Density Functional Perturbation Theory Research Articles

Structural phase transition of G-type antiferromagnetic SrMnO3 under pressure

Perovskite SrMnO3 is a versatile material with applications in transistors, supercapacitors, and spintronic devices. This study employs the DFT+U calculations to investigate the electronic and magnetic properties of two polymorphs: 4H-SrMnO3 and 3C-SrMnO3 under isotropic pressure conditions up to 60 GPa. A novel approach based on density functional perturbation theory (DFPT) is employed to precisely determine the on-site Hubbard U value, yielding a computed value of 4.552 eV, consistent with experimental reports. Below P = 14 GPa, the G-AFM phase of 4H-SrMnO3 is the most favored stable, and transitioning to the G-AFM phase of 3C-SrMnO3 beyond 14 GPa. Notably, we observed significant changes in the band gaps, exhibiting an increasing trend under isotropic pressure, while the magnetic moments of the Mn sites exhibit a decreasing trend from ≈2.8 μ B to ≈2.3 μ B for P = 0 GPa and 60 GPa, respectively. The projected density of states (PDOS) reveals the p-d hybridization contributions between the 3d-Mn and 2p-O covalent bonds, along with d-d Mott Hubbard interactions within the localized 3d orbitals of neighboring Mn-Mn atoms. Furthermore, a decrease in the Mn1-O-Mn2 distance prompts enhanced charge transfer which was facilitated by ligand holes, leading to a noticeable decrease in the magnetic moment of the Mn site in the G-AFM phase of SrMnO3.

From Bulk to one-dimensional MoS2 nanochains: evolution of electronic, mechanical, and optical properties

We theoretically investigate the stability of a MoS2 nanochain, reporting its electronic, mechanical, and optical properties. The nanochain presents a semiconductor structure with a minute band gap of 67m eV compared to the larger gap bulk and monolayer structures. It is more malleable, enduring a maximum compressive (tensile) strain of 6% (6.5%). It is dynamically stable, showing no negative frequencies along its Brillouin zone (BZ) path. The nanochain is thermally stable at 300K, making it possible to synthesize as a freestanding structure. The optical properties of the bulk, monolayer, and 1D MoS2 materials are evaluated using the time-dependent density functional perturbation theory (TDDFPT) and compared to those determined via the independent particle approximation (IPA). Along the nanochain’s periodic x direction, the reflectivity retains a maximum value of ∼68% in the infrared (IR) region. Furthermore, its optical conductivity also exhibits a peak within the IR regime. These two features make such nanochains suitable as coating materials in applications involving infrared radiation or can even be exploited as conductive substrates in near-IR devices.

Can Pressure be a Good Strategy for Optimizing Thermoelectric Performance of SnPS3${\rm SnPS}_3$?

AbstractExternal pressure can significantly alter the transport coefficients, power factor, and figure of merit because of its direct influence on the electronic structure, electron–phonon, and phonon–phonon couplings. This study delves into the electronic and thermal transport properties of at external pressures up to 30 GPa using first‐principles calculations and Boltzmann transport theory. The electron–phonon relaxation time is computed within the electron–phonon‐averaged (EPA) approximation, enabling exploration beyond the constant relaxation time approximation. The first‐principles calculations reveal an indirect bandgap of 1.76 (without pressure) and 0.12 eV (30 GPa). The density functional perturbation theory calculations confirm the dynamic stability of at external pressure up to 30 GPa. The electronic transport properties are improved by more than one order of magnitude at 30 GPa, consistent with experimental observations. The Peierls–Boltzmann transport calculations demonstrate the room temperature lattice thermal conductivity of 0.22 (without pressure) and 7.4 (at 30 GPa). The results emanate that exhibits of 0.71 at 900 K at a hole doping of 2 at zero pressure, which decreases with increasing pressure. The findings explore the effect of external pressure on both electronic and thermal transport properties of , warranting further experimental exploration of thermal transport properties at higher pressures.

Hafnium dinitride/poly(9-vinylcarbazole) heterojunction: Structural, electronic, optical and vibrational properties. A DFT and DFPT study

Photosensitivity of semiconducting polymers can be enhanced by blending donor and acceptor polymers to optimize photoinduced charge separation. With a motivation to understand microscopic aspects of HfN2 and PVK relevant to optoelectronic properties of transition metal-dinitride/semiconductor interfaces, we examined their structural, electronic, optical and vibrational properties using first-principles calculations based on density functional theory (DFT) and density functional perturbation theory (DFPT) levels of theory. We computed a large direct energy gap of 3.751 eV for PVK and an indirect narrow band gap of 1.396 eV in HfN2. The gap value computed for the composite structure, HfN2/PVK, was 0.958 eV. The heterojunction displayed an indirect energy gap. The charge transfer between HfN2 and PVK results in a polarized field within the interface region, which will benefit the separation of photogenerated carriers. The calculated density of electronic states, Lowdin charge transfer and charge density difference certify that this proposed layered nanoheterojunction is an excellent light-harvesting semiconductor. These findings indicate that the conjugated polymer PVK is a promising candidate as a non-noble metal co-catalyst for HfN2 photocatalysis. It also provides useful information for understanding the observed enhanced photocatalytic mechanisms in experiments.

Impact of Magnetism on (e-ph) self-energy and thermalization of hot carriers in 2D half-functionalized GeC-hybrid

The present work delves into the analysis of carrier scattering rates, emphasizing the effective strategy of chemical functionalization for introducing magnetism into two-dimensional, initially non-magnetic materials. Specifically, our investigation focuses on the distinct alterations observed in the electron linewidth within two half-fluorinated configurations, namely GeC-F and F-GeC. Leveraging density functional and many-body perturbation theories, our findings highlight a substantial increase in the linewidth upon inducing ferromagnetism and antiferromagnetism in the GeC hybrid. Additionally, we identify the significant contribution of different phonon modes to the linewidth. The outcomes reveal that the out-of-plane acoustic mode ZA exerts global control over the valence and conduction bands. Moreover, the relaxation time of electrons following sunlight illumination demonstrates that electron thermalization occurs within 1.5fs to 130fs for ferromagnetic GeC-F and within 2.32fs to 42fs for antiferromagnetic F-GeC. This study provides a detailed and accurate description of electron behavior at sub-picosecond timescales, a realm that is experimentally challenging to attain.

Anomalous superconductivity in Li/F modified two-dimensional molybdenene

Anomalous superconductivity in Li/F modified two-dimensional molybdenene

A new type of two-dimensional piezoelectric material: MOene based on Ti2O and its Janus structure

Two-dimensional (2D) piezoelectric materials have been widely studied because of their broad application prospects. In this paper, the piezoelectric properties of a MXene-like 2D material (MOene) and its Janus structure are reported by first-principles calculations. The dynamic stability of these 2D materials are verified based on phonon spectra. The semiconductor properties of materials are based on the calculation of energy bands. The origin of piezoelectric polarization of MOene and its Janus structure is explained based on differential charge density and Bader charge analysis. The piezoelectric stress tensor of the material is calculated based on density functional perturbation theory (DFPT). The theoretical prediction results show that the piezoelectric properties of MOene and its Janus structure are better than those of 2D 2H-MoS2, and the in-plane piezoelectric coefficient d11 of Ti2OH2 reaches 6.09 pm/V. Our research shows that MOene and its Janus structure add an important member to the 2D piezoelectric material library, and provide a new sample for the study of piezoelectric properties based on the 2D material library.

Effect of XC functionals and dispersion corrections on the DFT‐computed structural and vibrational properties of SrCl2–NaCl and ZrF4–LiF

AbstractDensity functional theory (DFT) calculations were performed to examine the impact of exchange–correlation (XC) functionals and van der Waals corrections (specifically the D3 method) on the structural and vibrational properties of the SrCl2–NaCl and ZrF4–LiF salt systems. Multiple XC functionals, including the local density approximation (LDA), the generalized gradient approximation using the Perdew–Burke–Ernzerhof (PBE) model, and its modified form suitable for solids (PBEsol), the dispersion‐corrected PBE‐D3 and PBEsol‐D3, were considered. Of these functionals, LDA was found to exhibit the highest degree of error, while PBEsol and PBE‐D3 displayed the least error. Underestimated lattice parameters compared with experimental values were observed to result in higher force constants, leading to an overprediction of vibrational frequencies. Conversely, an overestimation of lattice parameters was associated with lower vibrational frequencies. The methodology presented in this study yielded results that are in good agreement with experiment, irrespective of the method (finite differences vs. density functional perturbation theory) employed for calculating infrared and Raman spectra. It was further demonstrated that for alkali halides with weak Raman scattering, utilizing a supercell constructed from primitive cells better predicts Raman features than does the use of conventional cells.

Diamond two-phonon infrared absorption spectrum calculated from first principles using the finite displacement method

We calculate the two-phonon contribution to the dielectric susceptibility of diamond at room and elevated temperatures, utilizing a general ab initio model that can be extended to higher n phonon processes. We calculate the second-order dipole moments using a finite displacement method to calculate the derivatives of the Born effective charge tensor. Specifically, we use results obtained from density-functional theory and density-functional perturbation theory in this finite displacement method to calculate the dipole moments of diamond in the two-phonon case. We use the calculated dipole moment to determine the contribution of two-phonon processes to the dielectric susceptibility. We then calculate the absorption curve as a function of wave-number at room or elevated temperatures. Our results indicate that the calculated absorption is in good agreement with previous calculations, and that it increases in magnitude with temperature while maintaining a consistent shape.

Weakly Bound Complexes of γ-Butyrolactone with Water as Observed in Matrix Isolation FTIR and Theoretical Calculations.

A computational and spectroscopic analysis of weakly bound complexes of 1:1 γ-butyrolactone with water has been completed. In this work, multiple density-functional theories and perturbation theory were used to explore the energy-landscape of the complex. Four unique structures were identified in this analysis. One structure was characterized by the formation of a water to carbonyl hydrogen bond and the other three were formed from water to ester hydrogen bonds. The carbonyl-bound conformation was found to be the global minimum across a comprehensive panel of calculations. A wave function analysis demonstrated that the structures were additionally stabilized by weak van der Waals interactions. FTIR spectroscopy of matrix-isolated samples indicated the presence of at least two of the calculated geometries. The structures were identified to be a carbonyl-bound and at least one ester-bound structure. The transitions identified for the carbonyl-bound complex were noted to be significantly more intense than those of the ester bound, indicating greater prevalence in the matrix.

Mechanical and thermoelectric response of 18-valence electron half-Heusler tellurides XFeTe (X=Ti, Hf): A theoretical perspective

To address growing global energy needs, half Heusler compounds offer a cost-effective and efficient solution for power generation applications. We present a study on the structural, electronic, magnetic, phonon, mechanical and thermoelectric (TE) properties of two hH tellurides, XFeTe (X= Ti, Hf) with 18 valence electrons. The investigation employs Density functional theory (DFT), Semi-classical Boltzmann transport equations (BTE), Quasi-harmonic approximation (QHA), Density functional perturbation theory (DFPT) and Deformation potential theory (DPT). The equilibrium lattice constants are determined to be 5.882 Å and 6.041 Å for TiFeTe and HfFeTe respectively. The electronic structures of TiFeTe and HfFeTe are modeled with generalised gradient approximation (GGA), revealing indirect band gaps of 0.93 eV and 0.79 eV, respectively. The conduction bands in TiFeTe and HfFeTe exhibit remarkably low effective masses of 0.72 me and 0.61 me, respectively, resulting in higher carrier relaxation times. The compounds are dynamically and mechanically stable. TiFeTe is ductile whereas HfFeTe is brittle. The elastic constants analysis suggests that these compounds exhibit stiffness, better hardness, elastic anisotropy and high melting point. TE parameters are investigated across a temperature range from 400 K to 1200 K. At room temperature, TiFeTe and HfFeTe exhibit lattice thermal conductivities of 25.43 Wm−1K−1 and 29.29 Wm−1K−1, respectively. At 1200 K, zT for the n-type, p-type compositions are 1.79, 1.11 for TiFeTe and 1.35, 1.06 for HfFeTe, respectively. The n-type composition not only exhibits superior TE performance compared to the p-type but also shows the peak zT at more feasible doping levels.

A unified photo-excited GaAs model from ab initio simulation in terahertz regime

In this paper, we present a unified model for gallium arsenide (GaAs) based on ab initio simulations which characterizes its terahertz (THz) properties when excited by optical pump. We use density functional perturbation theory to calculate the dielectric properties of GaAs, and investigate the relaxation time of photo-excited GaAs through electron–phonon interactions. In light of the complexities arising from the mixed absorption mechanisms and the sensitivity of GaAs to laser parameters, we have developed a method that leverages time-dependent density functional theory and Boltzmann transport theory. This approach enables us to establish an accurate relationship between the pump laser intensity and the carrier concentration by introducing the percentage of excited electrons, facilitating the quantitative characterization of GaAs’s response under different optical pump intensities. Using the microscopic material parameters solved by first principles, we develop a unified Drude model to describe the macroscopic electromagnetic responses of photo-excited GaAs. We simulate several reported numerical examples of photo-excited GaAs, including a GaAs wafer and GaAs-based THz metamaterial modulators, to validate the proposed unified model as a reliable approach for predicting the THz properties of GaAs. The good agreement between the simulation and measurement results demonstrates that our model successfully captures the dynamic responses of photo-generated carriers and provides guidance for the design of optoelectronic devices based on GaAs. Furthermore, our modeling approach based on ab initio simulations is free from empirical parameters, providing a solid THz modeling method for other photo-excited semiconductor materials.

First-principles study of superconducting transition temperature of niobium and vanadium as a function of pressure

Ultra-high pressure has recently led to the observation of high Tc superconductors. The exploration of superconducting properties under high pressure conditions offers valuable insights into the fundamental mechanisms governing superconductivity and opens avenues for the synthesis of novel materials with enhanced superconducting properties. In this study, the superconducting transition temperature, Tc, for niobium and vanadium has been investigated to maximum pressures of 250.0 and 70.0 GPa, respectively. Density functional theory and density functional perturbation theory were used to undertake first-principles computations to investigate the variations of superconducting properties under pressure. For both the metals niobium (Nb) and vanadium (V), we observed that the density of states at the Fermi level decreased, while the overall phonon frequency generally increased with pressure. For Nb, we find that the electron–phonon coupling constant (λ) and the superconducting transition temperature (Tc) decrease steadily with pressure; however, for V, we find that λ and Tc decrease smoothly up to 26.0 GPa, and then gradually increase to 70.0 GPa pressure. The observed phonon spectrum nature for both Nb and V is in good agreement with the neutron scattering approach’s output.

Refractory and thermoelectric properties of stabilized Tricalcium aluminate allotrope: A first-principles calculation approach

Tricalcium aluminate (Ca3Al2O6) is an indispensable component of Portland cement due to its high strength and reactivity. This study investigates the equilibrium lattice structure as well as the electronic and refractory properties of a newly discovered allotrope of Ca3Al2O6 using density functional theory and random phase approximation methods. Using density functional perturbation theory, we investigate the phonon dispersion of the refined structure’s experimental stability. In addition, the lattice thermal conductivity and thermoelectric figure of merit are computed by solving the Boltzmann Transport Equation with the modified Debye Callaway model and the Relaxation time Approximation methods. The computed Figure of merit, determined by the carrier concentration, indicates a substantial enhancement of 40 to 100-fold, leading to a superior figure of merit of 0.25 and 0.6 when appropriately doped. As a result, we anticipate that stable allotropes of Ca3Al2O6 in the Pm3̄m phase will be useful as energy-harvesting, convective cooling and infrared radiation shield-based cement materials in the cement and concrete industries.

InAs–InP Superlattice Nanowires with Tunable Phonon Frequencies

AbstractThe control of heat conduction through the manipulation of phonons in solids is of fundamental interest and can be exploited in applications for thermoelectric conversion. In this context, the advent of novel semiconductor nanomaterials with high surface‐to‐volume ratio, e.g. nanowires, offer exciting perspectives, leading to significant leaps forwarding the efficiency of solid‐state thermoelectric converters after decades of stagnation. Beyond the high aspect ratio, the nanowire geometry offers unprecedented possibilities of materials combination and crystal phase engineering not achievable with 2D counterparts. In this work, the growth of long (up to 100 repetitions) wurtzite InAs/InP superlattice nanowires with homogeneous segment thicknesses is reported, with control down to the single digit of nanometer. By means of Raman scattering experiments, clear modifications of the phonon dispersion in superlattice nanowires are found, where both InAs‐like and InP‐like modes are present. The experimentally measured modes are well reproduced by density functional perturbation theory calculations. Remarkably, it is found that the phonon frequencies can be tuned by the superlattice periodicity, opening exciting perspectives for phonon engineering and thermoelectric applications.

Generation of thermal neutron scattering data for GH3535 alloy

Recent studies related to the thermal neutron scattering (TNS) data in reactors were generally focused on fuels and moderators, while the TNS data for structural materials has received little attention. The TNS effect of the GH3535 alloy, a nickel-based alloy widely used in molten salt reactor (MSR), is studied in this work. The partial phonon densities of states (PDOS) for the main metals (Ni, Mo, Cr and Fe) in the alloy, which are required for the TNS data generation, are calculated using the density functional perturbation theory (DFPT) based on a special quasi-random structure (SQS) of the alloy. The TNS cross sections for the main metals in the alloy are generated using the modified NJOY code. To preliminarily validate the TNS data, the calculated scattering cross section, combined with the evaluated absorption cross section of the alloy, is compared with the experimental total cross section, which shows a good agreement. Moreover, an integral neutronics experiment of neutron leakage spectrum is designed to further validate the TNS data. The neutron leakage spectrum is simulated by the Monte Carlo method, which can be compared with the future experimental result. Based on the experiment geometry modeling, the TNS effects of the GH3535 alloy are analyzed at multiple incident neutron energies, sample thicknesses and temperatures, respectively. It is found that the TNS effect of the GH3535 alloy on the neutron leakage spectrum cannot be ignored, and the scattering cross section of the alloy with high precision should be included in designs of MSR.

Demagnetizing Ferromagnetic Catalysts to the Sabatier Optimal of Haber-Bosch Process.

Achieving the Sabatier optimal of a chemical reaction has been the central topic in heterogeneous catalysis for a century. However, this ultimate goal was greatly hindered in previous catalyst design strategies since the active sites indeed changed. Fortunately, the magneto-catalytic effect (MCE) provides a promising solution to this long-standing challenge. Recent research suggests that the performance of ferromagnetic catalysts is capable to be promoted without changing its chemical structure. Herein, we use time-dependent density functional perturbation theory (TDDFPT) calculations to elucidate that a partially demagnetized (DM) ferromagnet could be a Sabatier optimal catalyst. Using ammonia synthesis as the model reaction, we determined the activity of Cobalt at each DM state by including the magnetic thermal excitations via magnon analysis, making the 55% DM Co to the genuine Sabatier optimal. As an essential but underexcavated phenomenon in heterogeneous catalysis, the MCE will open a new avenue to design high-performance catalysts.

Nanoscale Localized Phonons at Al2O3 Grain Boundaries.

Nanoscale defects like grain boundaries (GBs) would introduce local phonon modes and affect the bulk materials' thermal, electrical, optical, and mechanical properties. It is highly desirable to correlate the phonon modes and atomic arrangements for individual defects to precisely understand the structure-property relation. Here we investigated the localized phonon modes of Al2O3 GBs by combination of the vibrational electron energy loss spectroscopy (EELS) in scanning transmission electron microscope and density functional perturbation theory (DFPT). The differences between GB and bulk obtained from the vibrational EELS show that the GB exhibited more active vibration at the energy range of <50 meV and >80 meV, and further DFPT results proved the wide distribution of bond lengths at GB are the main factor for the emergence of local phonon modes. This research provides insights into the phonon-defect relation and would be of importance in the design and application of polycrystalline materials.

Lattice thermal conductivity in half-Heusler compounds XNiSn (X=Ti, Zr, Hf) using Slack's model

Reducing lattice thermal conductivity (κl) that reflects the material’s heat-carrying capacity through lattice phonon vibrations is crucial for optimizing the figure of merit (zT). Using density functional theory (DFT) and density functional perturbation theory (DFPT), present work considers the structural, electronic, magnetic, and phonon properties of the XNiSn (X=Ti, Zr, Hf ) half Heusler (hH) compounds. TiNiSn, ZrNiSn, and HfNiSn are identified as semiconductors with indirect band gaps of 0.43 eV, 0.47 eV and 0.39 eV, respectively, exhibiting dynamical stability. The temperature- dependent κl of hH XNiSn are compared using Slack’s model with two approaches for analyzing phonon velocity: elastic constants from quasi- harmonic approximation (QHA) as implemented in the Thermo_pw code and slope of phonon bands based on DFPT. At room temperature, TiNiSn, ZrNiSn and HfNiSn have κl values of 28.61 Wm−1K−1, 34.61 Wm−1K−1 and 29.85 Wm−1K−1 respectively using phonon velocity from slopes of phonon bands based on DFPT. These values show deviation of 1.48%, 6.29%, and 5.82% to those κl values 29.01 Wm−1K−1’, 36.79 Wm−1K−1 and 31.59 Wm−1K−1 for TiNiSn, ZrNiSn and HfNiSn respectively using QHA.

Electronic and vibrational properties of MgCu2O3

AbstractThe vibrational properties of orthorhombic MgCu2O3 compound have been investigated within the framework of the density functional perturbation theory (DFPT) as well as experimentally, to validate the computational results. MgCu2O3 was synthesized by solid‐state reaction and characterized by synchrotron‐based X‐ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The DFT + U‐based first principle calculations were performed to obtain the correct electronic ground state and the band structure of this compound. The same DFT + U methodology was employed along with DFPT calculations for obtaining vibrational properties: phonon density of states and phonon band structure. The atomic vibrations for each mode were also analyzed, and the Raman and the IR active modes are identified. Experimentally observed Raman and infrared (IR) spectra agree well with the computed ones.