A new insight on the role of Mg in calcite

Magnesium is a key determinant in calcite (CaCO₃) mineralization having a notable significance upon the carbonate growth. Magnesium is also present in CaCO₃ mineral scale formation which is a persistent and expensive problem in oil and gas production. In the present work, we attempt to treat CaCO₃ calcite mineral bulk inducing different concentrations of Mg impurity. The study is done using a quantum‐chemical computer code based on the Hartree–Fock method. It is found that small Mg impurity concentrations produce a significant distortion of the defect‐closest O atoms thus developing local microstructures being distinct to those of the original calcite structure. This structural alteration is accompanied by the electron density redistribution from the O 2p_z atomic orbitals (AOs) toward the O 2p_x and 2p_y AOs for impurity nearest oxygens. On the other hand, the high Mg impurity concentrations make the chemical bonding in calcite more ionic. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Mott–Wannier excitons in the tetragonal BaTiO3 lattice

Structural and electronic properties of excitons in the tetragonal BaTiO₃ crystal is studied using a quantum chemical method developed for crystals. The obtained defect structure corresponds to the so‐called Mott–Wannier‐type exciton having a considerable distance between the hole and electronic parts of the defect. Performed crystalline lattice relaxation shows displacements of atoms in an extensive defective region of up to 12 atoms. However, the calculated magnitudes of atomic movements are not large, normally not exceeding 0.08 Å. It is also observed that the self‐trapped exciton polarizes the lattice around it. Using the so‐called ΔSCF method, the luminescence energy due to the exciton is found to be equal to 0.94 eV. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Ab initio calculation of chromium oxide containing Ti dopant

First-principles computations based on the density functional theory within the generalised gradient approximation and introduced intra-atomic interaction term for strongly correlated electrons (DFT + U method) has been used in this work. Ti impurity doping in the α-Cr₂O₃ crystal has been carried out considering single defect model within the periodic crystalline structure. Atomic displacements, Bader charges on atoms have been computed showing that Ti dopant converts the chemical bonding in its neighbourhood into more ionic one. The defect-local microstructure is such as there exist general tendency of atomic rearrangements away with respect to the Ti imperfection. It is found that defect incorporation produces some local changes upon the band structure of the material and also induces a metallic state. That implies n-type electrical conductivity in the Ti-doped α-Cr₂O₃ crystals and relates our work directly to a number of experimental studies in this area. Our results provide evidence over change in magnetic moments in the vicinity of defect, which means that the chromium oxide doped with Ti impurity might not act as an antiferromagnetic substance.     

Hole polarons in pure BaTiO3 studied by computer modeling

Self‐trapped hole polarons in technologically important perovskite‐type ceramic of BaTiO₃ have been modeled by means of the quantum chemical method modified for crystal calculations. The computations are carried out in the self‐consistent field (SCF) manner using the embedded molecular cluster model. The spatial configuration of a hole polaron, displacement of defect‐surrounding atoms, and wave functions of the polaron ground and excited states are obtained and analyzed. The probability of spontaneous hole self‐trapping is estimated in the perfect lattice of the BaTiO₃ crystal by calculating the value of the hole self‐trapping energy as a difference of the atomic relaxation energy and the hole localization energy. This value is found to be negative, −1.49 eV, which demonstrates the preference of the self‐trapped polaron state. The calculated polaron absorption energy, 0.5 eV, is discussed in light of the available experimental data. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 358–366, 2000     

钾搀杂对叠氮化亚铜晶体能带结构影响的DFT研究

运用广义梯度近似(GGA)密度泛函理论（DFT）方法对钾搀杂叠氮化亚铜晶体的原子和电子结构以及缺陷形成能进行了研究。结果表明,钾搀杂破坏了叠氮酸根的对称性,导致不对称的原子位移。随着搀杂钾浓度的增加,搀杂叠氮化亚铜晶体的带隙逐渐增大。热力学上,杂质钾很容易被搀杂到叠氮化亚铜晶体中,而且钾还充当了缺陷团簇的成核中心。最后,从电子结构角度理解了搀杂钾浓度对叠氮化亚铜晶体感度的影响。     

First‐Principles Study on Doping of SnSe2 Monolayers

Doping is a vitally important technique that can be used to modulate the properties of two‐dimensional materials. In this work, by using first‐principles density functional calculations, we investigated the electrical properties of SnSe₂ monolayers by p‐type/n‐type and isoelectronic doping. Substitution at Sn/Se sites was found to be easy if the monolayer was grown under Sn‐/Se‐poor conditions. Substitutions at Sn sites with metallic atoms (e.g. Ga, Ge, In, Bi, Sb, Pb) resulted in positive substitution energies, which indicated that they were not effective doping candidates. For substitutions at Se sites with nonmetallic atoms, no promising candidates were found for p‐type doping (e.g., N, P, As). Among these, N and As showed positive substitution energies. Although P had a negative substitution energy under Sn‐rich conditions, it introduced trap states within the band gap. For n‐type doping (e.g., F, Cl, Br), all the calculated substitution energies were negative under both Sn‐ and Se‐rich conditions. Br was proven to be a promising candidate, because the impurity introduced a shallow donor level. Finally, for isoelectronic doping (e.g., O, S, Te), the intrinsic semiconducting features of the SnSe₂ monolayer did not change, and the contribution from the impurity to the states near the band edge increased with the atomic number.     

Quantum chemical investigation of lanthanum doping effects in tetragonal and rhombohedral PZT materials

Structural and electronic properties of lead–zirconate–titanate (PZT) materials doped with a lanthanum (La) impurity are studied using a quantum‐chemical approach based on the Hartree–Fock theory. Performed geometry optimization in the defective crystals shows that the atomic movements are predominantly outward with respect to the impurity position. It is found that the La impurity enhances a covalent character in the chemical bonding between the Ti and O atoms, as well as the Zr and O atoms situated in the neighborhood of the defect despite the fact that the La‐O interaction remains purely ionic. The occurrence of local energy levels within the band gap of the material is analyzed in light of the available experimental data on La concentration influence upon dielectric and piezoelectric properties in these crystals. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Étude Théorique de la Conductivité, Photoconductivité et de l'absorption Photoinduite de BaTiO3:Fe et KNbO3:Fe

Calculation of dark conductivity, and study of photoconductivity and light-induced absorption in BaTiO₃ :Fe and KNbO₃:Fe are performed at many light intensities. The results are interpreted in a two impurity level band model. This model allows to calculate the dark conductivity, to explain the behavior of the photoconductivity with the respect to the light intensities, and to study the light-induced absorption coefficient.     

First-principles study of the atomic structures,electronic properties,and surface stability of BaTiO3 (001) and (011) surfaces

The atomic structures, electronic properties, and surface stability of (001) and (011) surfaces of BaTiO₃ are studied by first-principles calculations. Four differently terminated BaTiO₃ surfaces are considered in this study, including (001)-BaO, (001)-TiO₂, (011)-BaTiO, and (011)-O₂ terminations. The relaxations and rumplings are calculated and discussed, finding that the first layer relaxes inwards, while the second layer relaxes outwards for (001) and (110) surfaces. The data obtained for electronic properties show that O_2p states in (001)-BaO/(001)-TiO₂ termination shift to the lower/higher energy region, leading to a wide/narrow band gap. And the new produced surface states are observed in (011) surface terminations, which is mainly attributed to the supplied electrons from outermost surface atoms, even O atoms are oxidized. Furthermore, the (001) surface of BaTiO₃ is found to be more stable than the (011) surface according to the predicted surface energy which is 0.86 and 2.92 J/m² for (001) and (011) surfaces, respectively. Of which, BaO termination is predicted to be more likely to cleavage from the (001) direction than the TiO₂ termination is.     

Bi掺杂锐钛矿相TiO2第一性原理计算

采用密度泛函理论(DFT)下的第一性原理平面波超软赝势方法计算了Bi掺杂前后锐钛矿相TiO2的电子结构和光学性质。结果分析发现:掺杂后Ti的电荷布居数下降,O的布居数增加;同时在TiO2禁带中引入了杂质能级,禁带宽度略微变大,但是杂质能级的作用抵消了禁带宽度变大带来的不利影响,使得掺杂后TiO2吸收带边红移并在可见光范围内吸收明显增强。     

Structural properties of PbTiO3 and PbZrxTi1−xO3: A quantum‐chemical study

We investigate the structural and electronic properties of pure and Zr‐doped PbTiO₃ crystals. The nature of atomic relaxation around the Zr impurity is studied through quantum‐chemical simulations based on the Hartree–Fock theory and a periodic large unit cell model adopted within the so‐called intermediate neglect of differential overlap approximation. The most stable defect configurations are predicted for different impurity concentrations. The results obtained are compared with those from other theoretical studies and a number of experimental measurements carried out on this technologically important perovskite‐type crystal. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

O deficiency in the rutile TiO2 (110) surface: Ab initio quantum chemical investigation of the electronic properties

The (110) surface of rutile TiO₂ (110) has been modeled using a density functional theory (DFT) plane‐wave pseudo‐potential method (CASTEP). In this study, 6 and 9 atomic‐layer slabs have been examined. The stoichiometric surface converges to a low‐spin solution in both cases with a density of states (DOS) similar to that for the bulk. O deficiencies are introduced by the removal of neutral O atoms thus leaving a neutral model with a surfeit of 2 e^? per vacancy. This results in the partial filling of the Ti t_2g conduction band orbitals and a compensatory shift in the Fermi level. The reduced surface converges to a high‐spin solution in all cases, with the excess spin located within the previously unoccupied Ti t_2g orbitals. Removal of the bridging surface O atoms results in an excess spin of 2 electrons per unit cell with approximately one‐half that for removal of in‐plane surface O atoms and subsurface O atoms. The removal of O atoms from the surface leads to an increase of the band gap, with the largest increase due to the removal of in‐plane 3‐fold coordinated surface O atoms, and the smallest one due to the removal of subsurface O atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

On the Origin of the Visible‐Light Activity of Titanium Dioxide Doped with Carbonate Species

Plane‐wave‐based pseudopotential density functional theory (DFT) calculations are used to elucidate the origin of the high photocatalytic efficiency of carbonate‐doped TiO₂. Two geometrically possible doping positions are considered, including interstitial and substitutional carbon atoms on Ti sites. From the optical absorption properties calculations, we believe that the formation of carbonates after doping with interstitial carbon atoms is crucial, whereas the contribution from the cationic doping on Ti sites is negligible. The carbonate species doped TiO₂ exhibits excellent absorption in the visible‐light region of 400–800 nm, in good agreement with experimental observations. Electronic structure analysis shows that the carbonate species introduce an impurity state from Ti 3d below the conduction band. Excitations from the impurity state to the conduction band may be responsible for the high visible‐light activity of the carbon doped TiO₂ materials.     

Hybridization: A Chemical Bonding Nature of Atoms

Both the bonding mode and geometry can serve as the chemical bonding nature of central cation, which is essentially determined by the atomic orbital‐hybridization. In this work, we focus on the possible chemical bonding scheme of central cations on the basis of a quantitative analysis of electron domain of an atom. Starting from the hybridization of outer atomic orbitals that are occupied by valence electrons, we studied the possible orbital hybridization scheme of atoms in the periodic table and the corresponding coordination number as well as possible molecular geometries. According to distinct hybrid orbital sets, the chemical bonding of central cations can be classified into three typical types, resulting in the cations with a variety of coordination numbers ranging from 2 to 16. Owing to different hybridization modes, the highest coordination number of cations in IA and IIA groups is larger than that in IB‐VIIIB groups, and the coordination number of lanthanide elements is most abundant. We also selected NaNO₃ , Fe(NO₃ )₃•9H₂O , Zn(NO₃ )₂•6H₂O , Y(NO₃ )₃•3H₂O , and La(NO₃ )₃•6H₂O as examples to confirm the direct relationship between chemical bonding characteristics and orbital hybrid set by IR spectra. The present study opens the door to reveal the chemical bonding nature of atoms on the basis of hybridization and will provide theoretical guides in structural design at an atomic level.     

A new method for estimating the dispersity of deposited metallic nanoparticles and extent of their interaction with the support matrices

Using a fully relativistic DV cluster method, we study the electronic structure of a large fragment of the crystal lattice of zircon ZrSiO₄ with a plutonium dopant atom replacing a Zr⁴⁺ zirconium atom. Three possible states of the impurity center are considered: Pu⁴⁺ (isovalent substitution), Pu³⁺ (non-isovalent substitution), and Pu³⁺ with an oxygen vacancy in the nearest environment that provides charge compensation. Relaxation of the ZrSiO₄ crystal lattice near a defect is simulated using a semi-empirical method of atomic pair potentials (GULP program). An analysis of overlap populations and effective charges on atoms shows that the chemical bonding of plutonium with a matrix is covalent, while isovalent substitution yields a more stable system than a Pu³⁺ impurity. In the presence of vacancies the structure of chemical bonding is intermediate with respect to substitutions Pu⁴⁺ → Zr⁴⁺ and Pu³⁺ → Zr⁴⁺.     

Fabrication of Predominantly Mn4+‐Doped TiO2 Nanoparticles under Equilibrium Conditions and Their Application as Visible‐Light Photocatalyts

The chemical state of a transition‐metal dopant in TiO₂ can intrinsically determine the performance of the doped material in applications such as photocatalysis and photovoltaics. In this study, manganese‐doped TiO₂ is fabricated by a near‐equilibrium process, in which the TiO₂ precursor powder precipitates from a hydrothermally obtained transparent mother solution. The doping level and subsequent thermal treatment influence the morphology and crystallization of the TiO₂ samples. FTIR spectroscopy and X‐ray photoelectron spectroscopy analyses indicate that the manganese dopant is substitutionally incorporated by replacing Ti⁴⁺ cations. The absorption band edge can be gradually shifted to 1.8 eV by increasing the nominal manganese content to 10 at %. Manganese atoms doped into the titanium lattice are associated with the dominant 4+ valence oxidation state, which introduces two curved, intermediate bands within the band gap and results in a significant enhancement in photoabsorption and the quantity of photogenerated hydroxyl radicals. Additionally, the high photocatalytic performance of manganese‐doped TiO₂ is also attributed to the low oxygen content, owing to the equilibrium fabrication conditions. This work provides an important strategy to control the chemical and defect states of dopants by using an equilibrium fabrication process.     

Geometrical Structure of the Gold–Iron(III) Oxide Interfacial Perimeter for CO Oxidation

The geometrical structure of the Au‐Fe₂O₃ interfacial perimeter, which is generally considered as the active sites for low‐temperature oxidation of CO, was examined. It was found that the activity of the Au/Fe₂O₃ catalysts not only depends on the number of the gold atoms at the interfacial perimeter but also strongly depends on the geometrical structure of these gold atoms, which is determined by the size of the gold particle. Aberration‐corrected scanning transmission electron microscopy images unambiguously suggested that the gold particles, transformed from a two‐dimensional flat shape to a well‐faceted truncated octahedron when the size slightly enlarged from 2.2 to 3.5 nm. Such a size‐induced shape evolution altered the chemical bonding environments of the gold atoms at the interfacial perimeters and consequently their catalytic activity. For Au particles with a mean size of 2.2 nm, the interfacial perimeter gold atoms possessed a higher degree of unsaturated coordination environment while for Au particles with a mean size of 3.5 nm the perimeter gold atoms mainly followed the atomic arrangements of Au {111} and {100} facets. Kinetic study, with respect to the reaction rate and the turnover frequency on the interfacial perimeter gold atom, found that the low‐coordinated perimeter gold atoms were intrinsically more active for CO oxidation. ¹⁸O isotopic titration and Infrared spectroscopy experiments verified that CO oxidation at room temperature occurred at the Au‐Fe₂O₃ interfacial perimeter, involving the participation of the lattice oxygen of Fe₂O₃ for activating O₂ and the gold atoms for CO adsorption and activation.     

Vanadium-doped BaTiO3 as high performance thermoelectric material: role of electronic structure engineering

It is well known that thermoelectric (TE) materials are the most sought-after ones to mitigate energy crisis. Development of an efficient non-toxic, economic, abundant, and stable TE material is quite difficult due to its complicated traits. BaTiO₃, a perovskite material shows a tremendous potential as a TE material due to its highly tunable electronic structure. Herein, for the first time we report use of dopant to improve the Seebeck coefficient of BaTiO₃. We used first-principles density functional theory calculations to study the effect of vanadium doping in BaTiO_3, and for the first time, we report that V acts as a resonant dopant in BaTiO₃. The study on effect of site occupancy reveals that V in Ba site distorts the density of states below the conduction band by introducing resonance level at the Fermi level. The transport property calculations based on Boltzmann's relation predicts V-doped BaTiO₃ to be a potential TE material. The results also provide new insights into development of BaTiO₃ as a multifunctional material.     

First‐principles calculations on the four phases of BaTiO3

The calculations based on linear combination of atomic orbitals basis functions as implemented in CRYSTAL09 computer code have been performed for cubic, tetragonal, orthorhombic, and rhombohedral modifications of BaTiO₃ crystal. Structural and electronic properties as well as phonon frequencies were obtained using local density approximation, generalized gradient approximation, and hybrid exchange‐correlation density functional theory (DFT) functionals for four stable phases of BaTiO₃. A comparison was made between the results of different DFT techniques. It is concluded that the hybrid PBE0 [J. P. Perdew, K. Burke, M. Ernzerhof, J. Chem. Phys. 1996, 105, 9982.] functional is able to predict correctly the structural stability and phonon properties both for cubic and ferroelectric phases of BaTiO₃. The comparative phonon symmetry analysis in BaTiO₃ four phases has been made basing on the site symmetry and irreducible representation indexes for the first time. © 2012 Wiley Periodicals, Inc.     

H‐doped PbTiO3: Structure and electronic properties

The geometry and electronic properties of the interstitial H atom in the tetragonal PbTiO₃ crystal have been studied using an advanced quantum chemical computer code developed for the modeling of crystals. The inserted H atom was found to bind to one of the O atoms and to form the hydroxyl, O? H group, with the inter‐atomic distance equal to 0.93 Å and 1.00 Å for the hydroxyls containing O atom in the dimerized and nondimerized Ti? O? Ti chains, respectively. Atomic displacements in the vicinity of O? H complex are calculated and analyzed in relation to the H‐produced changes upon the atomic charges in defective region. The role of H impurity on the ferroelectric polarization in the tetragonal PbTiO₃ is discussed in terms of the results obtained in our research and those presented in the other studies on this subject. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007