Enhancing photocatalytic properties of rutile TiO2 by codoping with N and metals - Ab initio study

Substitutional N to O and M to Ti (M = Pt, V, Sb) codoped rutile TiO2 was investigated using density functional theory (OFT) based calculations with both standard and hybrid exchange-correlation functionals. The band gaps calculated using generalized gradient approximation (GGA) exhibited narrowing compared to the pure rutile TiO2 in all the investigated cases. In contrast, the results obtained with hybrid exchange-correlation functional showed that there was no band gap narrowing, but doping induced localized states within the band gap just above the valence band, as well as below the conduction band for Pt doped TiO2. The presence of broad intermediate states (IS) in the band gap could enhance visible light absorption through a two step optical transition from the valence to the conduction band via the IS and at the same time lower recombination of the photo-generated charges. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Enhanced superconductivity and electron correlations in intercalated ZrTe3

Charge density waves (CDWs) with superconductivity, competing Fermi surface instabilities, and collective orders have captured much interest in two-dimensional van der Waals (vdW) materials. Understanding the CDW suppression mechanism, its connection to the emerging superconducting state, and electronic correlations provides opportunities for engineering the electronic properties of vdW heterostructures and thin-film devices. Using a combination of the thermal transport, x-ray photoemission spectroscopy, Raman measurements, and first-principles calculations, we observe an increase in electronic correlations of the conducting states as the CDW is suppressed in ZrTe3 with 5% Cu and Ni intercalation in the vdW gap. As superconductivity emerges, intercalation brings not only decoupling of quasi-one-dimensional conduction electrons with phonons as a consequence of intercalation-induced lattice expansion but also a drastic increase in Zr2+ at the expense of Zr4+ metal atoms. These observations not only demonstrate the potential of atomic intercalates in the vdW gap for ground-state tuning but also illustrate the crucial role of the Zr metal valence in the formation of collective electronic orders

Improving the photocatalytic activity of tetragonal BiVO4 with zircon-type structure through W doping; Ab initio calculations

In this paper we studied the effects of the concentration of W as a doped atom on the V lattice site on the electronic and optical properties of tetragonal zircon-type BiVO4 structure. The calculations were performed by the first-principles density functional theory WIEN2k code. The doping strategy was targeting V atoms substituted by W atoms using the same lattice parameters as pure BiVO4 structure. To avoid the self interaction of impurities, the supercell method was adopted ensuring a sufficient length between the impurities in all directions. For all considered concentrations, the optical properties in the visible light range of λ≥550nm are improved over the undoped BiVO4. © 2021 Elsevier B.V

Ab initio calculations of the structure, energetics and stability of AunTi (n=1-32) clusters

The stability and structure of titanium doped gold clusters (AunTi; n = 1-32) are studied by density functional theory calculations, as implemented in the first principles code SIESTA. The exchange and correlation effects were calculated within the generalized gradient approximation (GGA) parametrized by Perdew, Burke and Ernzerhof (PBE). We used norm conserving Troullier-Martins pseudopotentials for the 10-electron valence configuration of Ti and 11-electron valence configuration of Au. All calculations were spin-polarized. The global energy minimum geometries of the clusters were searched for by using the simulated annealing technique. The stability of the clusters is discussed on the basis of the binding energy per atom, second-order energy difference, vertical ionization potential, vertical electron affinities, HOMO-LUMO energy gap and vibrational frequencies. Based on the simultaneous criteria of high binding energy, high band gap, high vertical ionization potential, and low electron affinity, it is found that Au4Ti and Au14Ti clusters have a higher stability and are candidates for magic clusters, which confirms the already known results from previous works. The new result presented in this paper is that the Au20Ti and Au30Ti clusters have a higher stability too. In general, the clusters with even n are more stable than the clusters with odd n. Most of the clusters with even n are non-magnetic (total magnetic moment is zero). Our results also suggest that only the Au3Ti, Au7Ti and Au8Ti clusters have a planar structure. (C) 2016 Elsevier B.V. All rights reserved

First principles study of HfV2 and ZrV2 phases: Structural analysis and site preference of Cd and Ta dopants

We present first principles calculations of the electric field gradients (EFGs) in the pure cubic, tetragonal and orthorhombic phases of HfV2 and in the cubic phase of ZrV2. Band structure calculations of orthorhombic and tetragonal HfV2 are presented for the first time. EFGs are also calculated on the inserted Ta and Cd probe atoms, and results are compared with the existing experimental data from time differential perturbed angular correlations and nuclear magnetic resonance measurements. By this comparison, it was possible to determine the exact space group and crystallographic positions of orthorhombic HfV2, and the site preferences of the Ta and Cd probe atoms. We have also confirmed that the measured nonzero EFG in the Ta-doped cubic HfV2 structure originates from the displacement of Ta atoms from their equilibrium positions in the cubic environment. (C) 2012 Elsevier Ltd. All rights reserved

Zirconium aluminides studied with first principles calculations: Hyperfine interactions and site preference of dopants

By means of a density functional theory (DFT) based augmented plane waves plus local orbitals (APW ​+ ​lo) method, we study the electric field gradients (EFG) in Ta and Cd-doped Zr–Al intermetallics. Comparing the obtained results with the experimental data obtained from the perturbed angular correlation (PAC) measurements, we conclude that Ta atoms always replace Zr in all the investigated compounds. Our results confirmed the previous experimental assumption that Cd substitutes exclusively for Al in ZrAl3 and Zr2Al3. In the case of Zr2Al our calculations suggest that Cd can probably substitute on both Zr and Al lattice sites, while in Zr3Al, it is most likely to occupy Al position. The effects of the distance between the impurity atoms in the supercells and the deviation of the c/a ratio are also discussed

Hydrogen Storage in Martensite Ti-Zr-Ni Alloy: A Density Functional Theory Study

The hydrogen storage potential of TiNi-based shape memory alloys is an attractive but experimentally challenging issue. We employ the FP (L)APW+lo method, based on the density functional theory (DFT), in order to address the electronic structure of the low-temperature, martensitic phase of Ti0.67Zr0.33Ni and its hydrides. Further, the thermodynamics of hydride formation in the martensitic Ti-Zr-Ni alloys is studied, and some unanswered questions regarding the hydriding of martensite TiNi are resolved. The calculated formation energy of the orthorhombic beta- and gamma-hydrides of martensitic Ti0.67Zr0.33Ni is, respectively, -14.2 kJ/mol(H) and -29.6 kJ/mol(H), showing that isostructural (Ti,Zr)Ni hydrides with a larger amount of titanium have improved potential for hydrogen storage applications. Furthermore, based on the calculation results, it is unlikely that the hydriding of TiNi martensite would lead to the formation of orthorhombic beta-hydride in analogy to the pseudobinary compound. The formation of Ti0.67Zr0.33Ni hydrides leads to significant changes in electronic structure, causing shifting of some metal states to lower energies due to their interaction with hydrogen s states, and the increase of the number of states at Fermi energy. By modeling intermediate structures, the process of hydride formation in martensitic Ti0.67Zr0.33Ni is resolved to reveal the effects of crystal structure change, volume increase, and hydrogen-metal interactions on the band structure, charge transfer, and thermodynamics. A dominant, stabilizing effect in the process of hydride formation was found to come from the chemical interaction of hydrogen and metal atoms

Site occupation preferences in multicomponent semiconductors: the (Cd1-xxTe)-Te-Zn case

Various models of site occupation preferences (SOPs) in multicomponent semiconducting compounds of zincblende structure type are discussed for the Cd1-xZnxTe case using our new experimental and calculated data. In order to resolve the microscopic mechanism of the phenomenon, appropriate two-body potentials and stability of all nearest neighbor (NN) tetrahedral configurations that can arise in the structure are computed using the ab-initio Linear Combination of Atomic Orbitals (LCAO) method. The obtained results are related to some known material properties and existing systematics.Progress in Advanced Materials and Processes, 5th Conference of the Yugoslav-Materials-Research-Society (Yu-MRS 2003), Sep 15-19, 2003, Herceg Novi, Yugoslavi

Structure identification and site preference of Ta and Cd in Ti-Pd alloys: A first-principle study

By means of a density functional theory approach, we studied the electric field gradients (EFG) in Ta and Cd-doped Ti-Pd intermetallics. Our results confirmed the previous experimental findings that the TiPd2 low-temperature structure is orthorhombic and established that Ta substitutes for Ti in this structure. The temperature increase above 650 K changes the Ta impurity position in the lattice. Similar changes for the Cd doped system were not confirmed, as Cd is most likely to occupy Pd lattice sites in both low and high-temperature phases. In the case of TiPd, our calculations suggested that Ta substitutes for Ti in the low-temperature phase, while Cd probably can substitute on both Ti and Pd crystallographic sites. (C) 2012 Elsevier Ltd. All rights reserved

Hydrogen storage in Laves phases: First principles study of electronic structure and formation energies in HfV2 hydrides

We present first principles calculations of the electronic structure, enthalpies of formation and electric field gradients in C15 HfV2Hx (x = 1, 2, 3, 4). In C15 Laves phases, hydrogen can occupy three possible interstitial positions: 96g, 32e, and 8b. To determine which one of these interstitials is the most favorable for storing hydrogen, enthalpies of formation were calculated for every site, with different concentrations of hydrogen. In order to investigate the change in electronic structure before and after hydrogenation, we calculated the electric field gradients induced by hydrogen on the vanadium, and compared them with the existing nuclear magnetic resonance measurements. This comparison enabled us to study the distribution of hydrogen atoms in the crystal lattice, as well as the occupation of possible interstitials. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved