A DFT study of structural and electronic properties of anatase TiO2 phase with Ni and oxygen vacancies impurities

In the present work, first-principle theory using the functional density theory (DFT) was used in the ABINIT software package using the PBE pseudopotential (norm-conserving pseudopotentials). To determine the structural parameters such as lattice constant, Bulk modules, and energy formation, for the TiO2 anatase phase doped with substitutional and interstitial nickel impurity, oxygen vacancies (VO) are also included in the present work. For this study, the 2x1x1 supercells with 24 atoms and 2x2x1 with 48 atoms were used. Different types of Ni dopants and oxygen vacancies were considered for energy formation using the 2x2x1 supercell. Our results show that the values of network parameters, minimum energy, and Bulk modulus remain constant with the supercell's growth. With the inclusion of Ni in the supercell substituting the Ti-ions, the unit cell volume (V) exhibits a decrease in agreement with ionic radii mismatch between Ti and Ni atoms. However, when entry as an interstitial form a significant increase is shown. The preliminary results of the energy of formation analyzed for the Ni defects show that it is more probable for an interstitial Ni than for a substitutional Ni.

A DFT study of structural and electronic properties of anatase TiO2 phase with Ni and oxygen vacancies impurities

In the present work, first-principle theory using the functional density theory (DFT) was used in the ABINIT software package using the PBE pseudopotential (norm-conserving pseudopotentials). To determine the structural parameters such as lattice constant, Bulk modules, and energy formation, for the TiO2 anatase phase doped with substitutional and interstitial nickel impurity, oxygen vacancies (VO) are also included in the present work. For this study, the 2x1x1 supercells with 24 atoms and 2x2x1 with 48 atoms were used. Different types of Ni dopants and oxygen vacancies were considered for energy formation using the 2x2x1 supercell. Our results show that the values of network parameters, minimum energy, and Bulk modulus remain constant with the supercell's growth. With the inclusion of Ni in the supercell substituting the Ti-ions, the unit cell volume (V) exhibits a decrease in agreement with ionic radii mismatch between Ti and Ni atoms. However, when entry as an interstitial form a significant increase is shown. The preliminary results of the energy of formation analyzed for the Ni defects show that it is more probable for an interstitial Ni than for a substitutional Ni.

Synthesis of Europium Sulfides by CS2 Sulfurization and Heat Treatment

ABSTRACTThe formation of europium sulfides (Eu3S4 and EuS) via CS2 gas sulfurization of Eu2O3 powder was investigated. Single-phase Eu3S4 was obtained via CS2 gas sulfurization of Eu2O3 at 773 K for periods longer than 0.5 hr and EuS was generated by sulfurization at 1073 K for 8 hr. The higher sulfurization temperature and shorter sulfurization time accelerated the formation of high purity EuS. Synthetic pure Eu3S4 was treated under vacuum, rich-sulfur atmosphere and inert atmosphere, respectively. EuS was obtained when heat treatment was performed at 873 K under vacuum, at 973 K under CS2/Ar atmosphere, and at 1073 K under Ar atmosphere. Heat treatment of Eu3S4 generated a small amount of Eu2O2S due to impurity oxygen. Formation of Eu2O2S could be inhibited by heat treatment under CS2/Ar atmosphere.

Superconducting Parent Compound Pr2CuO4 Achieved by Special Post-Reduction

ABSTRACTIt is commonly believed that the parent compounds of high-Tc cupratres are, universally, charge transfer insulators and triggered by Mott physics. In our experiments using metal-organic decomposition (MOD), however, accumulating evidences show that the parent compounds of “electron-doped” superconductors, RE2-xCexCuO4 [RE = rare earth ion] with x = 0, are not Mott insulators but superconductors [1-5]. They have a Tc of 30 K and crystallize in the Nd2CuO4 (T’) structure. Most likely, the sharp contradiction between our results and commonly achieved data originates from the complicated oxygen chemistry in these materials. The as-synthesized specimens contain a fair amount of impurity interstitial oxygen. Throughout the reduction process it is required to remove exclusively impurity oxygen while preserving regular oxygen site occupied in order to obtain superconductivity. With decreasing x the constraints of the reduction process are getting more tight. In this study, we systematically investigated the post-annealing process using MBE-grown T’-Pr2CuO4 films. The MBE films were reduced ex-situ in a tubular furnace following a specially designed 2-step process, as in the case of MOD films. The films were annealed at Ta = 700 - 850°C in a reducing atmosphere (PO2 = 2 x 10−5 − 2 x 10−3 atm) and finally reduced at a lower temperature Tred = 450 – 700°C under vacuum (< 10−4 Torr). The film properties systematically changed with Ta, PO2, and Tred. The optimized Tred varies from 475°C to 650°C mainly depending on Ta, since the microstructure and grain size of the films are determined by Ta. Optimal superconducting properties are Tc of 26 K, while ρ(300 K) = 250 μΩcm, and RRR ~ 10. We believe the combination of thin-film synthesis and specially designed post-reduction process enabled us to obtain nearly intact CuO2 planes. Samples prepared by above-mentioned method unveiled the intrinsic properties of the parent compounds, which are not triggered by Mott physics. This result also agrees with the recent calculation result indicating the parent compounds with T’ structure are not charge transfer insulators [6-8].