Phonon Band Structure Research Articles

Cation substitution varieties of I-II2-III-VI4 semiconductors and their effects on electronic and phononic properties

Quaternary chalcogenides from the semiconducting I-II2-III-VI4 class of systems are relatively unexplored in terms of their electronic and vibrational properties. These multinary systems can be derived from simpler zincblende and wurtzite binary materials as they can accommodate different lattice structures with diverse cation atom arrangements. Here by using first principles simulations, we investigate the electronic and phonon properties of I-II2-III-VI4 systems in terms of their structural and atomic diversity. Common to the parents as well as unique to the specific atomic composition signatures are identified. Distinct structure-property relations for the electronic and phonon band structures are found in order to make predictions for the electronic and thermal transport in these materials. Property trends associated with the average atomic number and mass dissimilarities are identified, which can be used to understand the underlying transport properties in quaternary chalcogenides derived from binary materials.

Twists and Puckers: Tuning Crystal Chemistry in the La(AuxGe1-x)2 Compositional Series.

The physical properties of solid-state materials are closely tied to their crystal structure, yet our understanding of how competing structural arrangements energetically compare is limited. In this work, we explore how small differences in composition affect the structure in the La(AuxGe1-x)2 series of compounds, which comprises four unique structure types between LaGe2 and LaAu2. This family includes the previously unknown AlB2-type compound with stoichiometry La(Au0.375Ge0.625)2 as well as La(Au0.25Ge0.75)2, an intergrowth of the AlB2 and ThSi2 structure types. We then study the chemical forces driving the structure changes and use phonon band structure calculations and DFT-Chemical Pressure to evaluate atomic-size effects. These calculations show that the parent AlB2 structure type is disfavored in Au-rich compounds due to soft atomic motions along the c axis. The instability of AlB2-type LaAuGe is confirmed by the presence of imaginary modes in the phonon band structure that correspond to a "puckering" of the hexagonal AlB2-type lattice, resulting in the experimentally observed LiGaGe structure type. The impact of size effects is less clear for Au-poor compositions; instead, twisting the AlB2 structure type to form the ThSi2 type opens a pseudogap at the Fermi level in the electronic density of states. This investigation demonstrates how crystal structure in solid-state materials can be compositionally tuned based on balancing size and electronics when multiple structure types are in close thermodynamic competition.

Investigating the effect of electronic correlation on transport properties and phononic states of Vanadium

In the present work, we have tried to investigate the importance of electronic correlation on transport properties and phononic states of Vanadium (V). The temperature-dependent electrical resistivity (ρ) and electronic part of thermal conductivity (κe) due to electron–electron interactions (EEIs) and electron–phonon interactions (EPIs) are computed. The values of ρ due to EEIs are found to be extremely small in comparison to ρ due to EPIs. For instance, at 300 K, the calculated value of ρ due to EEIs (EPIs) is ∼0.859×10−3 (∼0.20) μΩ m. The magnitudes of κe due to EPIs are found to be in good agreement with the experimental results. These observations indicate the negligible importance of EEIs to these quantities for V. However, at 300 K, the value of Seebeck coefficient (S) at DFT+DMFT level (∼−0.547 μV K−1) is found to be entirely different than at DFT level (∼7.401μV K−1). Also, the DFT+DMFT value of S at 300 K is in good match with the available experimental data (−1.06 μV K−1[1], 1.0 μV K−1[2]). Apart from this, the study of phononic states at DFT and DFT+DMFT level is performed. The obtained phononic band structure and phonon DOS at DFT+DMFT differ to a good extent from that at DFT. The maximum energy of phononic state obtained at DFT (DFT+DMFT) is ∼ 33.83 (∼35.15) meV, where the result of DFT+DMFT is obtained more closer to the experimental data (35.15 [3], 36.98 [4] & 41.77 [5] meV). These results highlight the importance of electronic correlation on S & phononic states of simple correlated V metal.

Ab Initio Study of Structural, Electronic, and Thermal Properties of Pt/Pd-Based Alloys

Alloys are beneficial in numerous applications since they combine the desirable properties of different metals. In this regard, Pt/Pd alloys have been investigated as a replacement for Pt, which is the standard catalyst used in various catalytic processes. However, there are still gaps in our understanding of the structural, mechanical, and thermodynamic properties of Pt/Pd alloys. This study was conducted using density functional theory (DFT) calculations to investigate the electronic, elasticity, mechanical, and thermodynamic properties of Pt/Pd alloys and compared them to pristine Pt and Pd structures. The results indicate that the considered Pt/Pd alloy structures, PtPd3, PtPd, Pt3Pd, and Pt7Pd, are energetically favourable based on their formation energies. These structures also satisfy Born’s stability criteria and are elastically stable. The phonon density of states showed that the considered Pt/Pd alloy structures are dynamically stable, with no imaginary modes present. Additionally, the Pt atom dominates at lower frequencies, while the Pd atom dominates at higher frequencies, as seen in the phonon band structure. The electronic density of states revealed that the considered Pt/Pd alloy structures have a metallic character and are non-magnetic. These findings contribute to a better understanding of the properties and stability of Pt/Pd alloy structures that are relevant in various fields, including materials science and catalysis.

Comprehensive investigation of electronic structure, phonon spectrum and thermoelectric performance of LuMSb (M = Ni, Pd, Pt) half Heusler compounds from first principles.

We studied the structural, electronic, phonon spectrum and thermoelectric properties of ternary LuMSb (M = Ni, Pd, Pt) half Heusler compounds by using first principles method. The electronic properties are calculated via energy band structure and density of states by using GGA + U approximation. The calculations reveal that the replacement of Ni with Pd and Pt, energy gap decreases and LuNiSb, LuPdSb are found to have narrow indirect band gaps and exhibit semiconducting nature, while LuPtSb is found to be a gapless semiconductor. Phonon band structure calculations give only positive values of phonon frequency indicating the dynamically stability of these compounds. The thermoelectric properties have been computed using semi-classical Boltzmann transport theory. We found high Seebeck coefficient (S) and high power factor (PF) for LuNiSb and LuPdSb compounds in the whole temperature range. The ZT values of LuNiSb and LuPdSb are high in general and reach a maximum of 0.67 and 0.69 at 450 K, respectively, whereas 0.39 is the maximum ZT value for LuPtSb at the same temperature. These findings propose LuNiSb and LuPdSb compounds as promising materials for thermoelectric applications at room temperature.

Towards tunable graphene phononic crystals

Phononic crystals (PnCs) are artificially patterned media exhibiting bands of allowed and forbidden zones for phonons—in analogy to the electronic band structure of crystalline solids arising from the periodic arrangement of atoms. Many emerging applications of PnCs from solid-state simulators to quantum memories could benefit from the on-demand tunability of the phononic band structure. Here, we demonstrate the fabrication of suspended graphene PnCs in which the phononic band structure is controlled by mechanical tension applied electrostatically. We show signatures of a mechanically tunable phononic band gap. The experimental data supported by simulation suggests a phononic band gap at 28–33 MHz in equilibrium, which upshifts by 9 MHz under a mechanical tension of 3.1 N m−1. This is an essential step towards tunable phononics paving the way for more experiments on phononic systems based on 2D materials.

Computation insights of MoS2-CrXY (X≠Y[dbnd]S, Se, Te) van der waals heterostructure for Spintronic and photocatalytic water Splitting applications

van der Waals heterostructure (vdWH) is an attractive technique to enhance the behavior of two-dimensional materials for designing viable products for electronic, spintronics and clean energy source application. Inspired by the recent synthesized of CrSSe monolayer, we have calculated the structural, electronic, Rashba parameters and photocatalytic properties of MoS2-CrXY (X≠YS, Se, Te) vdWH by using density functional theory calculation. Two different model of MoS2-CrXY (X≠YS, Se, Te) vdWH are presented due to the alternative order of chalcogen atom attached to MoS2 layer. Phonon band structure, thermal stability and binding energies confirms, the most favorable stacking configuration is dynamically, thermal and energetically feasible. Indirect type-I (MoS2-CrXY (X≠YS, Se)) and type-II (MoS2–CrSeTe) band alignment are observed in understudy vdWH. Large Rashba spin splitting is observed in model 2. Electrostatic potential and Bader charge analysis show charges are transferred from MoS2 layer to CrXY layer, while the potential drop separate charge carrier (holes and electrons) at the interface. Work function and charge density difference are also calculated and presented. A red shift is observed in the position of excitonic peaks from model 1 to model 2. In last we investigated the performance of these vdWH for photocatalytic water splitting at pH = 0. Our results shows that MoS2-CrXY (X≠YS, Se, Te) vdWH may be promising for future nanoelectronics, spintronics and photocatalytic water splitting.

Tuning electronic and phononic states with hidden order in disordered crystals

Disorder in crystals is rarely random, and instead involves local correlations whose presence and nature are hidden from conventional crystallographic probes. This hidden order can sometimes be controlled, but its importance for physical properties of materials is not well understood. Using simple models for electronic and interatomic interactions, we show how crystals with identical average structures but different types of hidden order can have very different electronic and phononic band structures. Increasing the strength of local correlations within hidden-order states can open band gaps and tune mode (de)localisation—both mechanisms allowing for fundamental changes in physical properties without long-range symmetry breaking. Taken together, our results demonstrate how control over hidden order offers a new mechanism for tuning material properties, orthogonal to the conventional principles of (ordered) structure/property relationships.

Crystal Chemistry and First Principles Studies of Novel Superhard Tetragonal C7, C5N2, and C3N4

Tetragonal C7, C5N2, and C3N4, characterized by mixed tetrahedral and trigonal atomic hybridizations, have been devised based on crystal chemistry rationale and structural optimization calculations within density functional theory (DFT). Substitution of C(sp2) and C(sp3) in C7 for nitrogen yields α-C5N2 and β-C5N2, respectively, both of which are superhard, cohesive, and stable mechanically (elastic properties) and dynamically (phonon band structures). tet-C3N4 with both nitrogen sites within the C7 structure was found to be cohesive and classified as ductile with a Vickers hardness of 65 GPa. Due to the delocalization of π electrons of the sp2-like hybridized atoms, metallic behavior characterizes all four phases.

Entropic Control of Bonding, Guided by Chemical Pressure: Phase Transitions and 18-n+m Isomerism of IrIn3.

As with other electron counting rules, the 18-n rule of transition metal-main group (T-E) intermetallics offers a variety of potential interatomic connectivity patterns for any given electron count. What leads a compound to prefer one structure over others that satisfy this rule? Herein, we investigate this question as it relates to the two polymorphs of IrIn3: the high-temperature CoGa3-type and the low-temperature IrIn3-type forms. DFT-reversed approximation Molecular Orbital analysis reveals that both structures can be interpreted in terms of the 18-n rule but with different electron configurations. In the IrIn3 type, the Ir atoms obtain largely independent 18-electron configurations, while in the CoGa3 type, Ir-Ir isolobal bonds form as 1 electron/Ir atom is transferred to In-In interactions. The presence of a deep pseudogap for the CoGa3 type, but not for the IrIn3 type, suggests that it is electronically preferred. DFT-Chemical Pressure (CP) analysis shows that atomic packing provides another distinction between the structures. While both involve tensions between positive Ir-In CPs and negative In-In CPs, which call for the expansion and contraction of the structures, respectively, their distinct spatial arrangements create very different situations. In the CoGa3 type, the positive CPs create a framework that holds open large void spaces for In-based electrons (a scenario suitable for relatively small T atoms), while in the IrIn3 type the pressures are more homogenously distributed (a better solution for relatively large T atoms). The open spaces in the CoGa3 type result in quadrupolar CP features, a hallmark of low-frequency phonon modes and suggestive of higher vibrational entropy. Indeed, phonon band structure calculations for the two IrIn3 polymorphs indicate that the phase transition between them can largely be attributed to the entropic stabilization of the CoGa3-type phase due to soft motions associated with its CP quadrupoles. These CP-driven effects illustrate how the competition between global and local packing can shape how a structure realizes the 18-n rule and how the temperature can influence this balance.

Phononic Band Structure by Calculating Effective Parameters of One-Dimensional Metamaterials

Using a theory of homogenization that consists in the discretization of the inclusion of a binary phononic crystal in small volumes, in which the material parameters can be expanded in Fourier series, we have determined the dependence of the effective elastic parameters as a function of the frequency. In particular, the frequency dependence of all the elements that constitute the effective tensors of stiffness (moduli of elasticity) and density was analyzed for a 1D phononic crystal conformed of materials whose main characteristic is the high contrast between their elastic properties. In this dynamic case of homogenization, it was found that the effective parameters can reproduce the exact dispersion relations for the acoustic modes that propagate along the periodicity direction of the crystal. Particularly, in the second pass band (high-frequency branch) corresponding to the transverse vibrational modes, the homogenized elastic phononic crystal exhibits a metamaterial behavior because the effective C44-component (shear modulus) and dynamic mass density were found to be both negative. It is noteworthy that the study derived from this homogenization technique can lead to design of double negative metamaterial systems for potential applications.

Exploring the suitability of narrow-bandgap compounds for thermoelectric application

Different opinion exists regarding the role of bandgap size on thermoelectric variables. Motivated by the observation that the band shape can in a definite way affect thermoelectric properties, two compounds (BaSiSr and IrBiZr) have been evaluated for optimum thermoelectric application. The phonon spectrum, band structure, mechanical and thermoelectric properties of the two compounds were investigated using the density functional theory. The calculated bandgap size of the compounds are comparable (IrBiZr = 0.223 eV; BaSiSr = 0.220 eV). The valence energy edge peaked at the Fermi level for both compounds. The highest valence energy edge occurred at a single high symmetry point (Γ for IrBiZr) but at two symmetry points (X and W for BaSiSr). IrBiZr gave an overall superior performance over BaSiSr in the calculated elastic, thermodynamic and thermoelectric parameters. A lesser role for bandgap size can therefore be indicated in the design and optimization of novel thermoelectric materials in view of better results in favor of IrBiZr than BaSiSr.

Significant reduction of the lattice thermal conductivity in antifluorites via a split-anion approach

Searching for thermoelectrics with low lattice thermal conductivity ${\ensuremath{\kappa}}_{l}$ is crucial to address needs for waste heat recovery, but few light element dominated materials provide a low ${\ensuremath{\kappa}}_{l}$. Herein, we propose an effective strategy, i.e., a split-anion approach, to reduce the ${\ensuremath{\kappa}}_{l}$ of the antifluorite. After introducing the split-anion approach, the stable quasiantifluorite ${\mathrm{Li}}_{4}\mathrm{OSe}$ is obtained, and ${\ensuremath{\kappa}}_{l}$ is reduced by $\ensuremath{\sim}90%$ compared to the antifluorite ${\mathrm{Li}}_{2}\mathrm{Se}$. The ${\ensuremath{\kappa}}_{l}$ value of ${\mathrm{Li}}_{4}\mathrm{OSe}$ reaches as low as 0.49--1.75 W ${\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, the range of thermoelectric applications, despite numerous light Li atoms and a large mass mismatch among the constituent atoms. This significant reduction of the thermal conductivity mainly results from the soft phonon branches and large optical bandwidth induced by lattice symmetry breaking after the split-anion approach. These findings offer ample scope for tuning the phonon band structure and thermal transport by reducing lattice symmetry like in the split-anion approach, leading to ultralow ${\ensuremath{\kappa}}_{l}$ of materials.

Skyrmion formation in Ni-based Janus dihalide monolayers: Interplay between magnetic frustration and Dzyaloshinskii-Moriya interaction

We present a comprehensive theory of the magnetic phases in monolayer nickel-based Janus dihalides (NiIBr, NiICl, NiBrCl) through a combination of first-principles calculations and atomistic simulations. Phonon band structure calculations and finite-temperature molecular dynamics simulations show that Ni-dihalide Janus monolayers are stable. The parameters of the interacting spin Hamiltonian extracted from ab initio calculations show that nickel-dihalide Janus monolayers exhibit varying degrees of magnetic frustration together with a large Dzyaloshinskii-Moriya interaction due to their inherent inversion symmetry breaking. The atomistic simulations reveal a delicate interplay between these two competing magnetic interactions in giving rise to skyrmion formation.

DFT calculations of opto-electronic, mechanical, thermodynamic, and transport properties of XCeO3 (X = Mg, Ca, and Ba) perovskite

We report structural, electronic, mechanical, optical, thermodynamic, and thermoelectric properties of XCeO3 (X = Mg, Ca, and Ba) perovskite using density functional theory as inculcated in WIEN2K code. In the structural perspective these perovskites are found crystalized in their cubic phase Pm-3 m (221). The electronic profiles show semiconducting nature of all inspected materials. We have included various potentials like LDA, GGA-PBE, mBJ, mBJ + SOC and HSE06 to estimate energy band gap of the considered materials. We have estimated band gap of 2.266, 2.619, 2.439 eV for the MgCeO3, CaCeO3 and BaCeO3 perovskites. The mechanical and dynamical stabilities of these compounds are assured by calculating mechanical properties and phonon band structure, respectively. Further, the optical parameters such as reflectivity, absorption coefficient, dielectric function, optical conductivity and energy loss are examined corresponding to photon energy within the energy range of 0 to 13 eV. These materials are active in UV-region and can be used as UV-absorber. Furthermore, thermoelectric properties are examined by evaluating Seebeck coefficient, thermal and electronic conductivities, figure of merit and power factor. Thermoelectric efficiency shows increasing behavior corresponding to temperature. Materials show good thermoelectric efficiency at 1200 K equivalent to 0.64, 0.66, 0.7 for MgCeO3, CaCeO3, BaCeO3, respectively. The thermodynamic stability of XCeO3 is ensured by studying the behaviour of volume, bulk modulus, entropy, heat capacity, thermal expansion coefficient, Gr u¨ neisen constant and Debye temperature corresponding to temperature and pressure. The novelty in these materials is found for their suggestive applications as UV absorber, thermoelectric power generators, temperature resistant materials.

A first-principles study on two-dimensional tetragonal samarium nitride as a novel photocatalyst for hydrogen production

The prospect of using two-dimensional tetragonal samarium nitride (t-SmN) in photocatalytic applications is being reported. First principles calculations were performed in order to study structural, electronic, thermal and photocatalytic properties of the material. The phonon band structure and cohesive energy computed using density functional perturbation theory revealed dynamical stability of the monolayer. Ab-initio molecular dynamics (AIMD) simulations were carried out to check the thermal stability of the monolayer which pointed that the material is stable above 1000 K and chemically inert at room temperature. The electronic structure calculations revealed SmN as ferromagnetic semiconductor with a direct bandgap of 1.41eV that predicts applications of the material in spintronics. The band alignment pointed towards suitability of band offsets for electrochemical reduction of water splitting at neutral pH. The optical properties indicated decent light-harvesting ability of the material from visible and ultraviolet regions of solar spectrum. The micro-mechanisms of water decomposition and photocatalytic hydrogen formation on the SmN monolayer are unraveled to confirm water splitting and hydrogen generation.

Thermodynamics and robust n-type charge carrier density in Co-doped FeTe2: strain strategy

Herein, we investigated the combined effect of Co-doping and strain (biaxial [110] and hydrostatic [111]) on the thermodynamics and electronic structure of the FeTe2 motif using ab-initio calculations by considering the strong correlation effects. The pristine one has a non-magnetic semiconducting nature with an indirect band gap (E g ) of 0.384 eV. Interestingly, our results revealed that the Co-doping at the Fe site induced an n-type conductivity (i.e. few states are crossing the Fermi level from the valence to conduction band) in the system having a substantial charge carrier density magnitude of 0.14 × 1021 cm−3. The metallicity mainly comprises the Co-3d orbitals along with a significant contribution from Fe-3d states. Thermodynamic, mechanical, and dynamical stability of the Co-doped FeTe2 structure is confirmed by computing the formation energetic, elastic constants, and phonon band structure, respectively. Generally, an increasing and decreasing trend in E g value is evident against the applied compressive and tensile strains having ranged from −5% to +5% for the case of the undoped system, respectively. On the other hand, the Co-doped structure maintained its n-type conduction against considered both types of strains. Moreover, it is demonstrated that compressive strains strengthen the charge carrier density amplitude, while tensile strains show a negative impact. Hence, the present work displays that robust n-type conductivity and stable structure of Co-doped FeTe2 system, makes it a desirable candidate for device applications.

Exploring electronic, optical, and phononic properties of MgX (X = C, N, and O) monolayers using first principle calculations

The electronic, the thermal, and the optical properties of hexagonal MgX monolayers (where [Formula: see text] = [Formula: see text], [Formula: see text], and [Formula: see text]) are investigated via first principles studies. Ab-initio molecular dynamic, AIMD, simulations using NVT ensembles are performed to check the thermodynamic stability of the monolayers. We find that an MgO monolayer has semiconductor properties with a good thermodynamic stability, while the MgC and the MgN monolayers have metallic characters. The calculated phonon band structures of all the three considered monolayers show no imaginary nonphysical frequencies, thus indicating that they all have excellent dynamic stability. The MgO monolayer has a larger heat capacity then the MgC and the MgN monolayers. The metallic monolayers demonstrate optical response in the IR as a consequence of the metal properties, whereas the semiconducting MgO monolayer demonstrates an active optical response in the near-UV region. The optical response in the near-UV is beneficial for nanoelectronics and photoelectric applications. A semiconducting monolayer is a great choice for thermal management applications since its thermal properties are more attractive than those of the metallic monolayer in terms of heat capacity, which is related to the change in the internal energy of the system.

Exploring and machine learning structural instabilities in 2D materials

We address the problem of predicting the zero-temperature dynamical stability (DS) of a periodic crystal without computing its full phonon band structure. Here we report the evidence that DS can be inferred with good reliability from the phonon frequencies at the center and boundary of the Brillouin zone (BZ). This analysis represents a validation of the DS test employed by the Computational 2D Materials Database (C2DB). For 137 dynamically unstable 2D crystals, we displace the atoms along an unstable mode and relax the structure. This procedure yields a dynamically stable crystal in 49 cases. The elementary properties of these new structures are characterized using the C2DB workflow, and it is found that their properties can differ significantly from those of the original unstable crystals, e.g., band gaps are opened by 0.3 eV on average. All the crystal structures and properties are available in the C2DB. Finally, we train a classification model on the DS data for 3295 2D materials in the C2DB using a representation encoding the electronic structure of the crystal. We obtain an excellent receiver operating characteristic (ROC) curve with an area under the curve (AUC) of 0.90, showing that the classification model can drastically reduce computational efforts in high-throughput studies.

Moiré phonons in graphene/hexagonal boron nitride moiré superlattice

We theoretically study in-plane acoustic phonons of graphene/hexagonal boron nitride moir\'e superlattice by using a continuum model. We demonstrate that the original phonon bands of individual layers are strongly hybridized and reconstructed into moir\'e phonon bands consisting of dispersive bands and flat bands. The phonon band structure can be effectively described by a spring-mass network model to simulate the motion of moir\'e domain walls, where the flat-band modes are interpreted as vibrations of independent, decoupled strings. We also show that the moir\'e phonon has angular momentum due to the inversion symmetry breaking by hBN, with high amplitudes concentrated near narrow gap region. Finally, we apply the same approach to twisted bilayer graphene, and we find a notable difference between the origins of the flat-band modes in G/hBN and TBG, reflecting distinct geometric structures of domain pattern.