We carried out ab initio calculations for technologically important ABO3 perovskite, such as, SrT... more We carried out ab initio calculations for technologically important ABO3 perovskite, such as, SrTiO3, BaTiO3, SrZrO3 and PbZrO3 (001), (011) and (111) nano-surfaces, interfaces and bulk [Formula: see text]-centers. For SrTiO3, BaTiO3, SrZrO3 and PbZrO3 (001) nano-surfaces, as a rule, all first surface layer atoms relax inward, whereas all second layer atoms relax outward, and, typically, all third surface layer atoms, again, relax inward. Calculated (001) surface energies for SrTiO3, BaTiO3, SrZrO3 and PbZrO3 perovskites are almost equal for both BO2 and AO-terminations, and always smaller than the (011) and (111) surface energies. We discussed the results of our ab initio calculations dealing with BaTiO3/SrTiO3 and SrZrO3/PbZrO3 (001) interfaces. We analyzed systematic trends in ABO3 perovskite bulk [Formula: see text]-center ab initio calculations.
Proceedings of X International Scientific and Practical Conference "Electronics and Information Technologies", 2018
We performed first-principles calculations for technologically most important ABO 3 perovskites, ... more We performed first-principles calculations for technologically most important ABO 3 perovskites, such as, CaTiO 3 , SrTiO 3 , PbTiO 3 , BaTiO 3 , SrZrO 3 and PbZrO 3 (001), (011) and (111) surfaces, interfaces and bulk F-centers. For ABO 3 perovskite neutral (001) surfaces, in most cases, all upper surface layer atoms relax inward, whereas the all second surface layer atoms relax outward, and again, all third surface layer atoms relax inward. The relaxation pattern for YAlO 3 charged (001) surfaces is quite different from ABO 3 perovskite neutral (001) surfaces. The ABO 3 perovskite (001) surface energies are almost equal for both AO and BO 2-terminations, and always considerably smaller than the (011) and especially (111) surface energies. Systematic trends in BaTiO 3 , SrTiO 3 , SrZrO 3 and PbZrO 3 bulk F-center calculations are analyzed.
... Recommended Citation. Eglitis, R.; Kotomin, EA; Devreese, Jozef; Popov, AL - The atomic and e... more ... Recommended Citation. Eglitis, R.; Kotomin, EA; Devreese, Jozef; Popov, AL - The atomic and electronic structure of bound hole polarons and bipolarons in MgO-crystals - In: Proceedings of the International Conference on Defects in Insulating Materials. ...
We present and discuss the results of calculations of surface relaxations and rumplings for the (... more We present and discuss the results of calculations of surface relaxations and rumplings for the (001) and (011) surfaces of BaTiO3 and PbTiO3, using a hybrid B3PW description of exchange and correlation. On the (001) surfaces, we consider both AO (A = Ba or Pb) and TiO2 terminations. In the former case, the surface AO layer is found to relax inward for both materials, while outward relaxations of all atoms in the second layer are found at both kinds of (001) terminations and for both materials. The surface relaxation energies of BaO and TiO2 terminations on BaTiO3 (001) are found to be comparable, as are those of PbO and TiO2 on PbTiO3 (001), although in both cases the relaxation energy is slightly larger for the TiO2 termination. As for the (011) surfaces, we consider three types of surfaces, terminating on a TiO layer, a Ba or Pb layer, or an O layer. Here, the relaxation energies are much larger for the TiO-terminated than for the Ba or Pb-terminated surfaces. The relaxed surface energy for the O-terminated surface is about the same as the corresponding average of the TiO and Pb-terminated surfaces on PbTiO3, but much less than the average of the TiO and Ba-terminated surfaces on BaTiO3. We predict a considerable increase of the Ti-O chemical bond covalency near the BaTiO3 and PbTiO3 (011) surface as compared to both the bulk and the (001) surface.
We present the results of calculations of SrTiO 3 and CaTiO 3 polar (111) surface relaxations, ru... more We present the results of calculations of SrTiO 3 and CaTiO 3 polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO 3 or CaO 3) of the SrTiO 3 and CaTiO 3 (111) surfaces. According to our calculations, atoms of the first surface layer relax inwards for Ti-, SrO 3-, and CaO 3-terminated (111) surfaces of both materials. The only exception is outward relaxation of the SrO 3-terminated SrTiO 3 (111) surface upper layer Sr atom. For both Ti-terminated SrTiO 3 and CaTiO 3 (111) surfaces our calculated second layer Sr and Ca metal atom inward relaxations are approximately four and two times larger than the upper layer Ti atom inward relaxation. Our calculated optical band gap for the SrO 3-and Ti-terminated SrTiO 3 , as well as for CaO 3-terminated CaTiO 3 (111) surfaces, becomes smaller with respect to the bulk optical band gap. Our calculated surface energies for SrO 3-and CaO 3-terminated SrTiO 3 and CaTiO 3 (111) surfaces (6.30 and 5.86 eV) are considerably larger than the surface energies for Ti-terminated SrTiO 3 and CaTiO 3 (111) surfaces (4.99 and 4.18 eV). Our B3LYP calculations indicate a considerable increase of Ti-O chemical bond covalency near the SrTiO 3 and CaTiO 3 (111) surface (þ0.098e and þ0.094e) relative to the SrTiO 3 and CaTiO 3 bulk (þ0.088e and þ0.084e).
The accuracy of the Content should not be relied upon and should be independently verified with p... more The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
The semi-empirical Hartree-Fock intermediate neglect of differential overlap (INDO) method was us... more The semi-empirical Hartree-Fock intermediate neglect of differential overlap (INDO) method was used to explore the potential energy surface of adsorbed water molecules and to speculate, using fairly simple cluster models, on the reaction path followed by metal ions leaving the surface. As an example, INDO calculations were used to calculate the potential energy curve for a metal atom leaving the
The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Int... more The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Intermediate Neglect of the Differential Overlap (INDO) based on the Hartree-Fock formalism are combined for the study of the hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of KNbO3 perovskite crystals. The 40-atom and 320-atom supercells were used, respectively. We predict existence of both, one-site and two-site (molecular) polarons with close optical absorption energies (0.9 eV and 0.95 eV). The relevant experimental data are discussed.
ABSTRACT We present a first-principles study of BaTiO3/SrTiO3(001) interfaces taking into account... more ABSTRACT We present a first-principles study of BaTiO3/SrTiO3(001) interfaces taking into account non-stoichiometric compositions. By means of hybrid exchange–correlation functional within density functional theory (DFT) we demonstrate that charge redistribution in the interface region weakly affects the electronic structure of studied material, while change in the stoichiometry (termination of deposited BaTiO3(001) thin film) yields in significant shifts of band edges. The optical band gap of BaTiO3/SrTiO3(001) interface depends mostly on BaO or TiO2 termination of the upper layer. Based on results of our calculations we predict enhancement of the Ti–O chemical bond covalency near the BaTiO3/SrTiO3(001) interface as compared to the BaTiO3 bulk.
Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) ... more Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) and a supercell model, we calculated from the first principles the electronic structure of both ideal PbZrO 3 (001) surface (with ZrO 2-and PbO-terminations) and a neutral oxygen vacancy also called the F center. The atomic relaxation and electronic density redistributions are discussed. Thermodynamic analysis of pure surfaces indicates that ZrO 2 termination is energetically more favorable than PbO-termination. The O vacancy on the ZrO 2-surface attracts E0.3 e (0.7 e in the bulk PbZrO 3), while the remaining electron density from the missing O 2À ion is localized mostly on atoms nearest to a vacancy. The calculated defect formation energy is smaller than in the bulk which should lead to the vacancy segregation to the surface. Unlike Ti-based perovskites, the vacancy-induced (deep) energy level lies in PbZrO 3 in the middle of the band gap.
... The latter allows the tunnel cluster mode ± soft TO mode interaction which induces the ferroe... more ... The latter allows the tunnel cluster mode ± soft TO mode interaction which induces the ferroelectric phase transition. The co-existence of the displacive type dynamics discussed above in an incipientfer-roelectric and impurities with central peak behaviour is obtained. ...
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2002
We review results of our recent large-scale computer simulations of radiation-induced point defec... more We review results of our recent large-scale computer simulations of radiation-induced point defects, excitons and polarons in ABO 3 perovskite crystals, focusing mostly on KNbO 3 and KTaO 3 as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption, defect-induced electron density redistribution, and activation energies for defect migration. The majority of our results were obtained using the quantumchemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree-Fock formalism, as well as the shell model (SM). The main findings are compared with those obtained by means of ab initio density functional theory (FP-LMTO) first-principles calculations.
Materials Science in Semiconductor Processing, 2002
ABSTRACT We use quantum chemical method of the intermediate neglect of the differential overlap (... more ABSTRACT We use quantum chemical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree–Fock formalism for a large-scale modelling of the atomic and electronic structure of KNbxTa1−xO3 (KTN) perovskite solid solutions. Results for periodic defect models (large unit cell) of 40 and 320 atoms are compared with 135-atom INDO cluster calculations. Periodic Nb impurities in KTaO3 reveal clear off-center displacement beginning with the smallest calculated concentrations, so does an isolated Nb impurity in a cluster INDO calculation. The magnitude of Nb off-center displacement is close to the X-ray-absorption fine-structure observation (0.27a.u.). In contrast, Ta impurities in KNbO3 always remain on-center, due to higher ionicity of Ta as compared to Nb atom. Using the calculated energy gain due to the off-center displacements of Nb atoms for several concentrations of Nb in KTN, we construct the non-empirical Ginzburg–Landau-type functional for the excess energy. The coefficients of this functional are concentration dependent. This dependence allows to define the type of the concentration-induced phase transition in KTaO3 alloyed by Nb.
We report the results of ab initio calculations and analysis of systematic trends for the F cente... more We report the results of ab initio calculations and analysis of systematic trends for the F centers in the bulk and on the (001) surface in oxide perovskites, such as BaTiO 3 , SrTiO 3 , SrZrO 3 , and PbZrO 3 , with a corresponding comparison of the F centers in perovskites with those in alkaline earth metal fluorides (CaF 2 , BaF 2 , and SrF 2). It was found that in perovskites in both bulk F centers and those on their (001) surfaces, two nearest to the vacancy Ti or Zr atoms repel each other, while the next nearest O atoms relax towards the oxygen vacancy. It was also found that the obtained relaxations of atoms in the nearest neighborhood around the F center in ABO 3 perovskites are generally larger than in alkaline earth metal fluorides. The bulk and (001)-terminated surface F center ground states in BaTiO 3 , SrTiO 3 , and SrZrO 3 perovskites are located 0.23, 0.69, 1.12 eV, and 0.07, 0.25, 0.93 eV, respectively, below the conduction band bottom, indicating that the F center is a shallow donor. The vacancies in BaTiO 3 , SrZrO 3 , and PbZrO 3 are occupied with 1.103e, 1.25e, and 0.68e, respectively, whereas slightly smaller charges, only 1.052e, 1.10e, and 0.3e are localized inside the F center on the perovskite (001) surface. In contrast to the partly covalent ABO 3 perovskites, charge is well localized (around 80 %) inside the ionic CaF 2 , BaF 2 , and SrF 2 fluorine vacancy.
The singly B and N doped graphene systems are carefully studied. The highly concentrated dopants ... more The singly B and N doped graphene systems are carefully studied. The highly concentrated dopants cause a spin polarization effect in the systems. The spin polarization limits are affirmed in the singly B and N doped graphene systems through periodic hybrid density functional theory studies. The spin polarization effects must be considered indeed in the B and N doped graphene systems if the dopant concentration is above 3.1% and 1.4%, respectively. The system symmetry cooperating with the presence of the spin polarization brings half-metallic properties into the doping systems. The semiconducting channels in the half-metallic systems are in two different spin directions due to the different electron configurations of the B and N dopants in graphene.
Abstract The equation of state (EOS) from virial expansion (VE) is used in this work to pave the ... more Abstract The equation of state (EOS) from virial expansion (VE) is used in this work to pave the way for determining the fugacity coefficients of the hydrogen fluid at arbitrary temperature and pressure. The fugacity coefficients from our VE method have more physical meanings than the empirical values. In this way, the hydrogen storage capacity of a novel material model can be estimated by using few density functional theory (DFT) calculations with the aid of a continuum model. The efficient continuum model can provide a more accurate estimation of the hydrogen storage capacity than the pure DFT calculations. Furthermore, the expensive grand canonical ensemble (μNT) simulations combining with the quantum mechanics methods (i.e., QM/MD-μNT) are unnecessary within this method. The hydrogen fluid can be handled with our VE method at the temperature in the range of 160–773 K. The hydrogen storage capacity and the detailed thermodynamic information of a designed novel material can thereby be estimated by using this method with relatively high accuracy and low computing cost. As an example, the hydrogen storage capacities of the expanded bilayer graphene systems are presented. Our theoretical results agree with the experimental values very well.
Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) m... more Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) method, combined with the large unit cell periodic model argue for an existence of the self-trapped electrons in KNbO3 and KTaO3 perovskite crystals. An electron in the ground state occupies predominantly t2g orbital of a Nb4+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller modes where four nearest equatorial O atoms are displaced outwards and two oxygens shift inwards along the z axis. Triplet exciton is shown to be in a good approximation of a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron) which is very likely responsible for the 'green' luminescence observed in these crystals.
We carried out ab initio calculations for technologically important ABO3 perovskite, such as, SrT... more We carried out ab initio calculations for technologically important ABO3 perovskite, such as, SrTiO3, BaTiO3, SrZrO3 and PbZrO3 (001), (011) and (111) nano-surfaces, interfaces and bulk [Formula: see text]-centers. For SrTiO3, BaTiO3, SrZrO3 and PbZrO3 (001) nano-surfaces, as a rule, all first surface layer atoms relax inward, whereas all second layer atoms relax outward, and, typically, all third surface layer atoms, again, relax inward. Calculated (001) surface energies for SrTiO3, BaTiO3, SrZrO3 and PbZrO3 perovskites are almost equal for both BO2 and AO-terminations, and always smaller than the (011) and (111) surface energies. We discussed the results of our ab initio calculations dealing with BaTiO3/SrTiO3 and SrZrO3/PbZrO3 (001) interfaces. We analyzed systematic trends in ABO3 perovskite bulk [Formula: see text]-center ab initio calculations.
Proceedings of X International Scientific and Practical Conference "Electronics and Information Technologies", 2018
We performed first-principles calculations for technologically most important ABO 3 perovskites, ... more We performed first-principles calculations for technologically most important ABO 3 perovskites, such as, CaTiO 3 , SrTiO 3 , PbTiO 3 , BaTiO 3 , SrZrO 3 and PbZrO 3 (001), (011) and (111) surfaces, interfaces and bulk F-centers. For ABO 3 perovskite neutral (001) surfaces, in most cases, all upper surface layer atoms relax inward, whereas the all second surface layer atoms relax outward, and again, all third surface layer atoms relax inward. The relaxation pattern for YAlO 3 charged (001) surfaces is quite different from ABO 3 perovskite neutral (001) surfaces. The ABO 3 perovskite (001) surface energies are almost equal for both AO and BO 2-terminations, and always considerably smaller than the (011) and especially (111) surface energies. Systematic trends in BaTiO 3 , SrTiO 3 , SrZrO 3 and PbZrO 3 bulk F-center calculations are analyzed.
... Recommended Citation. Eglitis, R.; Kotomin, EA; Devreese, Jozef; Popov, AL - The atomic and e... more ... Recommended Citation. Eglitis, R.; Kotomin, EA; Devreese, Jozef; Popov, AL - The atomic and electronic structure of bound hole polarons and bipolarons in MgO-crystals - In: Proceedings of the International Conference on Defects in Insulating Materials. ...
We present and discuss the results of calculations of surface relaxations and rumplings for the (... more We present and discuss the results of calculations of surface relaxations and rumplings for the (001) and (011) surfaces of BaTiO3 and PbTiO3, using a hybrid B3PW description of exchange and correlation. On the (001) surfaces, we consider both AO (A = Ba or Pb) and TiO2 terminations. In the former case, the surface AO layer is found to relax inward for both materials, while outward relaxations of all atoms in the second layer are found at both kinds of (001) terminations and for both materials. The surface relaxation energies of BaO and TiO2 terminations on BaTiO3 (001) are found to be comparable, as are those of PbO and TiO2 on PbTiO3 (001), although in both cases the relaxation energy is slightly larger for the TiO2 termination. As for the (011) surfaces, we consider three types of surfaces, terminating on a TiO layer, a Ba or Pb layer, or an O layer. Here, the relaxation energies are much larger for the TiO-terminated than for the Ba or Pb-terminated surfaces. The relaxed surface energy for the O-terminated surface is about the same as the corresponding average of the TiO and Pb-terminated surfaces on PbTiO3, but much less than the average of the TiO and Ba-terminated surfaces on BaTiO3. We predict a considerable increase of the Ti-O chemical bond covalency near the BaTiO3 and PbTiO3 (011) surface as compared to both the bulk and the (001) surface.
We present the results of calculations of SrTiO 3 and CaTiO 3 polar (111) surface relaxations, ru... more We present the results of calculations of SrTiO 3 and CaTiO 3 polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO 3 or CaO 3) of the SrTiO 3 and CaTiO 3 (111) surfaces. According to our calculations, atoms of the first surface layer relax inwards for Ti-, SrO 3-, and CaO 3-terminated (111) surfaces of both materials. The only exception is outward relaxation of the SrO 3-terminated SrTiO 3 (111) surface upper layer Sr atom. For both Ti-terminated SrTiO 3 and CaTiO 3 (111) surfaces our calculated second layer Sr and Ca metal atom inward relaxations are approximately four and two times larger than the upper layer Ti atom inward relaxation. Our calculated optical band gap for the SrO 3-and Ti-terminated SrTiO 3 , as well as for CaO 3-terminated CaTiO 3 (111) surfaces, becomes smaller with respect to the bulk optical band gap. Our calculated surface energies for SrO 3-and CaO 3-terminated SrTiO 3 and CaTiO 3 (111) surfaces (6.30 and 5.86 eV) are considerably larger than the surface energies for Ti-terminated SrTiO 3 and CaTiO 3 (111) surfaces (4.99 and 4.18 eV). Our B3LYP calculations indicate a considerable increase of Ti-O chemical bond covalency near the SrTiO 3 and CaTiO 3 (111) surface (þ0.098e and þ0.094e) relative to the SrTiO 3 and CaTiO 3 bulk (þ0.088e and þ0.084e).
The accuracy of the Content should not be relied upon and should be independently verified with p... more The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
The semi-empirical Hartree-Fock intermediate neglect of differential overlap (INDO) method was us... more The semi-empirical Hartree-Fock intermediate neglect of differential overlap (INDO) method was used to explore the potential energy surface of adsorbed water molecules and to speculate, using fairly simple cluster models, on the reaction path followed by metal ions leaving the surface. As an example, INDO calculations were used to calculate the potential energy curve for a metal atom leaving the
The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Int... more The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Intermediate Neglect of the Differential Overlap (INDO) based on the Hartree-Fock formalism are combined for the study of the hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of KNbO3 perovskite crystals. The 40-atom and 320-atom supercells were used, respectively. We predict existence of both, one-site and two-site (molecular) polarons with close optical absorption energies (0.9 eV and 0.95 eV). The relevant experimental data are discussed.
ABSTRACT We present a first-principles study of BaTiO3/SrTiO3(001) interfaces taking into account... more ABSTRACT We present a first-principles study of BaTiO3/SrTiO3(001) interfaces taking into account non-stoichiometric compositions. By means of hybrid exchange–correlation functional within density functional theory (DFT) we demonstrate that charge redistribution in the interface region weakly affects the electronic structure of studied material, while change in the stoichiometry (termination of deposited BaTiO3(001) thin film) yields in significant shifts of band edges. The optical band gap of BaTiO3/SrTiO3(001) interface depends mostly on BaO or TiO2 termination of the upper layer. Based on results of our calculations we predict enhancement of the Ti–O chemical bond covalency near the BaTiO3/SrTiO3(001) interface as compared to the BaTiO3 bulk.
Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) ... more Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) and a supercell model, we calculated from the first principles the electronic structure of both ideal PbZrO 3 (001) surface (with ZrO 2-and PbO-terminations) and a neutral oxygen vacancy also called the F center. The atomic relaxation and electronic density redistributions are discussed. Thermodynamic analysis of pure surfaces indicates that ZrO 2 termination is energetically more favorable than PbO-termination. The O vacancy on the ZrO 2-surface attracts E0.3 e (0.7 e in the bulk PbZrO 3), while the remaining electron density from the missing O 2À ion is localized mostly on atoms nearest to a vacancy. The calculated defect formation energy is smaller than in the bulk which should lead to the vacancy segregation to the surface. Unlike Ti-based perovskites, the vacancy-induced (deep) energy level lies in PbZrO 3 in the middle of the band gap.
... The latter allows the tunnel cluster mode ± soft TO mode interaction which induces the ferroe... more ... The latter allows the tunnel cluster mode ± soft TO mode interaction which induces the ferroelectric phase transition. The co-existence of the displacive type dynamics discussed above in an incipientfer-roelectric and impurities with central peak behaviour is obtained. ...
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2002
We review results of our recent large-scale computer simulations of radiation-induced point defec... more We review results of our recent large-scale computer simulations of radiation-induced point defects, excitons and polarons in ABO 3 perovskite crystals, focusing mostly on KNbO 3 and KTaO 3 as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption, defect-induced electron density redistribution, and activation energies for defect migration. The majority of our results were obtained using the quantumchemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree-Fock formalism, as well as the shell model (SM). The main findings are compared with those obtained by means of ab initio density functional theory (FP-LMTO) first-principles calculations.
Materials Science in Semiconductor Processing, 2002
ABSTRACT We use quantum chemical method of the intermediate neglect of the differential overlap (... more ABSTRACT We use quantum chemical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree–Fock formalism for a large-scale modelling of the atomic and electronic structure of KNbxTa1−xO3 (KTN) perovskite solid solutions. Results for periodic defect models (large unit cell) of 40 and 320 atoms are compared with 135-atom INDO cluster calculations. Periodic Nb impurities in KTaO3 reveal clear off-center displacement beginning with the smallest calculated concentrations, so does an isolated Nb impurity in a cluster INDO calculation. The magnitude of Nb off-center displacement is close to the X-ray-absorption fine-structure observation (0.27a.u.). In contrast, Ta impurities in KNbO3 always remain on-center, due to higher ionicity of Ta as compared to Nb atom. Using the calculated energy gain due to the off-center displacements of Nb atoms for several concentrations of Nb in KTN, we construct the non-empirical Ginzburg–Landau-type functional for the excess energy. The coefficients of this functional are concentration dependent. This dependence allows to define the type of the concentration-induced phase transition in KTaO3 alloyed by Nb.
We report the results of ab initio calculations and analysis of systematic trends for the F cente... more We report the results of ab initio calculations and analysis of systematic trends for the F centers in the bulk and on the (001) surface in oxide perovskites, such as BaTiO 3 , SrTiO 3 , SrZrO 3 , and PbZrO 3 , with a corresponding comparison of the F centers in perovskites with those in alkaline earth metal fluorides (CaF 2 , BaF 2 , and SrF 2). It was found that in perovskites in both bulk F centers and those on their (001) surfaces, two nearest to the vacancy Ti or Zr atoms repel each other, while the next nearest O atoms relax towards the oxygen vacancy. It was also found that the obtained relaxations of atoms in the nearest neighborhood around the F center in ABO 3 perovskites are generally larger than in alkaline earth metal fluorides. The bulk and (001)-terminated surface F center ground states in BaTiO 3 , SrTiO 3 , and SrZrO 3 perovskites are located 0.23, 0.69, 1.12 eV, and 0.07, 0.25, 0.93 eV, respectively, below the conduction band bottom, indicating that the F center is a shallow donor. The vacancies in BaTiO 3 , SrZrO 3 , and PbZrO 3 are occupied with 1.103e, 1.25e, and 0.68e, respectively, whereas slightly smaller charges, only 1.052e, 1.10e, and 0.3e are localized inside the F center on the perovskite (001) surface. In contrast to the partly covalent ABO 3 perovskites, charge is well localized (around 80 %) inside the ionic CaF 2 , BaF 2 , and SrF 2 fluorine vacancy.
The singly B and N doped graphene systems are carefully studied. The highly concentrated dopants ... more The singly B and N doped graphene systems are carefully studied. The highly concentrated dopants cause a spin polarization effect in the systems. The spin polarization limits are affirmed in the singly B and N doped graphene systems through periodic hybrid density functional theory studies. The spin polarization effects must be considered indeed in the B and N doped graphene systems if the dopant concentration is above 3.1% and 1.4%, respectively. The system symmetry cooperating with the presence of the spin polarization brings half-metallic properties into the doping systems. The semiconducting channels in the half-metallic systems are in two different spin directions due to the different electron configurations of the B and N dopants in graphene.
Abstract The equation of state (EOS) from virial expansion (VE) is used in this work to pave the ... more Abstract The equation of state (EOS) from virial expansion (VE) is used in this work to pave the way for determining the fugacity coefficients of the hydrogen fluid at arbitrary temperature and pressure. The fugacity coefficients from our VE method have more physical meanings than the empirical values. In this way, the hydrogen storage capacity of a novel material model can be estimated by using few density functional theory (DFT) calculations with the aid of a continuum model. The efficient continuum model can provide a more accurate estimation of the hydrogen storage capacity than the pure DFT calculations. Furthermore, the expensive grand canonical ensemble (μNT) simulations combining with the quantum mechanics methods (i.e., QM/MD-μNT) are unnecessary within this method. The hydrogen fluid can be handled with our VE method at the temperature in the range of 160–773 K. The hydrogen storage capacity and the detailed thermodynamic information of a designed novel material can thereby be estimated by using this method with relatively high accuracy and low computing cost. As an example, the hydrogen storage capacities of the expanded bilayer graphene systems are presented. Our theoretical results agree with the experimental values very well.
Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) m... more Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) method, combined with the large unit cell periodic model argue for an existence of the self-trapped electrons in KNbO3 and KTaO3 perovskite crystals. An electron in the ground state occupies predominantly t2g orbital of a Nb4+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller modes where four nearest equatorial O atoms are displaced outwards and two oxygens shift inwards along the z axis. Triplet exciton is shown to be in a good approximation of a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron) which is very likely responsible for the 'green' luminescence observed in these crystals.
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Papers by R. Eglitis