Hexagonal Compounds Research Articles

Exchange interactions in hexagonal YMnO3 and LuMnO3 multiferroic compounds

In this work, spin density functional theory calculations are employed to obtain the exchange interactions in hexagonal YMnO3 and LuMnO3 multiferroic compounds, which have been a matter of debate in the literature. The exchange and correlation electronic effects are approximated using the local spin density approximation (LSDA) with the inclusion of the effective Hubbard U correction (Ueff). The intraplane superexchange interaction (Jnn) decreases with increasing Ueff. Moreover, the Jnn value with the best agreement with experiment is found for Ueff values between 2.55 and 4.0 ​eV. Without the addition of Ueff corrections, i.e., with standard LSDA calculations, the magnitude of Jnn is overestimated. The Bader topological analysis of the electron density at the bond critical point shows that the closed-shell interaction of the Mn–O3 and Mn–O4 bonds in both materials increases with increasing Ueff, which justifies the decrease of Jnn with increasing Ueff.. Additionally, both interplane and intraplane super-superexchange interactions are computed and compared with previous studies.

Crystal Growth of a New 8H Perovskite Sr8Os6.3O24 Exhibiting High TC Ferromagnetism

Single crystals of a new twinned hexagonal perovskite compound Sr8Os6.3O24 have been synthesized, and structural and magnetic properties have been determined. The compound crystallizes in a hexagonal cell with lattice parameters a = 9.6988(3) Å and c = 18.1657(5) Å. The structure is an eight-layered hexagonal B-site deficient perovskite with the layer sequence (ccch)2 and represents the first example of a hexagonal structure among 5d oxides adopting a twin option. The sample shows spontaneous ferromagnetic magnetization below 430 K with a small saturation moment of 0.11 μB/Os ion. This is the highest Curie temperature (TC) reported for any bulk perovskite containing only 5d ions at the B site.

High-throughput bandstructure simulations of van der Waals hetero-bilayers formed by 1T and 2H monolayers

Vertically stacked van der Waals heterostructures made of two-dimensional compounds are almost an infinite playground for the fabrication of nano-engineered materials for the most diverse applications. Unfortunately, high-throughput electronic structure theory, which often serves as a guidance for material design, is not practical in this case. In fact, the compositional and structural complexity of van der Waals heterostructures make the number of prototypes to calculate combinatorially large. In this work a method is developed to compute the bandstructure of van der Waals heterostructures with an arbitrary composition and geometry using minimal computational resources. Such scheme is applied to the systematic study of a library of two-dimensional hexagonal XY2 compounds. The method is based on the density functional theory and on the assumption that the inter-layer electronic interaction is limited to classical electrostatic screening. Our analysis enables us to identify and categorize a large range of van der Waals bilayer heterostructures with electronic band gaps of different nature ranging from 0.1 to 5.5 eV and various types of band line-up.

Extended B-Site Vacancy Content Range and Cation Ordering in Twinned Hexagonal Perovskites Ba8Cr4-xTa4+0.6xO24.

The new 8-layer twinned hexagonal solid solution Ba8Cr4-xTa4+0.6xO24 (x = 0.0-3.0) was isolated through the aliovalent substitution of Ta5+ for Cr3+ in Ba2CrTaO6, showing the widest B-site vacancy content range among the 8-layer twinned hexagonal perovskites. Ba8Cr4-xTa4+0.6xO24 forms a simple 8-layer hexagonal perovskite structure within 0.0 ≤ x < 2.4 and a tripled 8-layer hexagonal perovskite superstructure within 2.4 ≤ x ≤ 3.0. The latter shows expanded a and b axes by times in comparison to the simple 8-layer hexagonal perovskite structure owing to the partial face-sharing octahedral (FSO) B cation ordering along the ab plane. The B-cation and vacancy distributions in the tripled superstructure were characterized by neutron and X-ray powder diffraction and further confirmed by a scanning transmission electron microscopy-high angle annular dark field imaging and intensity profile analysis. The formation of 8-layer twinned hexagonal perovskites Ba8Cr4-xTa4+0.6xO24 in an extended solid solution range can be attributed to the presence of both covalent B-B and B-O-B bonding and B-site vacancies in the FSO sites. This work provides an effective way of combining covalent B-B and B-O-B bonding and vacancy creation as well as the cationic ordering in the FSO sites to reduce electrostatic repulsion, which could further enable the stabilization of new hexagonal perovskite compounds.

Chirality-induced linear response properties in noncoplanar Mn3Ge

Taking the noncollinear antiferromagnetic hexagonal Heusler compound ${\mathrm{Mn}}_{3}\mathrm{Ge}$ as a reference system, the contributions to linear response phenomena arising solely from the chiral coplanar and noncoplanar spin configurations are investigated. Orbital moments, x-ray absorption, and anomalous and spin Hall effects, as well as corresponding spin-orbit torques and Edelstein polarizations, are studied depending on a continuous variation of the polar angle relative to the kagome planes of corner-sharing triangles between the noncollinear antiferromagnetic and the ferromagnetic limits. By scaling the speed of light from the relativistic Dirac case to the nonrelativistic limit the chirality-induced or topological contributions can be identified by suppressing the spin-orbit coupling.

Natural hyperbolicity in layered hexagonal crystal structures

Discovering the physical requirements for meeting the indefinite permittivity in natural material as well as proposing a new natural hyperbolic media offer a possible route to significantly improve our knowledge and ability to confine and controlling light in optoelectronic devices. We demonstrate the hyperbolicity in a class of materials with hexagonal P6/mmm and P6$_{3}$/mmc layered crystal structures and its physical origin is thoroughly investigated. By utilizing density functional theory and solving the Bethe-Salpeter equation (BSE), we find that the layered crystal structure and symmetry imposed constraints in Li$_{3}$N gives rise to an exceedingly strong anisotropy in optical responses along in- and out-of-plane directions of the crystals making it a natural hyperbolic in a broad spectral range from the visible spectrum to the ultraviolet. More excitingly, the hyperbolicity relation to anisotropic interband absorption in addition to the impressive dependency of the conduction band to the lattice constant along the out-of-plane direction provide the hyperbolicity tunability in these hexagonal structures under strain, doping, and alloying. Our findings not only suggest a large family of real hexagonal compounds as a unique class of materials for realization of the highly tunable broad band hyperbolicity but also offers an approach to search for new hyperbolic

Voting Data-Driven Regression Learning for Accelerating Discovery of Advanced Functional Materials and Applications to Two-Dimensional Ferroelectric Materials.

Regression machine learning is widely applied to predict various materials. However, insufficient materials data usually leads to poor performance. Here, we develop a new voting data-driven method that could generally improve the performance of the regression learning model for accurately predicting properties of materials. We apply it to investigate a large family (2135) of two-dimensional hexagonal binary compounds focusing on ferroelectric properties and find that the performance of the model for electric polarization is indeed greatly improved, where 38 stable ferroelectrics with out-of-plane polarization including 31 metals and 7 semiconductors are screened out. By unsupervised learning, actionable information such as how the number and orbital radius of valence electrons, ionic polarizability, and electronegativity of constituent atoms affect polarization was extracted. Our voting data-driven method not only reduces the size of materials data for constructing a reliable learning model but also enables one to make precise predictions for targeted functional materials.

High-efficient ab initio Bayesian active learning method and applications in prediction of two-dimensional functional materials.

Beyond the conventional trial-and-error method, machine learning offers a great opportunity to accelerate the discovery of functional materials, but still often suffers from difficulties such as limited materials data and the unbalanced distribution of target properties. Here, we propose the ab initio Bayesian active learning method that combines active learning and high-throughput ab initio calculations to accelerate the prediction of desired functional materials with ultrahigh efficiency and accuracy. We apply it as an instance to a large family (3119) of two-dimensional hexagonal binary compounds with unbalanced materials properties, and accurately screen out the materials with maximal electric polarization and proper photovoltaic band gaps, respectively, whereas the computational costs are significantly reduced by only calculating a few tenths of the possible candidates in comparison with a random search. This approach shows the enormous advantages for the cases with unbalanced distribution of target properties. It can be readily applied to seek a broad range of advanced materials.

Structural, electronic, vibrational, mechanical and thermoelectric properties of 2D and bulk BaX (X=O, S, Se and Te) series under DFT and BTE framework

Abstract A thorough study on the structural, electronic, vibrational, mechanical, and thermoelectric properties of 3D (rock-salt) and 2D (hexagonal) barium chalcogenides (BaX; X = GrVI or O, S, Se, and Te) is carried out in the present work. For the first time, transport properties with respect to temperature (ranging from 50 to 800 K) for 3D (rock-salt or rs) and 2D (hexagonal or h) barium chalcogenides are studied. Together with their wide indirect and direct energy bandgaps, the dynamical and mechanical stability of both the rock-salt (rs) and hexagonal (h) BaX compounds motivates us to look at their temperature-dependent thermoelectric properties, viz. thermal or heat conductivity (κ), specific conductance or electrical conductivity (σ), thermopower or Seebeck coefficient (S) and figure of merit (ZT), under the framework of semi-classical Boltzmann transport equations (BTE). Phonon dispersion findings demonstrated the dynamic stability of BaX compounds, while via different elastic parameters, viz. Young's modulus and Poisson's ratio, the mechanical stability and elastic nature of these compounds are evaluated. As per the present investigation, it is noted that the GGA-PBE measurement provides stability in all the rs and h phases of BaX, which is in contrary to the previous report in which h-BaO, h-BaS and h-BaSe compounds were identified as unstable at the DFT-LDA (local density approximation) computation. For all the BaX except for rs-BaO and h-BaTe, the electronic band structure reveals indirect bandgap nature. The evaluation of effective mass, mobility and relaxation time has also been included in the electronic properties and, as per our observation, the maximum mobility for rs-BaSe, rs-BaTe and h-BaTe are confirmed. The BaSe in both of its rs (ZT > 0.8) and h (ZT > 0.75) phases has revealed to be outstanding thermoelectric materials in the present work. This investigation reveals excellent figure of merit (ZT) in the range of 0.5–1.1 for all the considered BaX compounds at the temperature of 800 K, suggesting its viability in thermoelectric generators and thermoelectric coolers.

Progress in the synthesis and applications of hexaaluminate-based catalysts

ABSTRACT The development of materials that can exhibit thermal resistance at very high temperatures, thus allowing them to be applied as catalysts and thermal insulators, amongst other possible uses, is a research subject of great interest. This is the case for hexaaluminates, a class of hexagonal aluminate compounds with a unique structure that are stable at very high temperatures up to 1600°C and exhibit exceptional resistance to sintering and thermal shock, thus making them attractive catalysts for high-temperature applications. In this review, the structure of hexaaluminates is presented first. The most recent advances in synthetic methods (sol-gel, reverse microemulsion, hydrothermal synthesis, carbon-templating, solution combustion synthesis, and freeze-drying methods) are discussed subsequently, with the aim of maximizing textural properties and including in their structure metals known to be active in catalytic applications, such as combustion of CH4, partial oxidation, and dry reforming of CH4 to produce synthetic gas, and the decomposition of N2O. Finally, other applications, such as their function as a thermal barrier, are also addressed.

Theoretical analysis of the structural, electronic, optical and thermodynamic properties of trigonal and hexagonal Cs3Sb2I9 compound

The structural, electronic, optical and thermodynamic properties of Cs3 Sb2 I9 compound with 0-D dimer form (hexagonal SP; P 63∕mmc, no. 194) and the 2-D layered form (trigonal SP; , no. 164) phases have been investigated and reported using both FP-LAPW and PP-PW methods. Besides, the thermodynamic properties of the materials of interest have been studied using the quasi-harmonic Debye model accommodating the lattice vibrations effects. The obtained lattice parameters for dimer and layered phase reveal very good accord with experiment. The computed electronic band structures show that in the dimer phase the material of interest is an indirect band-gap (k –Γ) semiconductor, whereas it is a direct band-gap (Γ–Γ) in the layered phase. The semiconducting material Cs3 Sb2 I9 of interest was found to be stable against volume change of 0 to +14%. Moreover, the optical properties of the material in question are also examined and discussed. The effect of pressure and temperature on the studied properties is found to be highly effective in tuning some of the macroscopic properties of the compound in question.

Electronic, magnetic and optical properties of monolayer (ML) hexagonal ZnSe on vacancy defects at Zn sites from DFT-1/2 approach

In this work, we have investigated the electronic, magnetic and optical properties of hexagonal monolayer ZnSe compound via atomic vacancy defects by using DFT-1/2 approximation. Our calculations revealed that monolayer ZnSe is a direct band semiconductor with the transition along Γ−Γ symmetry. The measured value of electronic band gap is ~2.68 eV in very good agreement with the experimental results. The optical spectra of hexagonal ZnSe is measured along the x- and z-axis as a function of wave length (λ). The absorption coefficient is highly anisotropic with majority of optical response lies along x-axis. The maximum absorption intensity is ~6.5 × 104cm−1 in violet-blue range of UV–Vis spectra. Our results of optical spectra comprehended the experimental results.

Structural, electronic, and optical properties of pristine and bilayers of hexagonal III-V binary compounds and their hydrogenated counterparts

Density functional theory calculations were performed to investigate the structural, electronic, and optical properties of two-dimensional hexagonal III-V binary compounds (h-MX, M = B, Al, Ga, and X = N, P and As), the fully hydrogenated counterparts (MXH2) and the MX/MXH2 van der Waals (vdW) heterostructures. The results show that hydrogenation can change the stability and electronic band dispersion of hexagonal III-V binary compounds, of which the fully hydrogenated h-AlN becomes dynamically unstable. The full hydrogenation induces an indirect-to-direct bandgap transition for hexagonal BN, GaN, GaP, GaAs, and the band gaps become tunable by applying the biaxial strain.These characteristics are conducive to design of strain-based 2D materials for application in nanoelectronic and optoelectronic devices. Furthermore, the different stacking pattern MX/MXH2 vdW heterostructures have been investigated. Of which, BN/BNH2, GaN/GaNH2, GaP/GaPH2 and GaAs/GaAsH2 were found to have the typical type-II band alignments, which can facilitate the effective separation of photogenerated electrons and holes. The calculated band alignment and enhanced optical absorption suggest that the GaP/GaPH2 and GaAs/GaAsH2 vdW heterostructures possess great potential to be high-performance optoelectronic device.

Metal-Free Hexagonal Perovskite High-Energetic Materials with NH3OH+/NH2NH3+ as B-Site Cations

Abstract Designing and synthesizing more advanced high-energetic materials for practical use via a simple synthetic route are two of the most important issues for the development of energetic materials. Through an elaborate design and rationally selected molecular components, two new metal-free hexagonal perovskite compounds, which are named as DAP-6 and DAP-7 with a general formula of (H2dabco)B(ClO4)3 (H2dabco2+ = 1,4-diazabicyclo[2.2.2]octane-1,4-diium), were fabricated via an easily scaled-up synthetic route using NH3OH+ and NH2NH3+ as B-site cations, respectively. Compared with their NH4+ analog ((H2dabco)(NH4)(ClO4)3; DAP-4), which has a cubic perovskite structure, DAP-6 and DAP-7 have higher crystal densities and enthalpies of formation, thus exhibiting higher calculated detonation performances. Specifically, DAP-7 has an ultrahigh thermal stability (decomposition temperatures (Td) = 375.3 °C), a high detonation velocity (D = 8.883 km·s−1), and a high detonation pressure (P = 35.8 GPa); therefore, it exhibits potential as a heat-resistant explosive. Similarly, DAP-6 has a high thermal stability (Td = 245.9 °C) and excellent detonation performance (D = 9.123 km·s−1, P = 38.1 GPa). Nevertheless, it also possesses a remarkably high detonation heat (Q = 6.35 kJ·g−1) and specific impulse (Isp = 265.3 s), which is superior to that of hexanitrohexaazaisowurtzitane (CL-20; Q = 6.23 kJ·g−1, Isp = 264.8 s). Thus, DAP-6 can serve as a promising high-performance energetic material for practical use.

The structural, magnetic and electronic properties of the ground state of the hexagonal LuMnO3 multiferroic

Non-collinear spin density functional theory calculations were employed to study structural, magnetic, and electronic properties of the hexagonal LuMnO3 compound in its ferroelectric phase. For that purpose, it was utilized full-potential linearized augmented plane wave plus local orbitals method, for the first time. The exchange and correlation effects were treated in the frame of the local spin density approximation (LSDA) including effective Hubbard (Ueff) correction. The principal objective of the work was to determine three properties of the LuMnO3 ground state: (1) the crystallographic space group (P63cm or P63), (2) the non-collinear magnetic configuration of the Mn spin moments (Γ1, Γ2, Γ3, Γ4, Γ1 + Γ2 or Γ3 + Γ4), and (3) the electronic structure. The results reveal that the P63cm structure with the Γ3 + Γ4 spin configuration (with ϕ = 80°) should be the ground state. The calculated indirect bandgap of 1.19 eV agrees well with the gap determined from optical conductivity measurements. Analysis of the details of the calculated electronic structure made it possible to interpret the origin of the most pronounced 1.7 eV peak in the optical conductivity spectrum, as well as to explain why it is registered only for the incident light polarized perpendicularly to the c-axis.

تحضیر عدد من مرکبات الکاربونیل الفا- بیتا غیر المشبعة من مرکبات البایرازولین فی الوسط الحامضی وتشخیصها طیفیا

This research includes the preparation of series of chalcone compounds using the Claisen – Schmidt method between Cyclohexane and a group of different aromatic aldehydes (2-Chlorobenzaldehyde) and (3-Nitrobenzaldehyde), (4-Methoxybenzaldehyde), (Vanillin), (4- Chlorobenzaldehyde), and (N,N- 4-Dimethylaminobenzaldehyde). In the second step, the chalcones were converted to pyrazoline compounds by reacting with hydrazine hydrate and with the presence of glacial acetic acid, and pyrazoline compounds containing the acetyl group which were utilized by their interaction with different aromatic benzaldehydes to obtain new alpha-beta unsaturated compounds, prepared in the acidic medium. And the last step ,a number of disproportionate and effective aromatic compounds were prepared with the expected vital efficacy by the reaction of the chalcones from the last step with hydrogen peroxide to give triple-oxirane compound (34-31) and reaction with hydrazine hydrate in the presence of glacial acetic acid to prepare pyrazoline compounds (30-27) and reaction with urea to give hexagonal pyrimidine compounds (34-31). The compounds prepared with the available physical and spectral methods were diagnosed, so the infrared spectrum, Nuclear magnetic resonance spectroscopy, melting point measurement, and color changes were measured.

Resistivity and magnetoresistance properties of R2NiSi3 (R = Gd, Dy, Ho, Er, Tm) compounds

The resistivity and magnetoresistance behaviour of the hexagonal intermetallic compounds R2NiSi3(R = Gd, Dy, Ho, Er, and Tm) are reported here. All the studied polycrystalline compounds exhibit metallic behaviour along with additional magnetic anomalies at low temperatures. A well-defined resistivity minima is observed in Gd2NiSi3and Dy2Ni0.87Si2.95at a temperature much higher than their respective magnetic transition temperatures. The anomaly has been ascribed to the charge carrier localization caused by magnetic precursor effect. Magnetic field induced crossover from positive to negative magnetoresistance (MR) behaviour associated with antiferromagnetic ground state is evidenced for Gd2NiSi3and Er2NiSi3in the low temperature region. Although Tm2Ni0.93Si2.93does not exhibit any long range magnetic order down to 2 K, a sudden drop in resistivity behaviour is observed below ~10 K. Presence of short range magnetic correlation observed in a wide temperature range, much beyond their respective magnetic ordering temperatures, has been argued to be responsible for achieving finite negative MR for all the compounds. A subtle resemblance between the observed transport anomalies and the magnetic properties of these systems have been discussed.

Neutron diffraction study on hydrostatic pressure regulated magnetostructural transition and magnetocaloric effect in MnNi1 – xFexSi1 – yGey alloys

Ni2In-type hexagonal compounds MM′X (M, M′ = transition metals, X = main element) involving magnetostructural transition (MST) have attracted much attention due to their giant magnetocaloric effect (MCE). Physical pressure, as an effective method, has been used to adjust the MST and the resultant MCE. Enhanced and diminished MCEs by hydrostatic pressure were both reported previously, but the underlying mechanism is unclear. Here, we report our neutron diffraction study on MST and MCE regulated by hydrostatic pressure in MnNi1 – xFexSi1 – yGey alloys. Careful refinements indicate that the martensitic phase shows a linear ferromagnetic structure with spin moment confined on Mn sites, which remains almost unchanged at pressures lower than 5 kbar even though slight compressions of Mn–Mn bond lengths can be identified. The MST keeps sharp under pressures lower than 5 kbar, while the derived volume change (ΔV/V) across MST reduces 7%, i.e., from 2.84% (0 kbar) to 2.63% (2.1 kbar), due to the effect of pressure on two-phase coexistence. Accordingly, the estimated lattice entropy change (ΔSLatt) based on the Debye approximation reduces by 10% from 37.1 J/kg K (0 kbar) to 33.5 J/kg K (2.1 kbar). These ΔSLatt values, driven by temperature, are all somewhat larger than the magnetic entropy change driven by a 5 T magnetic field. This result may imply that a magnetic field of 5 T is not sufficient for the MST to complete. As the pressure reaches 5 kbar, the MST notably slows down. This may originate from the extended temperature region of two-phase coexistence but not the decoupling of MST.

Effect of Au Doping on Elastic, Thermodynamic, and Electronic Properties of η-Cu6Sn5 Intermetallic

The effects of substitution of Au for Cu on the elastic, thermodynamic, and electronic properties of hexagonal η-Cu6Sn5 intermetallic compound (IMC) are investigated by first-principles calculations. The results show that Au atoms preferentially occupy Cu4 or Cu3 site to form η-Cu5Au1Sn5 or η-Cu4Au2Sn5 IMC, respectively. Doping Au in η-Cu6Sn5 IMC forms a more stable thermodynamic structure than pure phase. The ductility of η-Cu6Sn5 IMC increases after substitution of Au for Cu. However, doping Au weakens the Young’s modulus, shear modulus, hardness, and Debye temperature of η-Cu6Sn5 IMC. The results for the charge density difference, total density of states, and partial density of states show that doping Au atoms can stabilize the structure of η-Cu6Sn5 IMC.

First principles study of ferroelectric hexagonal compounds RInO3 (R = Dy, Er, and Ho): electronic structure, optical and dielectric properties.

The 4f-shell electrons of rare-earth ion R have a certain influence on both the electrical and optical properties of RInO3. We have studied the electronic structures, optical and dielectric properties of hexagonal compounds RInO3 (R = Dy, Ho, and Er) by performing first principles calculations. After optimization, the lattice parameters are in great agreement with the experiment with an error within 1%. Band structure calculations reveal decreasing band gaps with the increase of atomic number, yielding the highest conductivity for ErInO3, which has the smallest band gap in the present study. We have calculated the density of states of RInO3 and characterized the displacement of In and O. The dielectric properties and ionic contribution have been calculated and a detailed comparison has been made on the dielectric function including the static dielectric tensor. The absorption coefficient, the reflectivity, the refractive index, the conductivity and the electron energy-loss spectrum of hexagonal DyInO3, HoInO3 and DyInO3 have been calculated and analyzed.