigated. Herein, a Cr-doped Sb2Te film is fabricated by co-sputtering and the enhancement mechanism of Cr-doping is deeply investigated by experiments and first-principles calculations. The CrxSb2Te film exhibits excellent performances involving high crystallization temperature (238.9 °C) and good data retention (10 years @161.7 °C) for Cr0.56Sb2Te, low density change rate (3.8%) and high reversible switching speed (5 ns) for Cr0.29Sb2Te, providing a variety of options for different applications. The calculated results show that Cr atoms preferentially substitute the Sb1 site and the CrTe bonds display a stronger ionic bond character in contrast to the covalent bonded SbTe, thus improving the crystalline phase stability and crystallization temperature. Moreover, ab initio molecular dynamics simulations demonstrate that in the amorphous phase Cr atoms locate in defective octahedral coordination. Meanwhile, the homopolar SbSb wrong bond increased with Cr doping and the Cr atoms tend to form tight cluster in the amorphous phase which could benefit for the fast phase-change speed. Our present findings of Cr0.29Sb2Te PCRAM device clearly reveal the enhancement mechanism by Cr doping which provides guidance for developing high-performance PCRAM devices.

Computational study of the electronic structure of the Srm+Kr (m = 0, 1) van der Waals complexes

A computational study of the electronic structure of the SrKr and Sr+Kr molecular systems is presented in this paper. The theoretical approach is based on the pseudo-potential technique for Sr++Kr interaction and core-valence correlation for the one and two electrons-Sr++Kr interaction. The potential energy surfaces (PESs), spectroscopic parameters, electric dipole moments (EDM), and the vibrational levels’ spacing for all electronic states are calculated. The accuracy of the current spectroscopic results is discussed by comparing them to the available experimental and theoretical data. It is interesting to note that several avoided crossings (ACs) have occurred between the high-lying 2Σ+ excited states. Each curve exhibits ionic and neutral branches in the AC region, yielding the appearance of the ionic character and the illustration of charge transfer.

Experimental and Mechanistic Study of Synergistic Removal of Hg by Evaporation from Desulfurization Wastewater

The flue evaporation of desulfurization wastewater can solve the problem that it is difficult to remove some heavy metal ions and chloride ions by conventional methods. A large amount of chloride ions in desulfurization wastewater can also promote the catalytic oxidation removal of Hg in the flue gas. The migration character of chloride ions in the flue evaporation process of desulfurization wastewater was studied by using the coal-fired thermal state experimental platform. The concentrations of Hg0 and Hg2+ in the flue gas at the inlet and outlet of selective catalytic reduction denitration (SCR), electrostatic precipitator (ESP), and wet desulfurization (WFGD) devices were tested, and the synergistic removal of traditional pollutant removal equipment by flue evaporation of desulfurization wastewater was analyzed. The influence of Hg and the effect of the evaporation of desulfurization wastewater at different positions on the removal of Hg in the flue gas were compared and analyzed, and the catalytic mechanism of Hg on the SCR surface was further revealed. The results show that 10% chloride ions enter the flue gas after the desulfurization wastewater evaporates. The content of chlorine elements and evaporation temperature influence the evaporation of desulfurization wastewater. The mechanism of SCR catalytic oxidation of Hg0 was explored; oxygen atoms have catalytic oxidation effects on Hg0 at different positions in the V2O5 molecule in SCR; and chloride ions can enhance the catalytic oxidation of Hg0 by V2O5. The intermediate product HgCl is generated, which is finally converted into HgCl2. The oxidation efficiency of Hg0 in electrostatic precipitation (ESP) is increased from 3% to 18%, and the removal efficiency of Hg is increased from 5% to 10%. The removal efficiency of Hg2+ in WFGD is basically maintained at approximately 85%. In addition, a small amount of Hg2+ was restored to Hg0 in WFGD. The removal efficiency of Hg0 in the flue gas of evaporative desulfurization wastewater before SCR is 65%, and the removal efficiency of gaseous Hg is 62%. When the evaporative desulfurization wastewater before ESP, the synergistic removal efficiency of Hg0 is 39%, and the gaseous Hg removal efficiency is 39%, and the removal efficiency of Hg is 40%. Evaporation of the desulfurization wastewater before SCR was more conducive to the coordinated removal of Hg by the device.

Lanthanides Toxicity in Zebrafish Embryos Are Correlated to Their Atomic Number.

Rare earth elements (REEs) are critical metallic materials with a broad application in industry and biomedicine. The exponential increase in REEs utilization might elevate the toxicity to aquatic animals if they are released into the water due to uncareful handling. The specific objective of our study is to explore comprehensively the critical factor of a model Lanthanide complex electronic structures for the acute toxicity of REEs based on utilizing zebrafish as a model animal. Based on the 96 h LC50 test, we found that the majority of light REEs display lower LC50 values (4.19–25.17 ppm) than heavy REEs (10.30–41.83 ppm); indicating that they are atomic number dependent. Later, linear regression analyses further show that the average carbon charge on the aromatic ring (aromatic Cavg charge) can be the most significant electronic structural factor responsible for the Lanthanides’ toxicity in zebrafish embryos. Our results confirm a very strong correlation of LC50 to Lanthanide’s atomic numbers (r = 0.72), Milliken charge (r = 0.70), and aromatic Cavg charge (r = −0.85). This most significant correlation suggests a possible toxicity mechanism that the Lanthanide cation’s capability to stably bind to the aromatic ring on the residue of targeted proteins via a covalent chelating bond. Instead, the increasing ionic bond character can reduce REEs’ toxicity. In addition, Lanthanide toxicity was also evaluated by observing the disruption of photo motor response (PMR) activity in zebrafish embryos. Our study provides the first in vivo evidence to demonstrate the correlation between an atomic number of Lanthanide ions and the Lanthanide toxicity to zebrafish embryos.

Probing the structures and bonding of Ag/H end-capping acetylene and polyyne, AgC2nH− (n = 1, 2), a combined photoelectron imaging and quantum chemical calculation study

The electronic structures of AgC2nH− (n = 1, 2) are reported by using photoelectron imaging and ab initio calculations. The results indicate that all the ground states of anion and neutral AgC2nH have a linear geometry. The electron affinities (EAs) are measured to be 1.123 and 1.379 eV for AgC2H and AgC4H, respectively. The NRT analysis shows that ionic character dominates the Ag–C chemical bond in these neutral species. The calculated static dipole polarizability (α) and second hyperpolarizability (γ) of the molecules indicate that the cap of Ag could enhance the nonlinear optical responses.

On predicting bonding patterns of small clusters of alkaline-earth (Be, Mg) and triel (B, Al) fluorides: a balance between atomic size and electron-deficient character

The structures, bonding and stability of (MF2)m:(M´F3)n (M = Be, Mg; M´ = B, Al; m = 0,1,2; n = 0,1,2) clusters were obtained at the B3LYP/aug-cc-pVTZ level of theory. To understand trends across this set of closely related atoms, an analysis of the results obtained using atoms in molecules (AIM), electron localisation function (ELF) and electron density shift (EDS) approaches, permits to identify subtle dissimilarities when the first-row elements, Be and B, are replaced by the second-row counterparts, Mg and Al. For dimers this replacement involves an increase in the bonding enthalpies as a direct consequence of a much larger ionic character of the derivatives including second-row elements. For trimers and tetramers, rather stable structures involving penta-coordinated aluminium atoms are formed, which are not found for the B-containing analogues. In all clusters investigated the electronic environment around each monomer does not change significantly neither with nature of the monomers interacting with it or with the size of the cluster, though some small cooperative effects are observed when analyzing the binding enthalpies. The important consequence is that the stability of larger clusters can be easily predicted through a statistical treatment of the values obtained for the smaller ones.

Tailoring of structural, optical and electrical properties of anatase TiO2 via doping of cobalt and nitrogen ions

Pure (pristine) anatase and mono–doped and co–doped derivatives of TiO2 having nitrogen (N) and cobalt (Co) as dopants with respective fixed doping concentrations of 0.7 mol.% and 1.0 mol.% were synthesized using auto–combustion sol–gel technique. The doping effects at corresponding non–metal and transition metal sites of TiO2 on the basis of the structural, optical and electrical properties have been investigated. X–ray diffraction (XRD) measurement confirms the formation of pure anatase phase of TiO2 for all samples having I41/amd space group of tetragonal structure which has been also verified by the Raman spectroscopy measurement. Various crystallographic parameters have been calculated by performing Rietveld refinement of XRD data including average crystallite size that has been observed in the range of 10–15 nm. Pure anatase phase indicates the incorporation of Co2+ into TiO2 lattice which assists the substitution of N in place of oxygen in co–doped TiO2. The band gap tuning towards the visible region from 3.2 to 2.1 eV has been achieved with mono–doping and co–doping of the N and Co in TiO2 lattice. This can be described in terms of the formation of localized levels of N–2p and Co–3d states in mono–doping and an isolated intermediate band formation in co–doping case. Electrical properties have been investigated in details and explained as the synergetic effects of structural and inherent ionic characters of various dopants. The observed band gaps of all doped samples lie within the visible region which makes them pertinent as the solar energy harnessing materials for photocatalytic and photovoltaic applications.

Ion exchange for effective separation of 3-nitro-1,2,4-triazol-5-one (NTO) from wastewater

The explosive 3-nitro-1,2,4-triazol-5-one (NTO) presents a physiochemical challenge for treatment of munitions wastewater. Leveraging NTO’s ionic character in neutral pH wastewater allows for expanded treatment options. Four commercial drinking water anion exchange resins specific for NO3- and ClO4- were evaluated for NTO adsorption extent, adsorption kinetics, and regeneration potential. Batch studies demonstrated NTO adsorption to all resins tested (max 690 mg NTO/g resin) and that resins were regenerable with 6% NaCl. Adsorption capacities (88–99%) and desorption efficiencies (80–85%) of NTO from the resins remained stable over three loading cycles. Perchlorate selective resins adsorbed more NTO, with larger desorption efficiencies, than nitrate selective resins. Kinetic experiments demonstrated that equilibrium adsorption between NTO and resins occurs within 120 min of exposure, following the pseudo second-order model (K2 range 9.8 × 10−5 to 15 × 10−5 g resin/mg NTO/min). Intraparticle diffusion modeling suggested that boundary-layer diffusion was the predominant sorption mechanism in NTO adsorption to the resins compared to intraparticle diffusion. In synthetic wastewater mixtures of NTO, 2–4-dinitroanisole (DNAN), nitroguanidine (NQ), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), only NTO was exchanged to any great extent. This work suggests that perchlorate anion exchange resins may be a viable segregation technology for NTO from munitions wastewater as compared to activated carbon.

Niobium speciation in minerals revealed byL2,3-edges XANES spectroscopy

AbstractThe systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L2,3-edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter values for Nb5+, which range from 2.8 to 3.9 eV depending on Nb coordination and Nb—O distances. Rather than a 10Dq vs. R–5 relationship (where R represents the average Nb-O bond distance) expected in a point-charge model, we find a R–3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb-O bonds is equivalent, unlike more ionic 3d metal–O bonds. This systematic characterization of the L2,3-edges XANES spectral properties of Nb provides information on the mechanisms by which Nb5+ substitutes for Fe3+, Ti4+, or Ce4+ in oxides common in geological contexts. Whereas the substitution of Nb5+ for Ce4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb5+ for Ti4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb5+ for Fe3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L2,3-edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.

First-principles calculations to investigate Structural, electronic, and optical properties of MgF2 monolayer in 1T-phase and 2H-phase using hybrid functional

Full-potential linearized augmented plane-wave (FP-LAPW) method, in combination with the HSE06 hybrid functional, has been employed to investigate the structural, electronic, and optical properties of MgF 2 monolayer in 1T- and 2H-phase. The stability is examined via phonon dispersion curves and cohesive energy. In both phases, MgF 2 single layer exhibits insulator nature with energy gaps as large as 7.67(10.02) and 5.88(8.27) eV obtained by the WC(HSE06) functional, respectively. The density of states and valence charge distribution suggest ionic character of this two-dimensional (2D) material. Strain effect analyses indicate that the band gap decreases nearly linearly when switching the strain nature from compressive to tensile. Calculated dielectric function and optical properties including reflectivity, refractive index, and absorption coefficient assert the optical transparency in a wide energy range from infrared to ultraviolet. Results suggest that MgF 2 monolayer may hold prospect to be applied in the spectroscopy and medical imaging systems.

Prediction of nature of band gap of perovskite oxides (ABO3) using a machine learning approach

A material's electronic properties and technological utility depend on its band gap value and the nature of band gap (i.e. direct or indirect). This nature of band gaps is notoriously difficult to compute from first principles. In fact it is computationally intense to approximate and also rather time consuming. Hence its prediction represents a challenging problem. Machine learning based approach offers a promising and computationally efficient means to address this problem. Here we predict the nature of band gap for perovskite oxides (ABO3) with elemental composition, ionic radius, ionic character and electronegativity. We do this by training machine learning models on computationally generated datasets. Knowing the nature of the band gap of the perovskite oxides (whether direct or indirect) plays a pivotal role in determining whether the perovskite can be used for photovoltaic or photocatalytic applications. A total of 5329 perovskite oxides are considered in this study. Here, we determine the correlation between the nature of band gap and the composition of the perovskite oxide. A Random Forest algorithm is used for predicting the same since it yielded higher accuracy (∼91%) compared to the other Machine Learning models. The approach suggested here can be used to predict the nature of bandgap and can also aid in novel materials discovery within the family of perovskites. This is a robust, quick, and low-cost strategy to find novel materials for light harvesting applications in particular. Also we present feature ranking as it pertains to prediction of nature of bandgap and also discuss correlation between the features. We also show feature importance graphs and SHapley Additive exPlanations (SHAP) as is relevant for prediction of nature of band gaps. Using the approach reported, NaPuO3 and VPbO3 are discovered to be good candidates for solar cell materials (direct band gap∼1.5 eV). Novel composition predictions for targeted applications are the future and our model is a step ahead in this direction.

Polynuclear Iron(II) Pyridonates: Synthesis and Reactivity of Fe4 and Fe5 Clusters.

The combination of pyridonate ligands with transition metal ions enables the synthesis of an especially rich set of diverse coordination compounds involving various κ- and μ-bonding modes and higher nuclearities. With iron(II) ions, this chemical space is rather poorly explored beyond some biomimetic models of the pyridone iron-containing hydrogenase. Here, the topologically new Fe5 and Fe4 clusters, Fe5(LH)6[N(SiMe3)2]4 (1) and Fe4(LMe)6[N(SiMe3)2]2 (2), were synthesized (LH = 2-pyridonate; LMe = 6-methyl-2-pyridonate). Complex 1 contained an unprecedented diamondoid Fe@Fe4 tetrahedron with a central-to-peripheral Fe-Fe distance of ∼3.1 Å. The crystal structure of complex 2 displayed an Fe4O6 butterfly motif containing a planar Fe4 arrangement. Mössbauer spectroscopy confirmed the high-spin ferrous character of all iron ions. SQUID magnetometry reveals that the Fe(II) ions are involved in weak magnetic exchange coupling across the pyridonate bridges that results in antiferromagnetic interactions. The Fe4 cluster exhibits slow relaxation of magnetization under an applied magnetic field with an effective energy barrier of 38.5 K, rarely observed among the very rare examples of Fe(II) cluster-based single-molecule magnets. Studies of protolytic substitution of the amido ligands demonstrated the lability of the diamondoid Fe5 core in 1 and the stability of the Fe4 rhomboid in 2.

The Rigidity of the (BH4)-Anion Dispersed in Halides AX, A = Na, K; X = Cl, Br, I, and in MBH4 with M = Na, K, Rb, Cs

The B–H bond length of the borohydride anion (BH4−) in alkali metal borohydrides MBH4 with M = Na, K, Rb, Cs, and diluted in different alkali halide matrices, was investigated experimentally by infrared spectroscopy (FTIR) and theoretically using first principles calculations. The peak positions in IR absorption spectra of NaBH4 pressed at 754 MPa in halides NaX and KX with X = Cl, Br, I show significant variations indicating ion exchange effects between the halide and NaBH4. For NaBH4 in NaBr, NaI, KBr and KI pellets, the peak positions indicate that BH4− could be highly diluted in the AX matrix, which renders an isolation of BH4− in AX (i-BH4−). For NaBH4 in NaCl and KCl pellets, a solution of BH4− in AX occurred only after a further thermal treatment up to 450 °C. The observed peak positions are discussed with respect to the lattice parameter (a0), anion to cation ratio (R = rA/rX), standard enthalpy of formation (ΔfH) and ionic character (Ic) of the halides. A linear relation is obtained between ν3(i-BH4−) and the short-range lattice energies of AX. Density functional theory (DFT) calculations at generalized gradient approximation (GGA) level were used to calculate the IR vibrational frequencies ν4, ν3 and ν2 + ν4 for series of compositions Na(BH4)0.25X0.75 with X = Cl, Br, I, and MBH4. The theoretical and experimental results show the same trends, indicating the rigidity of the B–H bond length and the failure of Badger’s rule.

The Cesium Oxide Mercuride Cs18Hg8O6

AbstractBy reacting Cs with Cs2O and elemental mercury the new double salt Cs18Hg8O6 could be obtained. Its crystal structure (cubic, space group I23 with a=13.3920(10) Å, Z=2, R1=0.026/0.032 for I≥2σ(I)/all I, respectively) comprises isolated oxide anions in octahedral coordination by cesium cations next to the mercuride anion [Hg8]6−. This first isolated anion of mercury has cubic shape and is coordinated by cesium atoms capping the faces and the edges of the [Hg8]6− cube. The ionic character of the double salt is shown in DFT calculations of the electronic structure. Raman spectra show distinct features in the low energy region arising from the vibrations of the mercuride anion. Cs18Hg8O6 has very close structural analogies to the thallide oxide Cs18Tl8O6.

Understanding Hydrogenation Chemistry at MgB2 Reactive Edges from Ab Initio Molecular Dynamics.

Solid-state hydrogen storage materials often operate via transient, multistep chemical reactions at complex interfaces that are difficult to capture. Here, we use direct ab initio molecular dynamics simulations at accelerated temperatures and hydrogen pressures to probe the hydrogenation chemistry of the candidate material MgB2 without a priori assumption of reaction pathways. Focusing on highly reactive (101̅0) edge planes where initial hydrogen attack is likely to occur, we track mechanistic steps toward the formation of hydrogen-saturated BH4- units and key chemical intermediates, involving H2 dissociation, generation of functionalities and molecular complexes containing BH2 and BH3 motifs, and B-B bond breaking. The genesis of higher-order boron clustering is also observed. Different charge states and chemical environments at the B-rich and Mg-rich edge planes are found to produce different chemical pathways and preferred speciation, with implications for overall hydrogenation kinetics. The reaction processes rely on B-H bond polarization and fluctuations between ionic and covalent character, which are critically enabled by the presence of Mg2+ cations in the nearby interphase region. Our results provide guidance for devising kinetic improvement strategies for MgB2-based hydrogen storage materials, while also providing a template for exploring chemical pathways in other solid-state energy storage reactions.

Theoretical investigations of structural, electronic, optical and thermoelectric properties of oxide halide perovskite ACoO3 (A=Nd, Pr or La)

Perovskite oxides have emerged as promising alternatives for photovoltaic devices due to their astonishing optoelectronic properties. Nevertheless, a density functional theory study was performed on the electronic, optical, and thermoelectric properties of ACoO3 (A = Nd, Pr, and La) by TB-mBJ potentials. Based on the optimized structures, the observed results show that studied compounds display p-type semiconductor behavior with a bandgap of 1.95 eV, 1.08 eV, and 1.57 eV respectively. Hence, spherical charge density contours illustrate the ionic character of Nd/Pr/La/Co and O. The optical spectra are reported and discussed, the maximum absorption intensity for the LaCoO3 is ∼20 × 104 cm around 400 nm. Furthermore, the figure of merit, the electrical and thermal conductivity of each compound are investigated by applying Boltzmann transport theory, the high mobility of carriers enhances the Seebeck coefficient and electrical conductivity of ACoO3 (A = Nd, Pr, and La) materials. Our finding results make these materials favorable candidates for photovoltaic applications.

Atomistic analysis of 3D fracture fingerprints of mono- and bi-crystalline diamond and gold nanostructures

Gold and diamond are elements of advanced systems. Considering covalent and ionic character of gold versus completely covalent structure of diamond, visualization of fracture behavior of their nanocrystalline structures is the key to understanding their mechanical stability in order to smoothen manufacturing advanced nano-devices. Nevertheless, 3D patterns of their fracture are rarely investigated. In this work, we compare the mechanical properties of (111) diamond and (100) gold from molecular dynamics (MD) simulations perspective. The effects of the temperature, grain boundary, and pre-cracking on the Young's modulus, fracture stress, fracture strain, and stress–strain behavior are investigated. Overall, failure in diamond occurred along certain low-energy cleavage planes, as per a brittle fracture mode with no plastic deformation. Moreover, no significant reduction was detected in the fracture strength of diamond upon temperature rise up to 800 K; but at 1000 K it decreased by 10%., unlike 34% drop computed for gold. Compared to ideally perfect monocrystalline structure, the fracture strength and Young's modulus of the bicrystalline gold decreased by 60% and 8%, respectively. Moreover, the average maximal tensile stress was severely dependent on the strain magnitude. For the diamond, however, the tensile strength decreased about 31–38% depending on the crack initiation pattern, while Young's modulus decreased by nearly 24%. In the presence of defects, the maximal fracture stress of diamond and gold experienced a reduction by 33% and 54%, respectively.

The ratio of chemical bond components in metals and nonmetals and their classification into conductors, semiconductors and dielectrics

The article considers the influence of chemical bond components (covalent character CC, metallic character CM and ionic character CI, in %) in homo- and heteronuclear substances and materials on their classification into classes of electrical conductivity: conductors, semiconductors and dielectrics. Consideration of the influence of the chemical bond type (via CC, CM, CI) on the nature of the electrical conductivity properties of homo- and hetero compounds and respective materials showed the influence of all three components of the chemical bond on it. As a result, it became possible to classify homonuclear compounds of elements by electrical properties into three groups of materials: conductors - CC < CM; semiconductors - CC ≍ or > CM; dielectrics - CC > CM. A good correlation of the ratio of these components with the values of their electrical resistivity, the width of the band gap and the type of conductivity was established. It was found that the increase in electrical conductivity in heteronuclear binary materials, in general, is determined by the growth of CM.

DFT studies on PbO2 and binary PbO2/SnO2 thin films

Periodic quantum mechanics DFT calculations have been employed to investigate surface and electronic properties of β-PbO2 thin films and binary β-PbO2/SnO2 thin films with crystallographic planes (001), (010), (101), and (110) in both cases. The results show significant increases in the band gap energy of the pure PbO2 films compared to that of the bulk, due to a marked increase in the minimum energy of the conduction band. The relative surface stability follows the sequence (110) > (101) > (010) > (001). The surfaces become more unstable, and with a more accentuated ionic character after coating with SnO2, however, the relative stability of the surfaces does not change. Thus, the preferential growth direction and the crystalline shape of the coated films are maintained. The SnO2 coating causes significant changes in the band gap, with increases in the films with (001) and (010) surfaces, while a decrease is noticed in the band gap energy of the films with (110) and (101) surfaces.

Many-body effects for the Mg 2s XPS of MgO

The energy separation of the higher lying cation XPS binding energies, BEs, for MgO is examined. It is shown that the Hartree–Fock BEs overestimate the separation of the Mg BE(2s) and BE(2p), because an important many-body effect is neglected in the determination of the Hartree–Fock BEs. Once the many-body effect is taken into account, there is good agreement between theory and the XPS measurement. The character of this atomic many-body effect is established in terms of a nearly degenerate configuration that is needed to describe the wavefunction when an Mg 2s electron is ionized. The results provide additional evidence of the ionic character of the MgO crystal. Given the atomic character of the many-body effect, it is likely to apply also to the BE separations of other closed shell oxides.