PBEsol Functionals Research Articles

Pressure and temperature dependent ab-initio quasi-harmonic thermoelastic properties of tungsten

We present the ab-initio temperature and pressure dependent thermoelastic properties of body-centered cubic tungsten. The temperature dependent quasi-harmonic elastic constants (ECs) are computed at several reference volumes including both the phonon and the electronic excitations contribution to the free energy and interpolated at different temperatures and pressures. Good agreement with the experimental ECs on a single crystal at ambient pressure is found. The pressure and temperature dependence of the shear sound velocity measured on polycrystalline tungsten by Qi et al is also in agreement with theory. Some discrepancies are found instead for the compressional velocity at high temperature and this is attributed to the temperature derivative of the bulk modulus, higher in theory than in experiment. These conclusions are reached both by PBE and by PBEsol functionals. The two give elastic properties with a similar pressure and temperature dependence although the latter is closer to experiment at 0 K.

Schwarz P‐surface via isolated sp2 carbon heptagons: Design and properties

Described are the first two molecular designs of the triply periodic Schwarz P surface using merely the Schläfli t{3,7} pattern of sp2 -hybridized carbon atoms. Each atom is exactly part of one heptagon and two hexagon rings so that two heptagons do not share the same edge or vertex. Such pattern, called hyperbolic soccer ball obeys the isolated-heptagon rule with the minimum possible number of hexagons between heptagons similar to the isolation of pentagons from hexagons in the C60 fullerene. Both of the designed P surfaces are unbalanced, that is, they have two unequal sides, and belong to space groups P432 of the cubic system, and P4/ncc of the tetragonal system, respectively. Unit cells have a multiple of 24 heptagons similar to the only one previously known in literature Schwarzite with the hyperbolic soccer ball pattern-the D surface of Vanderbilt and Tersoff. The geometry of the periodic structures and unit cell parameters were fully optimized by DFT calculations using CASTEP software with PBE and PBESOL functionals under generalized gradient approximation. The effect of P and D surface dilution by hexagons on the calculated density, elastic and electronic properties is discussed.

Itinerant magnetism, electronic properties and half-metallicity of Co2ZrSn and Co2HfSn Heusler alloys

Half-metallic ferromagnetic alloys are attracting considerable interest for their potential applications in spintronic devices. Co-based Heusler alloys are considered to be promising half-metallic compounds as they combine suitable magnetic, electronic and transport properties with compositional versatility and high thermal stability. In this work, Co 2 ZrSn and Co 2 HfSn Heusler alloys were studied by combining experimental and ab-initio investigations in order to accurately estimate their electronic density of states in proximity of the Fermi level and to determine their magnetic and electronic properties. Magnetization measurements, performed between 2 K and 550 K, suggest for both alloys a gradual transition from localized ferromagnetism to weak itinerant ferromagnetism with increasing temperature, well described by a simple mean field model up to the Curie temperature (454 K in Co 2 ZrSn and 432 K in Co 2 HfSn) and beyond. Ab-initio calculations were performed using two exchange-correlational functionals, PBE and PBE optimized for solids (PBEsol), in order to assess the reproducibility of theoretical results. Overall, band structures and density of states (DOS) diagrams indicated, for both compounds, the presence of a half-metallic band gap in the minority spin sub-band. The dependence of magnetic moments and band gap width on cell parameters is discussed in detail. Calculated magnetic moments and cell parameters satisfactorily match the experimental results obtained in this work and previously reported in the literature. • Itinerant weak ferromagnetic behaviour is confirmed for both alloys as they follow the Edwards-Wohlfarth model. • The mean field model indicates that the density of states profiles at the Fermi level are smooth. • Ab-initio calculation performed with PBE and PBEsol functionals show half-metallic behaviour at 0 K. • A crossover from localized magnetism to itinerant magnetism as function of temperature was found for both alloys. • The M/T crossover can be attributed to a change in spin-flip mechanism at low temperature.

Electronic structure transition of cubic CsSnCl3 under pressure: effect of rPBE and PBEsol functionals and GW method

The antiperovskites based on metal halides have emerged as potential materials for advanced photovoltaic and electronic device applications. But the wide bandgap of non-toxic CsSnCl3 reduces its photovoltaic efficiency. Here, we report the change of electronic structure of CsSnCl3 at different pressure by using GGA-rPBE and GGA-PBEsol functionals and the GW method. We have shown that the prediction of electronic structure transition (semiconducting to metallic state) strongly depends on the exchange-correlation and the GW method gives the most reasonable values of the bandgap under pressure. The pressure increases the electronic density of states close to the Fermi level by pushing the valence electrons upward and thus, reduces the bandgap linearly. Afterward, we have also investigated the influence of pressure on absorption coefficient, and mechanical properties meticulously. Although the pressure shifts the absorption peak to lower photon energies, the absorption coefficient is slightly improved.

Structural, electronic and vibrational properties of LaF3 according to density functional theory and Raman spectroscopy

Crystal structure of LaF3 single crystal is refined in tysonite-type trigonal unit cell Pc1 using density functional theory calculations and Raman spectroscopy. It is shown that trigonal structure with Pc1 space group is more energy-efficient than hexagonal structure with space group P63cm. Simulated Raman spectra obtained using LDA approximation is in much better agreement with experimental data than that obtained with PBE and PBEsol functionals of GGA. The calculated frequency value of silent mode B2 in case of hexagonal structure P63cm was found to be imaginary (unstable mode), thus the energy surface obtains negative curvature with respect to the corresponding normal coordinates of the mode which leads to instability of the hexagonal structure in harmonic approximation. The A1g line at 214 cm−1 in Raman spectra of LaF3 related to the translation of F2 ions along c axis can be connected with F2 ionic conductivity.

PbI2 Thin-Films for Photoelectrochemical Hydrogen Evolution

PbI2 has become a highly prominent material in the field of photovoltaics as the key precursor in the fabrication of organo-halide perovskites films [1]. Despite intensive research in this area, the role of unconverted PbI2 in perovskite cells remains controversial. In fact, very little is known about the behaviour of PbI2 as a photocathode. In this contribution, we examine the photoelectrochemical properties of high quality PbI2 thin films obtained by gas phase anion replacement [2]. The preparation of PbI2 films is initiated by growing a thin PbS film on F:SnO2 electrodes, followed by spontaneous gas phase iodination at 200 ˚C. The structure and optical properties of the thin films were investigated by XRD, SEM, Raman Spectroscopy, diffuse reflectance and room temperature photoluminescence. The structural and spectroscopic information, supported by DFT calculations (PBESOL functionals), demonstrated the formation of the 2H- phase of PbI2 with a high degree of purity and a band gap of 2.4 eV. Electrochemical impedance spectroscopy in KI containing solutions showed a clear p-type behaviour with a flat band potential located 0.175 V vs Ag/AgCl and an effective majority carrier density of 1.6×1015 cm-3. Photoelectrochemical studies in Ar-saturated KI solutions were characterized by a nearly ideal semiconductor-electrolyte junction at low illumination levels. Incident-photon-to-current efficiency exhibits values close to 50%, which can be quantitatively rationalized in terms of reflection losses, the width of the space charge region and a minority carrier diffusion length of 80 nm. The remarkable feature of this photocathode is the absence of surface recombination even at potentials close to the flat band value. The efficient hydrogen photogeneration is achieved at neutral pH and in the absence of co-catalysts. We shall briefly discuss the origin of the nearly ideal semiconductor-electrolyte junction behaviour and potential routes for stabilising the PbI2 surface for solar fuel applications. References Yang and J. You, Nature 544 (2017) 155Tiwari and D.J. Fermin, Electrochim. Acta 254 (2017) 223

Finite-temperature properties of nonmagnetic transition metals: Comparison of the performance of constraint-based semilocal and nonlocal functionals

We assess the performance of nonempirical, truly nonlocal and semi-local functionals with regard to structural and thermal properties of $3d$, $4d$, and $5d$ non-magnetic transition metals. We focus on constraint-based functionals and consider the new consistent-exchange van der Waals density functional version vdW-DF-cx [Phys. Rev. B 89, 035412 (2014)], the semi-local PBE [Phys. Rev. Lett. 77, 3865 (1996)] and PBEsol functionals [Phys. Rev. Lett. 100, 136406 (2008)] as well as the AM05 meta-functional [Phys. Rev. B 72, 085108 (2005)]. Using the quasi-harmonic approximation structural parameters, elastic response, and thermal expansion at finite temperatures are computed and compared to experimental data. We also compute cohesive energies explicitly including zero-point vibrations. It is shown that overall vdW-DF-cx provides an accurate description of thermal properties and retains a level of transferability and accuracy that is comparable to or better than some of the best constraint-based semi-local functionals. Especially, with regard to the cohesive energies the consistent inclusion of spin polarization effects in the atoms turns out to be crucial and it is important to use the rigorous spin-vdW-DF-cx formulation [Phys. Rev. Lett. 115, 136402 (2015)]. This demonstrates that vdW-DF-cx has general-purpose character and can be used to study systems that have both sparse and dense electron distributions.

Effect of the Local Atomic Ordering on the Stability of β-Spodumene.

This study focuses on the relative energetic stability of β-spodumene configurations with different atomic ordering, evaluated using electronic structure methods based on static periodic density functional theory. We found that β-spodumene configurations with a framework containing exclusively Al-O-Si linkages are energetically the most stable, consistent with the aluminum avoidance principle. A correlation between the interstitial sites occupied by lithium and the stability of the configuration was established: highly stable configurations contain greater proportions of lithium associated with the edges of AlO4 tetrahedrons. The identified low-energy configurations have a band gap of ∼4.8 eV, and similar electronic band structures and densities of states. Both the PBE and PBEsol functionals predict small differences in the relative stabilities of the different configurations of β-spodumene. However, only PBEsol is able to reproduce the experimentally observed stability differences between α-spodumene and β-spodumene. β-Spodumene is the preferred polymorph at high temperatures, with the PBEsol inversion temperature from α- to β-spodumene predicted to occur at 1070 K.

First-principle study of O vacancy on LaNiO3 (001) surface

In the paper, based on the first principle of density functional theory, it is easier to form O vacancy on the surface by calculating vacancy formation energy, from which the adsorption problems of H2 molecule on LaNiO3 (001) surface with O vacancy are calculated and analyzed. By calculating the adsorption energy and dissociation energy of LaNiO3 (001) surface with O vacancy, it is found that there are two kinds of chemical adsorption modes on the surface of LaNiO3 (001):The first one is that two H atoms are adsorbed to the same O atom to form H2O molecules, which is the best adsorption position where the PBE and PBESOL functionals are applied to perform geometry optimization and property analysis, and it is found that the application of PBESOL functional is less ideal than the application of PBE functional and takes longer. At this point, the effects of H and O atoms result from the orbital hybridization effects of H 1s and O 2p, and there is a typical covalent bond between H and O. The second one is that two H atoms are respectively adsorbed to the two O atoms, forming two OH groups. Furthermore, H2 molecules on the surface of LaFeO3 (010) can also set off physical adsorption. The results show that the existence of O vacancy will lead to the increasing of adsorption energy of H, which is easier to reserve hydrogen; it also will reduce the conductivity of the surface, however, the surface conduction band with O vacancy will increase after absorbing H and its electrical conductivity will be enhanced; to appropriately increase the quantity of O vacancy of the surface will accelerate the decomposition of H2, but it does not have much effect on the electrical conductivity of the surface.

Thermodynamic and mechanical properties of lanthanum–magnesium phases from density functional theory

Thermodynamic and mechanical properties of the six known phases in the La–Mg phase diagram, viz. LaMg, LaMg2, LaMg3, La5Mg41, La2Mg17, and LaMg12, and their elemental antecedents, Mg and La, are computed with density functional theory (DFT) using the PBE and PBEsol exchange-correlation functionals. Phase stability analyses show that both LaMg2 and La5Mg41 are metastable at low temperatures which is consistent with experiments and vibrational spectra. We generalize an existing approach for computing the crystallographic dependence of Young's modulus and Poisson's ratio, which is presently limited to cubic systems, to address any space group symmetry using 0K elasticity tensor components (Cij) from DFT. Isothermal and isentropic Cij(T) are computed with the quasiharmonic approximation (QHA) as are the linear thermal expansion of the cubic compounds, the average linear thermal expansion for the non-cubic compounds, the bulk modulus, and the constant pressure heat capacity. A critical comparison of theoretical results from the PBE and PBEsol functionals is made with available experimental data.

Dependence of Structural and Electronic Properties of Uranium Monochalcogenides on Exchange–Correlation Energy Functionals

We study the dependence of the structural properties of uranium monochalcogenides, U X where X = S, Se, and Te, as well as their electronic ones on the exchange–correlation energy functionals within the spin density functional theory, carrying out all electron calculations by the fully relativistic full-potential linear-combination-of-atomic-orbitals method. We employ two functionals of the local spin density approximation (LSDA) and two functionals of the generalized gradient approximations (GGA); the former two are the Perdew–Zunger and Perdew–Wang functionals and the latter two are the Perdew–Burke–Ernzerhof (PBE) and PBEsol functionals. We also examine the effects of the relativistic correction to the LSDA exchange part of each functional. We find that, for lattice constants, bulk moduli, and cohesive energies, the results of the calculations using the PBE functional are in the best agreement with the experimental results. On the contrary, we find that calculated total magnetic moments and one-electro...