Calculated Phonon Spectra Research Articles

Theoretical Investigations on the Nanowires Assembled by the V1@Si12 Clusters

Based on the idea of “bottom-up” method, a series of one dimensional (1D) nanowires (NWs) assembled by V1@Si12 clusters were designed, and their structures, stabilities, magnetic and electronic properties were examined by means of density functional theory computations. We found that the 1D V1@Si12 NW connected through two inter-cluster Si–Si bonds along the radial direction of the Si12 drum exhibits good stabilities, antiferromagnetism and tunable electronic behavior under external strains; however, the NWs built along the axial direction of the Si12 drum do not have the dynamical stability as indicated by the imaginary frequencies in the calculated phonon spectrum. The radial assembly pattern could also be applicable to other Si-based building blocks, such as Ti1@Si12 and V3@Si12. Our theoretical investigations provide a new assembling way for 1D NWs with diverse magnetic and electronic properties.

Computational and experimental identification of hydrogen defect vibrational modes in zinc sulfide

The characteristic extrinsic infrared absorption in bulk chemical vapor deposited (CVD) ZnS has been unambiguously identified. High resolution absorption measurements and ab initio vibrational calculations identify the absorption of 1760–1580 cm−1 as due to defects of the form (ZnHn)S, with n = 3 being the highest concentration. Multiple absorption peaks in this spectral region are due to Zn-H stretching from different bond lengths. Phonon spectra calculations of hexagonal ZnO impurities do not account for the spectral position of the absorption without three-phonon processes and thus are unlikely to be responsible for the extrinsic infrared absorption in CVD ZnS. This work shows the complementary utility of computational and experimental work for identifying defect structures in semiconductors.

Computational Prediction of the Low‐Temperature Ferromagnetic Semiconducting 2D SiN Monolayer

Since the discovery of graphene, 2D materials have captured the minds of scientists because of their attractive and unique electronic properties. In particular, magnetic 2D materials have become a subject of extensive discussions today. Using density functional theory calculations, it is shown that 2D SiN sheet (built out of nonmetallic main group atoms) is a ferromagnetic semiconducting material with a magnetic moment 1 μB per unit cell and an indirect bandgap of 1.55 eV. Calculated phonon spectrum and conducted ab initio molecular dynamics simulation reveal thermal and dynamical stability of the designed material. It is shown that the ferromagnetic state is stable up to 20 K. Magnetism of silicon mononitride can be described by the presence of an unpaired electron located on silicon atoms. The semiconducting and ferromagnetic properties of SiN monolayer open many opportunities for its potential use in spintronic and nanoelectronic devices.

Structure, Mössbauer spectroscopy and vibration phonon spectra in valence-bond force-field model approach for distorted perovskites AFeO3 (A = La, Y)

Two distorted perovskite orthoferrites, AFeO3 (A = La, Y), were synthesized by solid-state reaction method and investigated using the experimental techniques, e.g., X-ray diffraction, Mössbauer spectra, and infrared and Raman spectra at room temperature. The internal hyperfine magnetic field at La-site is slightly larger than for Y-site due to the tilt angle and distortion of the oxygen octahedral, and Néel temperature dependence on the ionic radius of A-site ions. Vibration spectra were analysed employing the normal coordinate analysis method within the framework of a short-range valence-bond force-field model. The present lattice dynamical calculations of optical phonon spectra provide useful insights towards the dependence of phonon frequencies on different bond strengths caused by steric and magnetic effects of A-site cations. Mode mixing occurs due to coupling of different Ag and Bg modes in the Raman spectra of LaFeO3 and YFeO3. The assignment and comparison of the observed phonon spectra of LaFeO3 and YFeO3 to different symmetry modes differ strikingly from the other isomorphous rare-earth orthoferrites, which may be due to different degree of structural distortion and complexity of magnetic interactions between A and Fe ions through intermediate oxygen ions over the ferrite series.

Phase transitions and lattice dynamics in perovskite-type hydride

First principles calculations have been used to investigate the structural phase transitions and lattice dynamics in the alloy . The density functional perturbation theory and the virtual crystal approximation are employed, within the generalized gradient approximation. Our total energy and formation enthalpy results show that undergoes a phase transition from the orthorhombic Pnma phase to the polar R3c one at a lithium concentration of x = 0.7. Under pressure the system in the R3c structure exhibits a phase transition to the Pnma one and the values of the transition pressure are 0.32, 1.79 and 4.07 GPa for x = 0.7, 0.75 and 0.8, respectively. The calculated phonon dispersion curves for different lithium concentration show a softening of the B2u mode at the zone center ( point) which vanishes at x =0.8 in the polar R3c phase confirming the transition of the system from the non polar Pnma structure to the polar R3c one. The phonon frequencies at the zone center for the Raman-active and infrared-active modes are predicted for both structures for different lithium concentrations. The heat capacity is evaluated from the calculated phonon spectra and density of states in function of temperature and lithium concentration in both phases.

Structural, mechanical and electronic properties study on group 5 transition metals ternary mononitrides from first-principles calculations

Various possible structures of ternary mononitrides of group 5 transition metals (V0.5Ta0.5N, V0.5Nb0.5N and Nb0.5Ta0.5N) are systematically explored in 0–300 GPa pressure range through CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method. Three possible phases with Pmm2, Cmc21, and Fm-3m space group have been uncovered. Their dynamical, mechanical and thermodynamic stabilities are checked through calculations of phonon spectra, elastic constants and formation enthalpy. The elastic anisotropies are fully investigated by the dependence of linear bulk, Young's and shear moduli on the crystal orientations. To study the strength of these three compounds, the compressive, tensile and indentation shear strengths are systematically studied from the first-principles calculations. The mononitrides with Pmm2 and Cmc21 structures exhibit much better mechanical property relative to VN, NbN and TaN with traditional B1 (NaCl-like) structure. Our results reveal that the Pmm2 and Cmc21 phases of these three compounds possess the strength values up to 30 GPa under the Vickers indentation simulation calculations, which are comparable to well-known WB3, WB4 and ReB2 reported as potential superhard transition metal borides. Additionally, the electronic structures of these mononitrides are also studied to explore the intrinsic chemical bonding nature, and a novel transition of semiconductor-to-conductor from V0.5Ta0.5N, V0.5Nb0.5N to Nb0.5Ta0.5N is exposed for the Fm-3m phases of these compounds.

Discovery of a new crystal structure of LiBeF3 and its thermodynamic and optical properties

This article reports a newly discovered crystal structure of LiBeF3 from approximately 700 candidate structures by means of high-throughput DFT (Density Functional Theory) screening. The lowest-energy structure is monoclinic with the P2/c symmetry and its primitive cell contains 100 atoms. The calculated phonon spectrum appears no imaginary modes, which evidences the dynamic stability of this structure. Regarding its thermodynamic stability, the calculated electronic total energy shows that this structure is instable and may decompose into BeF2 and K2BeF4 at T = 0 K; however, above T = 260 K the LiBeF3 structure is stabilized, caused by extra free energy from phonon activation of lattice vibrations. The calculated electronic properties demonstrate that the new structure possesses an indirect band gap larger than 7.7 eV, indicating that the new structure could be a promising candidate for vacuum-ultraviolet-transparent lens in optical lithography steppers used in the semiconductor industry.

Titanium carbide sheet: metallic 2D structure with intrinsic and high catalytic activity for the hydrogen evolution reaction

Searching for a metallic material that possesses intrinsic and high catalytic activity is interesting for the hydrogen evolution reaction (HER) in the electrochemical reduction of water. In this work, via density functional theory, we identified a promising two-dimensional (2D) catalyst material, namely TiC (1 0 0) sheet, which is composed of two atomic layers. Phonon spectrum calculation shows that all vibrational modes of the TiC (1 0 0) sheet are real. Even at 1200 K, the geometrical structure of TiC (1 0 0) sheet is still well kept. Interestingly, TiC (1 0 0) sheet is metallic, which means that TiC (1 0 0) sheet can possess high conductivity. Moreover, the Gibbs free energy () of hydrogen on TiC (1 0 0) sheet at a low coverage is about 0.13 eV, which can be further optimized to an ideal value of 0 eV by introducing a strain. Besides, the effect of thickness on its catalytic activity is also assessed. We observed that TiC (1 0 0) sheet with a large thickness is also metallic. Calculated values of for four-layered and six-layered TiC (1 0 0) sheets at a moderate coverage can be about −0.06 and −0.04 eV, respectively. Our results highlight a new promising 2D catalyst material for the HER in the electrochemical reduction of water.

O-C8 carbon: A new allotrope of superhard carbon

An orthonormal crystal of carbon with PMMA space group (o-C8) was found to be a stable superhard carbon allotrope by particle swarm optimization algorithm and density functional calculations. The phonon spectrum calculations demonstrate that the o-C8 carbon phase is dynamically stable. The volume compression calculations show that it is highly incompressible, with bulk modulus of 298.6 GPa. The calculations demonstrate that it is a low-density superhard material with density of 2.993 g/cm3 and Vickers hardness of 82.4 GPa.

First-principles studies of structural, elastic, electronic, optical and lattice dynamical properties of XSnP2 (X = Zn, Cd and Hg) compounds

The structural, elastic, electronic, optical, lattice dynamical and thermodynamic properties of XSnP2 (X = Zn, Cd and Hg) compounds with chalcopyrite structure were studied using density functional theory. Including the newly predicted HgSnP2, XSnP2 compounds are thermodynamically, mechanically and dynamically stable. The calculated elastic constant, bulk modulus, shear modulus, Young's modulus and Poisson's ratio reveal that XSnP2 compounds are elastic anisotropic: ZnSnP2 is brittle, but CdSnP2 and HgSnP2 are ductile. The XSnP2 compounds are direct-gap semiconductors with band gaps of 2.062, 1.424 and 0.543 eV, respectively. All XSnP2 compounds have strong absorptions and high conductivities in the ultraviolet region. Especially, HgSnP2 has the greatest absorption coefficient and conductivity in the visible region, indicating that it has the strongest visible-light absorption capacity and the best photovoltaic activity. For XSnP2 compounds, the D2d group has five irreducible representations: A1, A2, B1, B2 and E. Modes A1 and A2 only involve displacements of P atoms, but the other three modes involve vibrations of X, Sn and P atoms. Based on the calculated phonon spectra of XSnP2, their thermodynamic properties were also analyzed in detail. The current work deepened the understanding of crystal structures and physical properties of XSnP2 compounds, and will aid in their potential application.

Phonon mode calculations and Raman spectroscopy of the bulk-insulating topological insulator BiSbTeSe2

The tetradymite compound BiSbTeSe$_2$, is one of the most bulk-insulating three-dimensional topological insulators, which makes it important in the topological insulator research. It is a member of the solid-solution system Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$, for which the local crystal structure, such as the occupation probabilities of each atomic site, is not well understood. We have investigated the temperature and polarization dependent spontaneous Raman scattering in BiSbTeSe$_2$, revealing a much higher number of lattice vibrational modes than predicted by group theoretical considerations for the space group R$\bar{3}$m corresponding to an ideally random solid-solution situation. The density functional calculations of phonon frequencies show a very good agreement with experimental data for parent material Bi$_2$Te$_3$, where no disorder effects were found. In comparison to Bi$_2$Te$_3$ the stacking disorder in BiSbTeSe$_2$, causes a discrepancy between theory and experiment. Combined analysis of experimental Raman spectra and DFT calculated phonon spectra for different types of atomic orders showed coexistence of different sequences of layers in the material and that those with Se in the center and a local order of Se-Bi-Se-Sb-Te, are the most favored.

A first-principles study of the properties of P-43m-Si3×2 (X = N, P and As)

The new P-43m-Si3P2 and P-43m-Si3As2 structures are predicted using the first-principles approach based density functional theory (DFT). The elastic constants, structural stability, phonon dispersion spectra, band structures, density of states, and optical properties of P-43m-Si3×2 (X=N, P and As) have been analyzed. The values of the elastic constants indicate that their structures are mechanically stable. Each elastic constant of the Si3N2 is greater than the corresponding elastic constants of Si3P2 and Si3As2. The Young's moduli, shear moduli, bulk moduli, Pugh ratios and Poisson's ratios of P-43m-Si3×2 are calculated at 0 GPa. Si3N2 has a larger Young's modulus, so it has higher hardness and good resistance to deformation. The bulk moduli of P-43m-Si3×2 are isotropic. The shear modulus of Si3As2 is anisotropic. The Pugh ratios of P-43m-Si3×2 are 0.50, 0.49 and 0.39, respectively. Their Poisson's ratios are 0.28, 0.29 and 0.33, respectively. The results show that they are brittle materials at zero pressure. The calculated phonon spectra confirm that they are dynamically stable. The calculated enthalpy of formation indicates their thermodynamic stability. The energy band gaps of P-43m-Si3×2 calculated by HSE06 hybrid function are 0.786, 0.955 and 0.343 eV, respectively. Si3N2 has a direct bandgap, Si3P2 and Si3As2 have indirect bandgaps. The dielectric functions, refractive indices, optical reflectance spectra, absorption coefficients, conductivities and loss functions of P-43m-Si3×2 are calculated. The calculated static dielectric constants of P-43m-Si3×2 are 5.207, 9.237 and 10.072, respectively. The maximum values of the loss functions of P-43m-Si3×2 are 6.408, 5.672 and 5.276 eV, respectively.

Stability and physical properties tuning via interstitials chemical engineering of Zr5Sn3: a first-principles study

Hexagonal binary intermetallics A5B3 has a unique A6 octahedra chain structure, providing space for interstitial chemical engineering the physical, mechanical, electrical, and chemical properties without change in the basic structure of crystal. Because of the engineering importance of Zr–Sn alloy, here, we investigate the influence of 24 interstitial alloying elements X (X = B, C, N, O, Al, Si, P, S, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Nb, and Sn) on stability and properties of hexagonal Zr5Sn3 via first-principles calculations. A general trend is that the additional element with small atom size and high electronegativity is favorable as interstitials in Zr5Sn3. The calculated formation enthalpy and the elastic constants suggest that these Zr5Sn3X structures are thermodynamically and mechanically stable. The calculated phonon spectra indicate that Zr5Sn3X structures are dynamically stable except X = V, Cr, Mn, Zn, and Nb. We show that their electronic structures including bonding characters have strong correlation with the stability and mechanical properties. With strong covalent bonds, Zr5Sn3B has the highest Young’s modulus, bulk modulus, shear modulus, Debye temperature, and microhardness. The addition of alloying elements decreases the anisotropy except X = O, Sc, Ti, V and Nb. All the additive elements increase the specific heat capacity of Zr5Sn3. Our results could be helpful in designing and improve the performance of Zr–Sn alloy on demand.

First-principles study of ultrathin molybdenum sulfides nanowires: Electronic and catalytic hydrogen evolution properties

Molybdenum sulfides nanomaterials, such as one-dimensional (1D) nanotubes, nanoribbons, and two-dimensional (2D) nanosheets, have attracted intensive research interests for their novel electronic, optical, and catalytic properties. On the basis of first-principles calculation, here, we report a new series of 1D ultrathin molybdenum sulfides nanowires, including Mo2S6, Mo3S6 and Mo6S10 nanowires. Our results demonstrate that these ultrathin nanowires are both thermal and lattices dynamically stable, confirmed with the calculated phonon spectrum and Born-Oppenheimer molecular dynamic simulation at the temperature up to 600 K. The calculated elastic constant is 21.33, 103.22, and 163.00 eV/Å for Mo2S6, Mo3S6, and Mo6S10 nanowires, respectively. Mo2S6 and Mo3S6 nanowires are semiconductors with band gap of 1.55 and 0.46 eV, while Mo6S10 nanowires is metal, implying their potential applications in electronics and optoelectronics. In particular, ultrathin molybdenum sulfides nanowires can be used as catalysts for hydrogen evolution reaction. The calculated Gibbs free energy change for hydrogen evolution is about −0.05 eV for Mo2S6 nanowire, comparable with those of Pt and H-MoS2. The prediction of these 1D molybdenum sulfides nanowires may enrich the 1D family molybdenum sulfides and make a supplement to understand the high performance of hydrogen evolution reaction in transition-metal dichalcogenides.

Stability of 2D Alkaline-Earth Metal Silicides, Germanides and Stannides

By means of ab initio calculations, we have estimated stability of 2D Me[Formula: see text] ([Formula: see text], Ca, Sr, Ba and [Formula: see text], Ge, Sn) in the T and Td phases, which are similar to the ones of 2D transition metal chalcogenides, in addition to their phonon spectra. The T phase is found to be more stable for 2D Ca[Formula: see text], Sr[Formula: see text] and Ba[Formula: see text], whereas the Td phase is predicted to be the ground state for 2D Mg[Formula: see text]. We have also discussed that imaginary frequencies in the calculated phonon spectra of 2D Me[Formula: see text], which appeared in the vicinity of the [Formula: see text] point, were not necessarily associated with the dynamic instability.

Van der Waals heterostructures of P, BSe, and SiC monolayers

Electronic structure, optical, and photocatalytic properties of P, BSe, and SiC monolayers and their van der Waals heterostructures are investigated by (hybrid) first-principle calculations. The stability of the heterostructures and their corresponding induced-strain/unstrain monolayers are confirmed by the phonon spectra calculations. Similar to the corresponding parent monolayers, P-BSe (BSe-SiC) heterostructures are indirect type-II (type-I) bandgap semiconductors. A tensile strain of 10% (2%) transforms P-BSe (BSe-SiC) to type-I (type-II) direct bandgap nature. Interestingly, irrespective of the corresponding monolayers, the P-SiC heterostructure is a direct bandgap (type-II) semiconductor. The calculated electron and hole carrier mobilities of these heterostructures are in the range of 1.2×104cm2/Vs to 68.56×104cm2/Vs. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, where the lowest energy transitions are dominated by excitons. The valence and conduction band edges straddle the standard redox potentials in P-BSe, BSe-SiC, and P-SiC (strained) heterostructures, making them promising candidates for water splitting in the acidic solution. An induced compressive strain of 3.5% makes P suitable for water splitting at pH=0.

Thermal bridging of graphene nanosheets via covalent molecular junctions: A non-equilibrium Green’s functions–density functional tight-binding study

Despite the uniquely high thermal conductivity of graphene is well known, the exploitation of graphene into thermally conductive nanomaterials and devices is limited by the inefficiency of thermal contacts between the individual nanosheets. A fascinating yet experimentally challenging route to enhance thermal conductance at contacts between graphene nanosheets is through molecular junctions, allowing covalently connecting nanosheets otherwise interacting only via weak Van der Waals forces. Beside the bare existence of covalent connections, the choice of molecular structures to be used as thermal junctions should be guided by their vibrational properties, in terms of phonon transfer through the molecular junction. In this paper, density functional tight-binding combined with Green functions formalism was applied for the calculation of thermal conductance and phonon spectra of several different aliphatic and aromatic molecular junctions between graphene nanosheets. Effect of molecular junction length, conformation, and aromaticity were studied in detail and correlated with phonon tunnelling spectra. The theoretical insight provided by this work can guide future experimental studies to select suitable molecular junctions in order to enhance the thermal transport by suppressing the interfacial thermal resistances, particularly attractive for various systems, including graphene nanopapers and graphene polymer nanocomposites, as well as related devices. In a broader view, the possibility to design molecular junctions to control phonon transport currently appears as an efficient way to produce phononic devices and controlling heat management in nanostructures.

Correction to: Strain-modulated mechanical, electronic, and thermal transport properties of two-dimensional PdS2 from first-principles investigations

Strains can effectively modulate the electronic structure and thermal conductivity of materials. In this work, the electronic structure and thermal conductivity properties of PdS2 under different strains were investigated using first-principles calculation combined with iterative method for solving Boltzmann transport equation theory. Through phonon spectrum calculations, it is found that PdS2 is thermodynamically stable in the strain range from 0 to 10%. Interestingly, the strained single-layer PdS2 transforms from an indirect bandgap semiconductor to a quasi-direct bandgap semiconductor, and the bandgap of PdS2 decreases to 1.41 eV (decreased by 20.8%). Due to the softening of the phonons and the decreasing of phonons group velocity, the thermal conductivity is reduced with the applied biaxial area strains. Thermal transport investigations reveal that the in-plane thermal conductivity of unstrained PdS2 is 32.32 Wm− 1 K− 1. When the area strain reaches 10%, the thermal conductivity of PdS2 is reduced by nearly twice the ratio of strain. The sensitive strain dependence of bandgap and thermal conductivity indicates that PdS2 can flexibly select the substrate and match the substrate, and the thermoelectric coefficient can be effectively adjusted, indicating that PdS2 has good application prospects in thermoelectric, photoelectric, and catalytic materials.

High-pressure structure prediction and high-temperature structural stability of periclase

Periclase is the terminal component of the ferropericlase, and its chemical composition is MgO. It is well known that there exists a huge difference between the melting curves of MgO determined experimentally and theoretically. A feasible way to clarify the nature of the melting temperature is to investigate the possible new phase of MgO. Meanwhile, it is very important to study the new phase and the influence of temperature on structural stability of MgO in high-pressure condensed matter physics and geophysics. In the present work, we study in detail the phase stability and the possible existing structures of MgO, which include the structure predicted by particle swarm optimization algorithm through using the first-principles pseudopotential density functional method. We find that MgO crystallizes into a rocksalt structure in a pressure range from 0 to 580 GPa and that the CsCl-type structure is of a high-pressure phase at up to 800 GPa. Although an NiAs-type hexagonal phase perhaps explains the volume discontinuity at (170 ± 10) GPa along the MgO Hugoniot in a shock-compression experiment (Zhang L, Fei Y W 2008 <i>Geophys. Res. Lett</i>. <b>35</b> L13302) and a wurtzite phase perhaps explains the huge difference between the melting curves of MgO determined experimentally and theoretically (Aguado A, Madden P A 2005 <i>Phys. Rev. Lett</i>. <b>94</b> 068501), neither of them is existent in the entire range of pressures studied, according to the thermodynamic stability calculations. The calculations of phonon spectra indicate that the B3, B4, B8<sub>1</sub>, B8<sub>2</sub>, and <i>P</i>3<i>m</i>1 phases of MgO are dynamically stable at zero pressure. That is to say, all of the predicted structures are the metastable structures of MgO. In addition, the high-temperature structural stability of MgO is investigated by using very similar Lewis-Catlow and Stoneham-Sangster shell model potential based on the classical molecular dynamics (MD) simulations. In order to take into account the non-central force in crystal, the breathing shell model is also introduced in simulation. The thermodynamic melting curves are estimated on the basis of the thermal instability MD simulations and compared with the available experimental data and other theoretical results in the pressure range of 0－150 GPa.

Lattice dynamics of Bi3TeBO9 microcrystals: μ-Raman/IR spectroscopic investigation and ab initio analysis

A Bi3TeBO9 crystal is an excellent nonlinear optical material as well as an efficient host matrix for luminescent rare earth ions. Investigated Bi3TeBO9 microcrystals were synthesized by means of the modified Pechini method. XRD measurements of Bi3TeBO9 confirmed its hexagonal structure with noncentrosymmetric P63 space group. The vibrational properties of Bi3TeBO9 were studied using μ-Raman and FT-IR/MIR spectroscopy. The theoretical lattice dynamics study of Bi3TeBO9 crystal at room temperature comprises the group theory analysis and the ab initio calculations of phonon mode spectrum in Brillouin zone centre. The unpolarized Raman and IR spectra were analyzed in details on the basis of group theory predictions and the results of ab initio calculations of phonon spectra.