First-principles Density Functional Theoretical Calculations Research Articles

Pressure tuning of structure, magnetic frustration, and carrier conduction in the Kitaev spin liquid candidate Cu2IrO3

The layered honeycomb lattice iridate Cu$_2$IrO$_3$ is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressure-induced structural evolution of Cu$_2$IrO$_3$ by powder x-ray diffraction (PXRD) up to $\sim$17 GPa and Raman scattering measurements up to $\sim$25 GPa. A structural phase transition (monoclinic $C2/c \: \rightarrow$ triclinic $P\bar{1}$) is observed with a broad mixed phase pressure range ($\sim$4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4--300 K show resilient non-metallic $R$($T$) behaviour with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale $T_0$ suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. Using first-principles density functional theoretical (DFT) calculations, we find that at ambient pressure $\rm Cu_2IrO_3$ exists in monoclinic $P2_1/c$ phase which is energetically lower than $C2/c$ phase (both the structures are consistent with experimental XRD pattern). DFT reveals structural transition from $P2_1/c$ to $P\bar{1}$ structure at 7 GPa (involving dimerization of Ir-Ir bonds) in agreement with experimentally observed transition pressure.

Impact of oxygen vacancy occupancy on piezo-catalytic activity of BaTiO3 nanobelt

BaTiO3 nanobelt featuring controlled oxygen vacancies, namely BTO-OV-X (X represents time (h) of vacuum heat-treatment), were prepared to systematically investigate the effect of oxygen vacancies occupancy on pizeocatalytic performance for degradation of organic pollutant. Remarkably, the creation of oxygen vacancies on BaTiO3 can mediate the piezocatalytic activity. The piezocatalytic activity exhibited a volcano-type trend with increasing oxygen vacancies. Results from first-principles density functional theoretical (DFT) calculations and O2-temperature programmed desorption (O2-TPD) measurement indicated that the presence of oxygen vacancies could efficiently adsorb and activate O2 on the surface of BaTiO3 nanobelt and consequently enhance piezocatalytic activity. Importantly, with the aid of piezoresponse force microscopy (PFM) measurement, the intrinsic reason of volcano-type trend of oxygen vacancy-activity was well unveiled. This work reveals the effect and mechanism of oxygen vacancy occupancy on enhancing piezocatalytic activity of BaTiO3 nanobelt and will shed light on design of efficient piezocatalysts in the future.

Phonon dispersion, Raman spectra, and evidence for spin-phonon coupling in MnV2O4 from first principles

MnV$_2$O$_4$ in the spinel structure is known to exhibit coupled orbital and spin ordering, and its Raman spectra show interesting anomalies in its low-temperature phase. With a goal to explain this behavior involving coupled spins and phonons, we determine here the spin-phonon couplings in MnV$_2$O$_4$ from a theoretical analysis of its phonon spectra and their dependence on spin-ordering and electron correlations, obtained from first-principles density functional theoretical calculations. Using these in an analysis based on a Landau-like theory, we uncover the mechanism governing the Raman anomalies observed in its low-temperature phase.

First-principles phonon-based model and theory of martensitic phase transformation in NiTi shape memory alloy

Remarkable shape memory property of Nitinol (NiTi) originates from a diffusion-free reversible phase transition it exhibits upon cooling from the cubic austenite to a low-symmetry martensite structure. Atomistic mechanisms of this martensitic transformation (MT) and consequent emergence of its various low-symmetry structures are not understood yet. Starting with first-principles density functional theoretical calculations, we present here a phonon-based model and its statistical mechanical analysis to obtain atomistic insights into martensitic phases and transformation in NiTi. We uncover seven order parameters that are relevant to the MT in NiTi. From Monte Carlo simulations of an effective model Hamiltonian derived to capture its low energy landscape, we determine its soft phonons and establish the cell-doubling M5′ mode as the primary order parameter. Using Landau theoretical analysis, we show that relative strengths of its third-order coupling with secondary order parameters (e.g. strain) determine the symmetry of low-T structures emerging at its MT. These couplings can be used as the descriptors of stability of martensitic phases that will guide the strategies to improve the shape memory of NiTi through substitutional alloying.

Realization of Both n- and p-Type GeTe Thermoelectrics: Electronic Structure Modulation by AgBiSe2 Alloying.

Successful applications of a thermoelectric material require simultaneous development of compatible n- and p-type counterparts. While the thermoelectric performance of p-type GeTe has been improved tremendously in recent years, it has been a challenge to find a compatible n-type GeTe counterpart due to the prevalence of intrinsic Ge vacancies. Herein, we have shown that alloying of AgBiSe2 with GeTe results in an intriguing evolution in its crystal and electronic structures, resulting in n-type thermoelectric properties. We have demonstrated that the ambient rhombohedral structure of pristine GeTe transforms into cubic phase in (GeTe)100-x(AgBiSe2)x for x ≥ 25, with concurrent change from its p-type electronic character to n-type character in electronic transport properties. Such change in structural and electronic properties is confirmed from the nonmonotonic variation of band gap, unit cell volume, electrical conductivity, and Seebeck coefficient, all of which show an inflection point around x ∼ 20, as well as from the temperature variations of synchrotron powder X-ray diffractions and differential scanning calorimetry. First-principles density functional theoretical (DFT) calculations explain that the shift toward n-type electronic character with increasing AgBiSe2 concentration arises due to increasing contribution of Bi p orbitals in the conduction band edge of (GeTe)100-x(AgBiSe2)x. This cubic n-type phase has promising thermoelectric properties with a band gap of ∼0.25 eV and ultralow lattice thermal conductivity that ranges between 0.3 and 0.6 W/mK. Further, we have shown that (GeTe)100-x(AgBiSe2)x has promising thermoelectric performance in the mid-temperature range (400-500 K) with maximum thermoelectric figure of merit, zT, reaching ∼1.3 in p-type (GeTe)80(AgBiSe2)20 at 467 K and ∼0.6 in n-type (GeTe)50(AgBiSe2)50 at 500 K.

Density Functional Theory Study of Defect Induced Ferromagnetism and Half-Metallicity in CaI2 Based Monolayer for Spintronics Applications

With the rapid advancement of electronics and spintronics industries, the demand for highly efficient materials has been increased for the application of high speed devices and circuits. In the recent past, Ca-based systems have been studied for superconducting properties. Using first-principles density functional theoretical calculations, we have investigated another Ca-based system (CaI2) for its half-metallic properties, which can be promising for ultrafast nonscattering transport. In the domain of spintronics, main group based half-metallic materials have attracted much attention due to their long spin-relaxation time. Here, in this study, we report for the first time ferromagnetism and half-metallicity in calcium iodide (CaI2) based materials with a wide spin-up gap of ∼3.84 eV. Such high spin-up gap in Ca-based half-metallic material is very promising since it can provide nonscattering transport. Among all, Ca0.67δ0.33I2 has a similar pattern to the most well established and thinnest magnet, CrI3 monolayer. Our further investigation predicts that the Ca0.89δ0.11I2 system has a ferromagnetic ground state with Curie temperature of ∼238 K (XY model), which is considerably higher than the reported CrI3 monolayer (45 K). Further to this, such system has a high magnetic anisotropy energy (∼14.11 meV), which indicates that it can prohibit spin fluctuation. Therefore, we report here that alkaline earth metal halide based magnetic materials can be promising for next generation spintronics devices.

Franck Condon shift assessment in 2D MoS2

Optical spectroscopy (OS) techniques are often coupled with first-principles density functional theoretical (DFT) calculations for determining the precise influence of defects on the electronic and structural properties of two-dimensional (2D) transition metal dichalcogenides. Such calculations are carried out presuming there is little or no effect of vibrational transitions on the observed electronic spectrum. However, if the effect of change in vibrational energy (Franck Condon (FC) shift) associated with such a transition is large, it could possibly lead to a different origin for the observed peak. One such instance is the attribution of the 0.75 eV cathodoluminescence peak by Fabbri et al (2016 Nat. Commun. 7 13044) to an optical transition from an S vacancy level in the band gap, under the assumption that the FC shift is negligible. Here, by first principles constrained DFT calculations using hybrid HSE06 functional we show that this combined prediction of OS and DFT calculations is valid for 2D MoS2 since the FC shift associated with electronic transitions from a sulfur vacancy is indeed small ~28 meV. Based on our calculations we conclude that it is reasonable to make a direct connection between DFT calculations and optical spectroscopy techniques in this material, hence, establishing a one to one relation between defect related emission bands and electronic transitions from the defect levels.

Tuning planar fault energies of Ni3Al with substitutional alloying: First-principles description for guiding rational alloy design

We determine effects of substitutional alloying, at Al or Ni sublattice, with 3d, 4d and 5d series of transition elements on the energies of anti-phase boundary (APB), superlattice intrinsic stacking fault (SISF) and unstable stacking fault (USF) of Ni3Al using first-principles density functional theoretical calculations. We show that USF and APB energies are maximal for the systems wherein elements having half-filled d-orbitals are substituted at the Al site. Analysis of the data on alloys studied reveals a correlation between SISF and APB energies, which along with d-orbital occupancy can be useful as descriptors in rational design of improved Ni-Al alloys.

Pressure-dependent semiconductor to semimetal and Lifshitz transitions in 2H-MoTe2: Raman and first-principles studies

High pressure Raman spectroscopy of bulk 2H-MoTe2 up to ∼29 GPa is shown to reveal two phase transitions (at ∼6 and 16.5 GPa), which are analyzed using first-principles density functional theoretical calculations. The transition at 6 GPa is marked by changes in the pressure coefficients of A1g and Raman mode frequencies as well as in their relative intensity. Our calculations show that this is an isostructural semiconductor to a semimetal transition. The transition at ∼16.5 GPa is identified with the changes in linewidths of the Raman modes as well as in the pressure coefficients of their frequencies. Our theoretical analysis clearly shows that the structure remains the same up to 30 GPa. However, the topology of the Fermi-surface evolves as a function of pressure, and abrupt appearance of electron and hole pockets at GPa marks a Lifshitz transition.

Activation of Oxygen in Ce2Zr2O7+x across Pyrochlore to Fluorite Structural Transformation: First-Principles Analysis

Cationic substitution of zirconium in ceria (CeO2) greatly improves its oxygen storage capacity (OSC) and thermal stability. Although the fluorite structure of Ce0.5Zr0.5O2 (Ce2Zr2O8) exhibits good oxygen storage and release properties, its reduced counterpart Ce2Zr2O7 in the pyrochlore structure derived from the fluorite structure does not. Here, we present an analysis of the structural evolution of Ce2Zr2O7+z from pyrochlore-Ce2Zr2O7 (z = 0) to fluorite-Ce2Zr2O8 (z = 1) using first-principles density functional theoretical calculations and bond-valence theory and correlate the consequent activation of oxygen to the observed oxygen storage capacity. The gradual addition of oxygen atoms to the otherwise ordered vacant tetrahedral sites for anions in pyrochlore-Ce2Zr2O7 structure will lead to a transition to fluorite-Ce2Zr2O8 structure, and we demonstrate that this transition involves an increase in the number of weakly bonded, activated oxygen sites that are pertinent to higher OSC observed for the fluori...

Density Functional Investigation of the Inclusion of Gold Clusters on a CH3S Self-Assembled Lattice on Au(111)

We employ first-principles density functional theoretical calculations to address the inclusion of gold (Au) clusters in a well-packed CH3S self-assembled lattice. We compute CH3S adsorption energies to quantify the energetic stability of the self-assembly and gold adsorption and dissolution energies to characterize the structural stability of a series of Au clusters adsorbed at the SAM-Au interface. Our results indicate that the inclusion of Au clusters with less than four Au atoms in the SAM-Au interface enhances the binding of CH3S species. In contrast, larger Au clusters destabilize the self-assembly. We attribute this effect to the low-coordinated gold atoms in the cluster. For small clusters, these low-coordinated sites have significantly different electronic properties compared to larger islands, which makes the binding with the self-assembly energetically more favorable. Our results further indicate that Au clusters in the SAM-Au interface are thermodynamically unstable and they will tend to dissolve, producing Au adatoms incorporated in the self-assembly in the form of CH3S-Au-SCH3 species. This is due to the strong S-Au bond which stabilizes single Au adatoms in the self-assembly. Our results provide solid insight into the impact of adatom islands at the CH3S-Au interface.

Quantum spin Hall insulator phase in monolayer WTe2 by uniaxial strain

Monolayer WTe2, which is predicted to be large-gap quantum spin Hall (QSH) insulators with distorted 1T (1T’) structure, attracts rapidly growing interests. However, the intrinsic semimetallic nature of the monolayer 1T’-WTe2 limits their direct applications based on QSH effect. By first-principles density functional theoretical calculations, we demonstrate a phase transition from semimetal to QSH insulator under the uniaxial strains along a and b axis in monolayer 1T’-WTe2. The electronic phase transition results from the geometric structure deformation upon the uniaxial strains. This suggests monolayer 1T’-WTe2 as a promising material for application in strain-tunable topological quantum electronics.

Effect of Pressure on Valence and Structural Properties of YbFe2Ge2 Heavy Fermion Compound--A Combined Inelastic X-ray Spectroscopy, X-ray Diffraction, and Theoretical Investigation.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

Theoretical investigation on the solution behaviors of Ba and Zr in uranium dinitride

The in-pile performance of ceramic fuels is significantly affected by the fission products. In this work, we have performed first-principles density functional theoretical calculations to study the interaction between metallic fission products (barium and zirconium) and the uranium dinitride UN2 matrix. The thermodynamic properties and bonding nature of Ba and Zr atoms in different incorporation configurations indicate that Zr is more soluble in UN2 matrix than Ba. With increasing the concentration of the impurity atoms, Zr-doped UN2 exhibits a slight tendency to contract, while Ba-doped UN2 tends to swell. Based on the competition between steric effect and chemical interaction, various incorporation trends for Ba and Zr in UN2 as well as in UN have been understood.

First-principles theory, coarse-grained models, and simulations of ferroelectrics.

CONSPECTUS: A ferroelectric crystal exhibits macroscopic electric dipole or polarization arising from spontaneous ordering of its atomic-scale dipoles that breaks inversion symmetry. Changes in applied pressure or electric field generate changes in electric polarization in a ferroelectric, defining its piezoelectric and dielectric properties, respectively, which make it useful as an electromechanical sensor and actuator in a number of applications. In addition, a characteristic of a ferroelectric is the presence of domains or states with different symmetry equivalent orientations of spontaneous polarization that are switchable with large enough applied electric field, a nonlinear property that makes it useful for applications in nonvolatile memory devices. Central to these properties of a ferroelectric are the phase transitions it undergoes as a function of temperature that involve lowering of the symmetry of its high temperature centrosymmetric paraelectric phase. Ferroelectricity arises from a delicate balance between short and long-range interatomic interactions, and hence the resulting properties are quite sensitive to chemistry, strains, and electric charges associated with its interface with substrate and electrodes. First-principles density functional theoretical (DFT) calculations have been very effective in capturing this and predicting material and environment specific properties of ferroelectrics, leading to fundamental insights into origins of ferroelectricity in oxides and chalcogenides uncovering a precise picture of electronic hybridization, topology, and mechanisms. However, use of DFT in molecular dynamics for detailed prediction of ferroelectric phase transitions and associated temperature dependent properties has been limited due to large length and time scales of the processes involved. To this end, it is quite appealing to start with input from DFT calculations and construct material-specific models that are realistic yet simple for use in large-scale simulations while capturing the relevant microscopic interactions quantitatively. In this Account, we first summarize the insights obtained into chemical mechanisms of ferroelectricity using first-principles DFT calculations. We then discuss the principles of construction of first-principles model Hamiltonians for ferroelectric phase transitions in perovskite oxides, which involve coarse-graining in time domain by integrating out high frequency phonons. Molecular dynamics simulations of the resulting model are shown to give quantitative predictions of material-specific ferroelectric transition behavior in bulk as well as nanoscale ferroelectric structures. A free energy landscape obtained through coarse-graining in real-space provides deeper understanding of ferroelectric transitions, domains, and states with inhomogeneous order and points out the key role of microscopic coupling between phonons and strain. We conclude with a discussion of the multiscale modeling strategy elucidated here and its application to other materials such as shape memory alloys.

High tunability of the work function of (001) surface of ReO3 with O-vacancies: First principles analysis

Physical and chemical properties of transition metal oxides are central to the emerging field of oxide electronics. However, they are greatly influenced by defects, particularly, oxygen vacancies, which are always present in oxides. Here, we show how the control of oxygen vacancies at (001) surface of ReO3 can be used to tune its work function from 7 to 3 eV, based on first-principles density functional theoretical calculations of its structure, electronic and magnetic properties. The work function is shown to correlate directly with the stability of surface and exhibit a linear dependence on surface energy. We further assess the stability of ReO3 surface by determining its phonon dispersion, and explain how the surface stresses effectively strengthen structural instability leading to size dependence of its pressure dependent structural phase transitions observed experimentally. Our results highlight how significantly oxygen vacancies alter the work function of a metallic oxide and has important consequences to development of electronic devices and catalysts based on oxide heterostructures.

First-Principles Analysis of Potential-Dependent Proton Coupled Electron Transfer between Polypyridyl–Ruthenium Complexes and Oxygen-Modified Graphene Electrodes

Proton coupled electron transfer reactions which are pervasive throughout electrochemistry control a number of energy conversion strategies. First-principles density functional theoretical calculations are used herein to examine proton coupled electron transfer between a homogeneous mononuclear polypyridyl–ruthenium catalyst used in the catalytic oxidation of water and surface ketone groups on oxygen-modified graphene electrode surfaces. The potential-dependent interface energies were calculated for two proton transfer states: RuIIIOH···O═C–graphene and RuIV═O···HO–C–graphene. The reactivity for interfacial proton coupled electron transfer was found to be controlled by functional groups that terminate surface defect sites as well as graphene edge sites. The energy gap between the two proton transfer states becomes smaller as the number of surface ketone groups increases. Ab initio molecular dynamics simulations clearly show that increases in the number of surface ketone groups increase the hydrophilicity ...

Theoretical Study of Thermodynamics Properties and Bulk Modulus of SiC under High Pressure and Temperature

In extended pressure and temperature ranges, a theoretical study of the isothermal bulk modulus of SiC in B3 structure under high pressure and temperature is carried out by means of first-principles density functional theoretical calculations combined with the quasi-harmonic Debye model. Through the quasi-harmonic Debye model, the isothermal bulk modulus and its first and second pressure derivatives are successfully obtained. The thermodynamics properties of 3C-SiC are investigated in the pressure range of 0–100 GPa and the temperature range of 0–2000 K.

First principles analysis of graphene and its ability to maintain long-ranged interaction with H2S

We determine the chemical activity of (a) carbon site of pristine graphene, (b) Stone–Wales (SW) defect site, and (c) BN-sites of BN-doped graphene towards adsorption of a toxic gas H2S, through comparative analysis based on first-principles density functional theoretical calculations incorporating van der Waals (vdW) interactions. While the adsorption of H2S is weak at both C and BN sites with a binding energy of 15kJ/mol, it is significantly stronger at the Stone–Wales defect site with a much higher binding energy of 26kJ/mol. This is clearly reflected in the contrasting orientation of H2S molecule in the relaxed geometries: the sulfur atom of H2S is closer to graphene (at a distance 3.14Å) during physisorption at C and BN sites, while the molecule's H atoms come closer to graphene (at a distance 2.84Å) during physisorption at the Stone–Wales defect site. The origin of the stronger binding interaction between H2S and a SW defect site is attributed to two possible reasons: (i) an increase in the vdW interaction; and (ii) the lowering of both energy of the HOMO level and the total energy of the H2S molecule in attaining a stable configuration. Our findings are compared to the available theoretical results and their technological relevance is further discussed.

First-principles study of field emission properties for ZnO nanotuber capped and codoped with N-M(Cd, Mg)

The first-principles density-functional theoretical calculations are performed to investigate the effects of N doped and N, M (Cd, Mg) codoped on the geometrical structures and field emission properties of capped (9, 0) zinc oxide nanotubes (ZnONT). The results show that the N could improve the stability of the structure of capped side. With the increase of the applied electric field, the density of states (DOS) shifts towards the low energy position, the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and the effective work function decrease drastically, and the electrons congregate to the capped side. The analyses of DOS/local DOS, HOMO/LUMO, and Mulliken population indicate that the field emission properties of N, Cd-Codoped ZnONT are improved, but those of N, Mg-codoped ZnONT are worsened.