Calculated Phonon Spectra Research Articles

Oxidation of two-dimensional electrides: Structural transition and the formation of half-metallic channels protected by oxide layers

Based on first-principles calculations, we performed a systematic study of the energetic stability, structural characterization, and electronic properties of the fully oxidized ${A}_{2}B$ electrenes, with the following combinations: (i) $A$ = Ca, Sr, and Ba for $B$ = N; (ii) $A$ = Sr and Ba for $B$ = P; and ${\mathrm{Y}}_{2}\mathrm{C}$ and ${\mathrm{Ba}}_{2}\mathrm{As}$. We have considered one side oxidation of single layer electrenes (O/${A}_{2}B$), and two side oxidation of bilayer electrenes (O/${({A}_{2}B)}_{2}$/O). We show that the hexagonal lattice of the pristine host is no longer the ground state structure in the (fully) oxidized systems. Our total energy results reveal an exothermic structural transition from hexagonal to tetragonal ($\mathrm{h}\ensuremath{\rightarrow}\mathrm{t}$) geometry, resulting in layered tetragonal structures [(${A\mathrm{O}AB)}^{\mathrm{t}}$ and ${(A\text{O}{(AB)}_{2}A\text{O})}^{\mathrm{t}}$]. Phonon spectra calculations and molecular dynamic simulations show that the O/${A}_{2}B$ and O/${({A}_{2}B)}_{2}$/O systems, with $A$ = Ba, Ca, Sr, and $B$ = N, become dynamically and structurally stable upon such a $\mathrm{h}\ensuremath{\rightarrow}\mathrm{t}$ transition. Further structural characterizations were performed based on simulations of the near edge x-ray absorption spectroscopy at the nitrogen $K$ edge. Finally, the electronic band structure and transport calculations reveal the formation of half-metallic bands spreading out through the $A\mathrm{N}$ layers, which in turn are shielded by oxide $A\mathrm{O}$ sheets. These findings indicate that ($A\mathrm{O}A\mathrm{N}$)${}^{\mathrm{t}}$ and ${(A\mathrm{O}{(A\mathrm{N})}_{2}A\mathrm{O})}^{\mathrm{t}}$ are quite interesting platforms for application in spintronics; since the half-metallic channels along the $A\mathrm{N}$ or ${(A\text{N})}_{2}$ layers (core) are protected against the environment conditions by the oxidized $A\text{O}$ sheets (cover shells).

Structural, Elastic, Electronic, Magnetic, and Thermoelectric Properties of the IrFeSi Half-Heusler Compound: A First-Principles Study

The emerging need for new functional materials has recently been engaged in a large quest to predict the alternative green and low-cost energy industry. To this end, thermoelectric as well as spintronic applications remain challenging. In this study, we present, for the first time, a density functional calculation on the IrFeSi half-Heusler compound. The calculation gives a deep analysis of the stability within the calculation of phonon spectra and elastic constants as well as Gibbs energy. The electronic and magnetic properties show that the investigated compound is a half-metallic ferromagnetic material. We show that IrFeSi has an integer magnetic moment of 3[Formula: see text][Formula: see text] in good agreement with the Slater–Pauling (SP) rule. By means of the Boltzman theory, we find that the investigated compound exhibits an interesting thermoelectric performance at room temperature.

Effect of Bi substitution on the cationic vacancy ordering in SrMoO4-based complex oxides: Structure and properties

The electronic and crystal structures of the Sr1−3xBi2xФxMoO4 series (0.025 ≤ x ≤ 0.30, where Ф represents cation vacancies) were studied. The 0.025 ≤ x ≤ 0.15 compositions showed a defect scheelite structure. Powder X-ray and neutron diffraction patterns for compositions with 0.15 < x ≤ 0.225 exhibited a tetragonal supercell with asup ≈ √5a, csup ≈ c where a and c are the tetragonal scheelite cell parameters. Strong distortion of MoO4 polyhedra was shown by Raman spectroscopy and described by the calculated phonon spectra of “virtual crystals” of Bi2+[MoO4] and Ф2+[MoO4]. The electronic structures were calculated for both types of structure. The decrease of the calculated energy gap is consistent with experimental data from Kubelka-Munk measurements. The electroconductive properties were measured by A.C. impedance spectroscopy. For Sr1−3xBi2xФxMoO4 compositions conductivity increases with increasing x-value, with maximum values of conductivity at 948 K of 2.82 × 10−5S cm−1 for the x = 0.20 composition.

First principle design of new thermoelectrics from TiNiSn based pentanary alloys based on 18 valence electron rule

In this study we have reported electronic structure, lattice dynamics, and thermoelectric (TE) transport properties of a new family of pentanary substituted TiNiSn systems using the 18 valence electron count (VEC) rule. We have modeled the pentanary substituted TiNiSn by supercell approach with the aliovalent substitution, inspite of the traditional isoelectronic substitution. We have performed the detailed analysis of electronic structure, lattice dynamics, and TE transport properties of selected systems from this family. From our calculated band structures and density of states we show that by preserving the 18 VEC through aliovalent substitutions at Ti site of TiNiSn semiconducting behavior can be achieved and hence one can tune the band structure and band gap to maximize the thermoelectric figure of merit (ZT) value. Two approaches have been used for calculating the lattice thermal conductivity (κL), one by fully solving the linearized phonon Boltzmann transport (LBTE) equation from first-principles anharmonic lattice dynamics calculations implemented in Phono3py code and other using Slack’s equation with calculated Debye temperature and Grüneisen parameter using the calculated elastic constant values. At high temperatures the calculated κL and ZT values from both these methods show very good agreement. The calculated κL values decreases from parent TiNiSn to pentanary substituted TiNiSn systems as expected due to fluctuation in atomic mass. The substitution of atoms with different mass creates more phonon scattering centers and hence lower the κL value. The calculated κL for Hf containing systems La0.25Hf0.5V0.25NiSn and non Hf containing system La0.25Zr0.5V0.25NiSn calculated from Phono3py (Slack’s equation) are found to be 0.37 (1.04) and 0.16 (0.95) W/mK, at 550K, respectively and the corresponding ZT value are found to be 0.54 (0.4) and 0.77 (0.53). Among the considered systems, the calculated phonon spectra and heat capacity show that La0.25Hf0.5V0.25NiSn has more optical-acoustic band mixing which creates more phonon–phonon scattering and hence lower the κL value and maximizing the ZT. Based on the calculated results we conclude that one can design high efficiency thermoelectric materials by considering 18 VEC rule with aliovalent substitution.

Structural, electronic, and optical properties of C3B and C3B0·5N0.5 monolayers: A many-body study

Performing first-principles calculations, the structural, electronic, and optical properties of C 3 B and C 3 B 0·5 N 0.5 monolayers are investigated systematically. The calculations of cohesive energies, molecular dynamics, and phonon spectra show the energetic , thermal, and dynamical stabilities of C 3 B and C 3 B 0·5 N 0.5 monolayers. The C 3 B 0·5 N 0.5 (C 3 B) is predicted to be semiconductors with moderate direct (indirect) gap of 2.376 (1.948) eV at G 0 W 0 level. The observed systems both have good absorptions for the visible light . Compared with that of the C 3 B, first absorption peak of the C 3 B 0·5 N 0.5 is remarkably red-shifted, suggesting an enhanced absorption for the near-infrared light. The corresponding bright excitons have large binding energies, being 0.913 eV for the C 3 B and 0.704 eV for the C 3 B 0·5 N 0.5 , respectively. The relatively small binding energy of exciton in the C 3 B 0·5 N 0.5 is very beneficial for the separation of the photogenerated electron-hole pair. More importantly, the C 3 B 0·5 N 0.5 monolayer can achieve a large light absorption coefficient beyond 10 5 cm −1 for the sunlight irradiation. These findings enrich the deep understanding of C 3 B and C 3 B 0·5 N 0.5 monolayers, and indicate their potential applications in optoelectronics. Figure: Comparable optical absorptions of C 3 B and C 3 B 0·5 N 0.5 monolayers obtained by the G 0 W 0 +BSE method. • Electronic and optical properties of C 3 B and C 3 B 0·5 N 0.5 are investigated by the G 0 W 0 +BSE method. • The two compounds exhibits semiconducting properties with moderate indirect/direct bandgap. • Large exciton binding energies in two compounds show that these excitons have good stabilities against external force. • The two compounds have good optical properties that make them promising for optoelectronic applications.

Comparison of features arising in phonon spectra of crystals belonging to the argyrodite family for various combinations of orbits filled with Ag (Cu) atoms

The paper is devoted to model phonon spectra calculations of argyrodite family crystals, namely: Ag7GeSe5I and Cu7GeSe5I (Ag7SiS5I and Cu7SiS5I). The specificity of the crystalline structure of these crystals is realization of variable partial populating different crystallographic orbits with Cu, I and Ag atoms (in 24 (g) and 48 (h) Positions Multiplicity, Wyckoff letter, Site symmetry of the space group ), which are associated with high superionic conductivity in crystals of this family. Compared in this paper are model phonon spectra calculated with account of three different configurations of orbital populations with Cu and Ag atoms (6 + 1, 5 + 2 and 4 + 3), by choosing the same lattice model for the unit cell and the set of force constants of each crystal. The calculations were performed by finding the eigenvalues of the generalized dynamic matrix perturbed by the modulation function of the mass defect obtained in the concept over spatial symmetry. It is shown that the phonon spectra do not undergo significant qualitative changes, which can serve as indirect evidence of the possibility to realize the jumping mechanism of super-ionic conductivity in these crystals in the high-temperature phase.

Prediction of topological Dirac semimetal in Ca-based Zintl layered compounds CaM2X2 (M\u2009=\u2009Zn or Cd; X\u2009=\u2009N, P, As, Sb, or Bi)

Topological Dirac materials are attracting a lot of attention because they offer exotic physical phenomena. An exhaustive search coupled with first-principles calculations was implemented to investigate 10 Zintl compounds with a chemical formula of CaM2X2 (M = Zn or Cd, X = N, P, As, Sb, or Bi) under three crystal structures: CaAl2Si2-, ThCr2Si2-, and BaCu2S2-type crystal phases. All of the materials were found to energetically prefer the CaAl2Si2-type structure based on total ground state energy calculations. Symmetry-based indicators are used to evaluate their topological properties. Interestingly, we found that CaM2Bi2 (M = Zn or Cd) are topological crystalline insulators. Further calculations under the hybrid functional approach and analysis using k · p model reveal that they exhibit topological Dirac semimetal (TDSM) states, where the four-fold degenerate Dirac points are located along the high symmetry line in-between Г to A points. These findings are verified through Green's function surface state calculations under HSE06. Finally, phonon spectra calculations revealed that CaCd2Bi2 is thermodynamically stable. The Zintl phase of AM2X2 compounds have not been identified in any topological material databases, thus can be a new playground in the search for new topological materials.

The difference on the physical properties between CsPbX3 and Cs2PbX6: A comparative study

A complete theoretical investigation is performed to study the physical properties of all-inorganic halide perovskites CsPbX3 and Cs2PbX6 (X ​= ​Cl, Br, I). The various physical properties are compared and discussed. The results show that the bond length Pb−X of Cs2PbX6 is shorter than that of CsPbX3. Six compounds are ductile materials and mechanically stable. According to the phonon spectrum calculations, Cs2PbCl6 and Cs2PbBr6 are dynamically stable, while Cs2PbI6 is unstable. The band gap of CsPbX3 is larger than that of Cs2PbX6. The strong spin−orbit coupling effect is found for CsPbX3, while it has little influence on the band structure of Cs2PbX6. The mobility between the electron and hole varies from CsPbX3 to Cs2PbX6. The different in physical properties mainly results from the different valence states of lead. The results further indicate that Cs2PbBr6 has an ideal band gap value and shows a stronger light absorption than that of CsPbI3 between 450 and 800 ​nm, which is expected to be a promising candidate for perovskite solar cells. Therefore, our study can provide further interest for exploring the potential of all-inorganic stable lead-based double perovskites in optoelectronic devices.

Electronic structures and optical properties of monolayer borophenes

In this paper, we theoretically investigated the electronic and optical properties of monolayer borophene, including the electronic energy band, density of states (DOS), dielectric function, and absorption spectra and the charge distribution. The calculated phonon spectra and phononic DOS confirm that the four kinds of monolayer borophene structures can stably exist. Two-dimensional (2D) borophene exhibits apparent optical anisotropy in visible and near infrared (NIR) regions. Our results provide a reliable theoretical base for the application of monolayer borophene in optoelectronic devices.

Route to a novel tetragonal carbon allotrope via T-carbon

Through first-principles calculations, we employed anisotropic compression to T-carbon and identified several carbon allotropes. Among them, we predict a novel body-centered tetragonal carbon allotrope with space group I41/amd (No. 141), in which sixteen carbon atoms comprise four- and five- and eight-membered rings within a unit cell; we name it as bct-C16-II. Phonon spectrum calculations exhibit that it is dynamically stable under pressure up to at least 100 GPa. This bct-C16-II is energetically more stable than T-carbon and the previously proposed bct-C16-I, although it has higher energy than graphite or diamond. Our calculations also show that bct-C16-II may be super-hard with Vickers hardness around 55 GPa, comparable to cubic boron nitride. Its ideal tensile and shear strength is calculated to be around 75 and 43 GPa, respectively. We simulated the transformation pathway of importance from T-carbon to this novel bct-C16-II phase. The energy barrier is estimated to be around 0.187 eV/atom, which provides a possible approach to synthesize it. The achievement of other carbon allotropes indicates that the tetrahedral units in T-carbon are not very strong under anisotropic strains that T-carbon might be considered as a good precursor to synthesize other carbon materials. The discovery of this new phase may enrich our knowledge of carbon materials and their formation mechanisms.

First principles study of monolayer Sb2S2Te and a mathematical model of a thin-film thermoelectric generator with maximum power point tracing

The result of the structural, electronic and thermoelectric performance of monolayer Sb2S2Te, which is a bran-new material, is calculated by using the method of full-potential linearized augmented plane-wave (FP-LAPW), and the exchange-correlation potential selected in this paper is generalized gradient approximation (GGA) scheme. There are no negative frequencies in the calculated phonon spectra, which demonstrates that dynamics of monolayer Sb2S2Te is stable in theory. The average value of lattice thermal conductivity (κl) is smaller than that of other thermoelectric (TE) materials, and monolayer Sb2S2Te has larger figure of merit (ZT) comparing with other well-known materials. According to the TE parameters (Seebeck coefficient S, electrical conductivity σ, electronic thermal conductivity κe and lattice thermal conductivity κl) of monolayer Sb2S2Te, an idealized model of thermoelectric generator (TEG) and a realistic model of maximum power point tracking (MPPT) are built. In addition, this paper studies the influence of different temperature zones, different temperature differences, cross sectional area, height and the number of P–N junction on output power and conversion efficiency of TEG. The results of physical calculation and simulation models show that the conversion efficiency of TEG of monolayer Sb2S2Te is larger than or close to that of other well-known materials, and the tracking accuracy of MPPT proposed in this paper is higher than or close to that of other researches.

The new stable phases of Fe2Pd crystal alloy and their properties

This work presents a combination of particle swarm optimization crystal structure search method and first-principles calculations to investigate new stable crystal structures of Fe2Pd. Three structurally stable tetragonal phases (with space groups I4/mmm, P4/mmm, P4/nmm) and a structurally stable cubic phase (C15 Laves phase (Fd3¯m)) are predicted at 0 GPa. The enthalpy of formation reveals that these four phases are thermodynamically stable in the ferromagnetic state, and the tetragonal P4/nmm phase has the highest stability. Calculated phonon spectra indicates that the remaining three predicted structures are dynamically stable except for I4/mmm structure. From the elastic constant, all of the predicted structures are mechanically stable phases. Among these studied structures, the three tetragonal phases with similar values of bulk modulus have high uniaxial magneto-crystalline anisotropy, high thermal expansion coefficient, large bulk modulus, and good ductility. However, the cubic Fd3¯m phase is characterized by low magneto-crystalline anisotropy, small thermal expansion coefficient, high shear and Young’s modulus, and brittleness. Finally, the high-pressure stability of these four structures is investigated. The theoretical prediction indicates a structural transition from tetragonal P4/nmm phase to ductile Fd3¯m structure at 26.8 GPa, while the I4/mmm is the most stable above 80 GPa.

Effect of stress regulation on electronic structure and optical properties of TiOCl&lt;sub&gt;2&lt;/sub&gt; monolayer

Based on first-principles calculations, the electronic structure, the transport and optical properties of TiOCl&lt;sub&gt;2&lt;/sub&gt; monolayer are systematically investigated. The vibrational, thermodynamic, and mechanical properties of TiOCl&lt;sub&gt;2&lt;/sub&gt; monolayer are studied by phonon spectrum, molecular dynamics and elastic constants calculations. All these results indicate that the TiOCl&lt;sub&gt;2&lt;/sub&gt; monolayer possesses good structural stability at room temperature and excellent mechanical properties. The electronic structure analysis shows that the TiOCl&lt;sub&gt;2&lt;/sub&gt; is an indirect band gap (1.92 eV) semiconductor. Its band structure can be significantly affected by in-plane stress. Specifically, the TiOCl&lt;sub&gt;2&lt;/sub&gt; monolayer undergoes an indirect-to-direct band gap transition under –4% uniaxial stress along the &lt;i&gt;a&lt;/i&gt;-axis and the gap size decreases to 1.66 eV. Moreover, the TiOCl&lt;sub&gt;2&lt;/sub&gt; monolayer exhibits obvious anisotropy characteristics, and its electron mobility is 803 cm&lt;sup&gt;2&lt;/sup&gt;·V&lt;sup&gt;–1&lt;/sup&gt;·s&lt;sup&gt;–1&lt;/sup&gt; along the &lt;i&gt;b&lt;/i&gt;-axis, whereas the hole mobility reaches 2537 cm&lt;sup&gt;2&lt;/sup&gt;·V&lt;sup&gt;–1&lt;/sup&gt;·s&lt;sup&gt;–1&lt;/sup&gt; along the &lt;i&gt;a&lt;/i&gt;-axis. The wave peaks (valleys) of the absorptivity, reflectivity and transmittance shift toward the violet part of the visible band by the stress. All these appealing properties make the TiOCl&lt;sub&gt;2&lt;/sub&gt; monolayer a promising candidate for applications in optoelectronic devices.

Динамика решетки и нелинейный отклик ОЦК-титана на деформации при высокой температуре в методе ab initio молекулярной динамики

The lattice dynamics and the response to deformations of the bcc titanium lattice at high temperatures are studied using the method of ab initio molecular dynamics. The results of calculation of phonon spectra using the harmonic Hamiltonian and the Hamiltonian containing third-order force constants are compared. It is shown that the lattice dynamics of bcc titanium cannot be correctly described without taking into account the anharmonic terms. Stress-strain curves are calculated for a given deformation scheme η1= η11= η, η6=2η12= η. For this deformation scheme, analytical expressions are given for the dependence of the nonzero components of the stress tensor on the magnitude of the deformation, taking into account the elastic constants up to the fourth order inclusive. All second-order elastic constants, three third-order elastic constants, and four fourth-order elastic constants of bcc titanium are calculated.

Self-energy corrected band-gap tuning induced by strain in the hexagonal boron phosphide monolayer

We performed a first-principles study of the electronic behavior of a 2D hexagonal boron phosphide monolayer (2D-h-BP). The system was deformed isotropically by applying a simultaneous tensile strain along the a and b crystal axes. We analyzed the band-gap evolution as function of the deformation percentage, ranging from 1% to 8%. Results show that the system behaves as a direct band-gap semiconductor, with the valence band maximum and conduction band minimum located at the K point (1/3, 1/3, 0) of the Brillouin zone. This behavior is unchanged despite the strain application. The band gap underestimation, as computed within the standard DFT, was corrected by applying the G0W0 approach. Trends in the band-gap behavior are the same within both approaches: for low deformation percentages, the band-gap grows linearly with a small slope, and at higher values, it grows very slow with a tendency to achieve saturation. Therefore, the band-gap is less sensitive to tensile strain for deformations near 8%. The origin of this band gap behavior is explained in terms of the projected density of states and charge densities, and it can be attributed to Coulomb interactions, and charge redistributions due to the applied tensile strain. To study the carrier mobility, we computed the electron and hole effective masses, finding high mobility for both carriers. Finally, the stability analysis of each strained system includes the calculation of phonon spectra, to assure the dynamical stability, the computation of elastic constants to evaluate the mechanical stability, and cohesive energies for exploring the thermodynamical stability. Results indicate that the boron phosphide monolayer is stable under the calculated tensile strains.

First-principles calculations of phase transition, elasticity, phonon spectra, and thermodynamic properties for CeO2 polymorphs

In this work, the structural transformation, elastic behavior, phonon spectra and thermodynamic properties of CeO2 under compressive and tensile loading were investigated by first-principles calculations based on density generalized function theory (DFT). Our calculated structural parameters and transition pressure under compressive condition are in good agreement with the experimental values. Under tensile condition, the phase transitions of CeO2 from fluorite-type (space group Fm3¯m) phase to cotunnite-type (space group Pnma), rutile-type (space group P 42/mnm) and marcasite-type (space group Pnnm) phases were predicted to occur at 23.2 ​GPa, −7.69 ​GPa, and −7.75 ​GPa respectively. The mechanical stability criteria are verified in terms of elastic constants and shows that the cotunnite and fluorite structures of CeO2 are mechanically stable in the studied pressure range, while the rutile and marcasite structures turn out to be mechanical unstable under high pressure. The calculated phonon spectra of CeO2 polymorphs demonstrate that all phases considered in present work are dynamically stable under ambient pressure. Furthermore, the calculated heat capacity, isothermal bulk modulus, Gibbs free energy, thermally expanding coefficient of CeO2 polymorphs as a function of temperature are discussed and compared with available experimental data.

Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe3 and monoclinic-TaS3

NbSe3 and monoclinic-TaS3 (m-TaS3) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions at and associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron–hole response function. Because of the stronger inter-chain interactions, the FS and electron–hole response function are considerably more complex for NbSe3 than m-TaS3; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron–hole response for NbSe3 are found with the (0a *, 0c *) and (1/2a *, 1/2c *) transverse components at and , respectively, whereas the second maximum is not observed for m-TaS3 at . Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transverse a * components in NbSe3. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling along c * in NbSe3 and along both a * and c * for m-TaS3. Phonon spectrum calculations reveal the formation of a giant 2k F Kohn anomaly for m-TaS3. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides.

Synthesis and characterization of new indium gallium selenides of the InSe-GaSe system

The new solid solution phases crystallized in the hexagonal and tetragonal system were revealed in the GaSe-InSe system, and a new orthorhombic phase with a new type of structural blocks has been proposed. The structural features of the In1-xGaxSe solid solutions (where 0 ​≤ ​x ​≤ ​1) were studied using X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA), and Raman spectroscopy. The chemical interaction between Ga and In atoms in the hexagonal In1-xGaxSe phases was identified. The reversible phase transition from hexagonal to the tetragonal structure was found to occur in the In0.7Ga0.3Te phase. Using the Density Functional Theory (DFT) method, the electronic, optical, and vibrational properties for the Pnnm orthorhombic InGaSe2 phase were studied. Modified Becke-Johnson exchange potential and generalized gradient approximations (mBJ-GGA) were used to calculate the electronic band structure, dielectric function, and density of states (DOS) of InGaSe2, which solve the band-gap problem found within GGA approach. The InGaSe2 stoichiometric phase was found to be a direct band-gap semiconductor and the band gap of this compound can be tuned via the different stacking sequences of InSe/GaSe layers. The optical absorption edges which are observed in the visible light region make this compound promising material for photovoltaic applications. The calculated phonon spectra of the InGaSe2 revealed that the frequencies of the optic modes are close to frequencies of acoustic modes, which carry the heat flow, and in this case, the acoustic modes will be strongly scattered by low-frequency optic modes. The latter event may lead to a low lattice thermal conductivity that is important for thermoelectric applications. The phonon dispersion of this compound shows no imaginary modes, suggesting that it is dynamically stable.

Intrinsic magnetism in two-dimensional Cr2X2Se (X = I, Br, Cl) monolayers

A novel family of two-dimensional Cr2X2Se (X = I, Br, Cl) monolayers with intrinsic magnetism has been predicted using first-principles calculations. The monolayer with the five atomic sublayers has two structures: structure I with ABCAB-type atomic sublayers and structure II with ABCBA-type atomic sublayers. It is found that the lowest energy states are ferromagnetic (FM) half metal for the structure I, and antiferromagnetic (AFM) semiconductor for the structure II, respectively. Phonon spectrum calculations and molecular dynamics simulations show that these lowest energy magnetic states are stable. Monte Carlo simulations based on the Ising model show that the Curie temperatures are about 94, 156 and 93 K for Cr2I2Se, Cr2Br2Se and Cr2Cl2Se monolayers in the structure I, respectively, which are above 77 K of the liquid-nitrogen transition temperature. The large magnetocrystalline anisotropy energies of about 1879, 68, 28 μeV per Cr atom for Cr2X2Se (X = I, Br, Cl) monolayers in the structure I show their importance in magnetic memory and storage. The magnetization direction is out of plane in Cr2I2Se and Cr2Br2Se, but is in plane in Cr2Cl2Se. Our study implies a new route in 2D electronics and spintronics.

2D ternary nitrides XNY (X=Ti, Zr, Hf; Y[dbnd]F, Cl, Br) with applications as photoelectric and photocatalytic materials featuring mechanical and optical anisotropy: A DFT study

Using density functional theory, we systematically studied the stability, electronic structure and optical properties of two-dimensional ternary nitrides XNY (X ​= ​Ti, Zr, Hf; YF, Cl, Br). Through phonon spectrum calculations and ab initio molecular dynamics simulations, we found that XNY is dynamic and thermodynamically stable. The calculated results of cohesive energy indicate the possibility of synthesis experimentally. XNY is a direct band gap semiconductor, and the band gap is larger than the 1.23 ​eV required for water splitting. In addition, XNY has anisotropic mechanical and optical properties. In addition, XNY has absorption of visible light, and the band-edge arrangement of ZrNF and HfNCl is feasible for water photo oxidation and photo reduction. These comprehensive properties make 2D ternary nitride XNY a candidate material for direction-dependent optoelectronic devices, and it is expected to be applied in the field of photo catalysis.