Hexagonal Compounds Research Articles

Fluorescence temperature dependent behaviors of Eu3+/Mn4+ co-doping cubic and hexagonal ZnO-TiO2 compounds: Application in high sensitive optical thermometers

ZnTiO3:Eu3+,Mn4+ phosphors with hexagonal and cubic structure are synthesized by solvothermal method. The structure of ZnTiO3 depends on precursors and the annealing temperature. Inverse spinel Zn2TiO4 phase reveals the highest thermostability compared with cubic ZnTiO3 and hexagonal ZnTiO3 phases. The different phases of ZnTiO3 crystals exhibit different photoluminescence properties. The forbidden transition of 4A2g→2T2g for Mn4+ is observed in Zn2TiO4 and hexagonal ZnTiO3 (h-ZnTiO3) due to the decreased symmetry. The zero photon line (ZPL) assigned to 2Eg→4A2g transition of Mn4+ is absent in all type ZnTiO3 phases. A sharp emission peak at 714 nm assigned to ν6 (Stokes emission) mode of Mn4+ in h-ZnTiO3 is present when the temperature was below 270 K, and increases sharply in intensity with the temperature decreasing. The similar PL spectra of cubic ZnTiO3 and Zn2TiO4 co-doped with Eu3+ and Mn4+ show a wide emission band composed of Stokes and anti-Stokes modes. The calculated nephelauxetic parameter increases from 0.84 to 1.03 and 1.18 for Mn4+ in h-ZnTiO3, cubic ZnTiO3 and Zn2TiO4, respectively, indicating the covalency decreasing, which causes the red shift of ZPL in h-ZnTiO3. The Mn4+ emissions show stronger temperature dependence than that of Eu3+, especially for that in h-ZnTiO3 matrix. Based on the fluorescence intensity ratio between IEu3+ and IMn4+, the highest relative temperature sensitivity of 2.46 %K−1 is observed in cubic ZnTiO3 (c-ZnTiO3) and Zn2TiO4. Under the excitation at 550 nm, the highest relative sensitivity of 2.9 %K−1 is achieved in c- and h-ZnTiO3 mixed phases.

Weakly coupled Type-II superconductivity in a breathing Kagome metal ZrRe2

We present a comprehensive investigation of the superconducting properties of ZrRe2, a Re-based hexagonal Laves compounds. ZrRe2 crystallizes in a C14-type structure (space group P63/mmc), where the Re atoms form a breathing Kagome lattice, with cell parameters a = b = 5.2682(5) Å and c = 8.63045 Å. Through transport measurements, the superconducting transition is observed with Tconset = 6.44 K and Tczero = 6.06 K. In magnetization measurements(ZFC-FC) superconducting transition occurs at 6.12 K. The measured lower and upper critical fields are 6.27 mT and 7.84 T, respectively. Measurements of the specific heat capacity confirm the presence of bulk superconductivity, with a normalized specific heat change of ΔCe/γTc=1.63 and an electron-phonon strength of λep=0.69. DFT calculations revealed that the band structure of ZrRe2 is intricate and without van Hove singularity.

Superconductivity in Nb5Ir3N: a nitrogen-filled electride

The hexagonal Mn5Si3-type compounds possess the capability to accommodate specific atoms in the interstices, thereby creating filled Mn5Si3-type structures. In Nb-based Mn5Si3-type system, interstitial atoms like carbon (C) or oxygen (O) have been identified to induce or enhance superconductivity. However, the compounds filled with nitrogen (N) are scarce, and the existence of a N-filled superconductor remains unknown. Here, we report the discovery of a novel ternary nitride superconductor, Nb5Ir3N, synthesized via incorporating N into the electride Nb5Ir3. The crystal structure of Nb5Ir3N conforms to the filled Mn5Si3-type, belonging to the P63/mcm space group (No. 193), with cell parameters a = b = 7.8398(2) Å and c = 5.1108(1) Å. Electrical resistivity and magnetic susceptibility demonstrate that Nb5Ir3N is a type-II superconductor with a T c of 8.7 K. The estimated lower and upper critical fields are 11.0 mT and 12.16 T, respectively. Moreover, specific heat measurements confirm the bulk superconductivity with enhanced electron–phonon coupling in Nb5Ir3N, as demonstrated by the normalized specific heat jump ΔC e/γT c ∼ 1.59. First-principles calculations emphasize the strong spin–orbit coupling in Nb5Ir3N.

Static Magnetic Properties and Morphological Behaviour of Mn and Zn Substituted Ca-hexaferrite by Ceramic Technic

Synthesis of M-type hexagonal ferrite compounds, Ca(CoTi)0.5(MnZn)x/2Fe11-xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) prepared by ceramic technic. Mn and Zn ions substituted to find the magnetic characterizationat magnetic field of 10 kOe. XRD and SEM study have been done for getting the hexagonal structure of these samples. Due to the substitution of Mn and Zn ions saturation magnetization increases and magnetocrystalline anisotropy decreases. Due to the sub-latice site distribution the variations in magnetic parameters have been observed. Because of Mn and Zn ions substitution Curie temperature decreases by weakening of superexchange interaction. These materials can be used for various applications such aspermanent magnetic material, magnetic recording media, ferrofluids, sensors, microwave absorbing materials, ceramic magnets in loud speakers and rotors in small DC motors by studying various magnetic parameters from VSM characterization.

The Ce-Ni-Si System Revisited: More Homologue Compounds?

The nickel-rich region of the system Ce-Ni-Si has been reinvestigated utilizing X-ray single-crystal, powder, and electron diffraction as well as electron microprobe and thermal analyses. Two novel hexagonal compounds, τ-Ce20+xNi36+ySi30-z and τ'-Ce30+xNi50+ySi42-z, were identified. The crystal structure of τ-Ce20+xNi36+ySi30-z was derived from single-crystal X-ray diffraction and found to be isotypic with the Sm10Ni20.8P15-type structure (S.G. P63/m, x = 1.8, y = 3.0, z = 1.8, a = 2.07156(2) nm, c = 0.39990(1) nm, RF = 0.048). Rietveld refinement of τ'-Ce30+xNi50+ySi42-z revealed isotypism with Tb15Ni28P21 (S.G. P63/m, a = 2.46926(13) nm, c = 0.40019(3) nm, RF = 0.058). The compound Ce3Ni4Si2 from X-ray single-crystal analysis was found to crystallize in a novel structure type with monoclinic unit cells (S.G. C2/c, a = 1.54708(3) nm, b = 0.58677(1) nm, c = 0.74331(1) nm, β = 102.985(1)°, RF = 0.017). This compound belongs to a new homologue series in the RE-Ni-Si system (RE = La and Ce) with general formula of RE(3×2n)Ni(3×2n+1)Si(2n+1); n = 0,1, ..., ∞. The crystal structure of this series is characterized by alternating numbers (2n) of corner-sharing Si-polyhedral blocks sandwiched between zigzag nickel chains. Higher-order members of this series are produced by the formation of more corner-sharing Si-polyhedral blocks due to removal of nickel chains.

Ba6RE2Ti4O17 (RE = Nd, Sm, Gd, Dy-Yb): A Family of Rare-Earth-Based Layered Triangular Lattice Magnets.

The exploration of new rare-earth (RE)-based triangular-lattice materials plays a significant role in motivating the discovery of exotic magnetic states. Herein, we report a family of hexagonal perovskite compounds Ba6RE2Ti4O17 (RE = Nd, Sm, Gd, Dy-Yb) with a space group of P63/mmc, where magnetic RE3+ ions are distributed on the parallel triangular-lattice layers within the ab-plane and stacked in an 'AA'-type fashion along the c-axis. The low-temperature magnetic characterizations indicate that all synthesized Ba6RE2Ti4O17 compounds exhibit dominant antiferromagnetic (AFM) interactions and the absence of magnetic order down to 1.8 K. The isothermal magnetization and electron spin resonance results reveal the distinct magnetic anisotropy for the compounds with different RE ions. Moreover, the as-grown Ba6Nd2Ti4O17 single crystals exhibit Ising-like magnetic anisotropy with a magnetic easy-axis perpendicular to the triangle-lattice plane and no long-range magnetic order down to 80 mK, as the quantum spin liquid candidate with dominant Ising-type interactions.

Towards 2D Borane Chemistry in Hexagonal Cyclic Compounds.

A comprehensive comparison between known benzene mono-substituted compounds R-Ph and the corresponding isoelectronic unknown R-cyclohexaborane(12) molecules is carried out from a geometric and electronic structure point of view, with R={H, BH2, CH3, NH2, OH, F ; AlH2, SiH3, PH2, SH, Cl ; NO2, OCH3}. We suggest new chemical names for the 2D borane compounds and analyze the geometric and electronic structure carbon vs. boron comparatives by means of HOMO-LUMO gaps, bonding schemes, electron density topological properties and predicted NMR chemical shifts. The predictions on the properties in planar hexagonal cyclic boranes may help in the design of synthesis procedures for these yet-unkown compounds.

Superconductivity in a breathing kagome metals ROs2 (R = Sc, Y, Lu)

We have successfully synthesized three osmium-based hexagonal Laves compounds ROs2 (R = Sc, Y, Lu), and discussed their physical properties. LeBail refinement of pXRD data confirms that all compounds crystallize in the hexagonal centrosymmetric MgZn2-type structure (P63/mmc, No. 194). The refined lattice parameters are a = b = 5.1791(1) Å and c = 8.4841(2) Å for ScOs2, a = b = 5.2571(3) Å and c = 8.6613(2) Å for LuOs2 and a = b = 5.3067(6) Å and c = 8.7904(1) Å for YOs2. ROs2 Laves phases can be viewed as a stacking of kagome nets interleaved with triangular layers. Temperature-dependent magnetic susceptibility, resistivity and heat capacity measurements confirm bulk superconductivity at critical temperatures, Tc, of 5.36, 4.55, and 3.47 K for ScOs2, YOs2, and LuOs2, respectively. We have shown that all investigated Laves compounds are weakly-coupled type-II superconductors. DFT calculations revealed that the band structure of ROs2 is intricate due to multiple interacting d orbitals of Os and R. Nonetheless, the kagome-derived bands maintain their overall shape, and the Fermi level crosses a number of bands that originate from the kagome flat bands, broadened by interlayer interaction. As a result, ROs2 can be classified as (breathing) kagome metal superconductors.

Ab initio investigations of crystal transition for graphite intercalation compounds during aluminum electrolysiss

This paper studies the crystal transition of graphite intercalation compounds during aluminum electrolysis. Transmission electron microscopy (TEM) analysis demonstrates that the transition induces the structural degradation of carbon cathodes. Buckling of basal planes in the hexagonal compounds contributes to the crystal transition of intercalation compounds from hexagonal to rhombohedral modification. First-principle techniques are used to calculate the structural and electronic properties of graphite intercalation compounds by density-functional theory using the plane wave Pseudo-potential method. The results have demonstrated a close relationship between lattice mismatch and intercalation compounds. The electronic band structure close to the Fermi level has been investigated. The main effect of intercalation compounds is to transfer charge from alkali metal s electron to the carbon pz orbitals, rendering the system metallic. With a decrease of band gap, the crystal transition induces an increase in the interlayer spacing and C–C bond lengths and deteriorates the structural stability of carbon cathodes. The overwhelming majority of contribution to the bands neighboring the Fermi level is from C pz orbital in both intercalation compounds. These findings provide new insights into the mechanisms of carbon cathodes deformation, damage and fracture behavior.

Electronic and Excitonic Properties of MSi2Z4 Monolayers (Small 19/2023)

MSi2Z4 Monolayers In article number 2206444, Agnieszka B. Kuc and co-workers study from first-principles monolayers of a new family of hexagonal MSi2Z4 compounds. These host 2D excitons with large response to magnetic field, with a broad range of g-factors values, making them attractive for opto-, spin- and valleytronic devices.

Nature of magnetic transitions and evidence for magnetoelastic coupling in the biskyrmion-host hexagonal compound MnNiGa

The hexagonal compound MnNiGa with Ni2In-type structure has emerged as an important biskyrmion-host system in which these textures are super-stable over a wide temperature range extending from 16 K to the Curie temperature TC ∼350 K under moderate fields. Both the intensity of the external magnetic field and the presence of magnetoelastic coupling are known to play key role in the stability of the skyrmionic textures. Here we present the results of the first comprehensive investigation of the order of the phase transitions and its field dependence along with magnetoelastic (i.e., spin-lattice) coupling using a combined magnetization and high-resolution synchrotron x-ray powder diffraction (SXRPD) study of MnNiGa. The temperature dependence of the magnetization M(T) exhibits a paramagnetic to collinear ferromagnetic (FM) transition at TC (∼347 ± 0.5 K) with thermal hysteresis and negative slope in the Arrott plots across the TC suggesting its first order character. The M(T) plot in the collinear FM phase exhibits an anomalously decreasing behaviour after reaching its peak value at 300 K followed by a step-like change below 210 K due to an additional spin reorientation transition (SRT) to a non-collinear FM phase with TSRT∼200 ± 1 K. The SRT also shows thermal hysteresis characteristic of a first order phase transition. Our field dependent studies reveal stabilization of the collinear FM phase to higher temperatures and destabilization of the SRT transition setting an upper limit on the magnetic field for the observation of biskyrmions in this system. The Rietveld analysis of the SXRPD data reveals anomalies in the unit cell parameters at TC and TSRT without any change in the crystal symmetry. The modelling of the temperature dependence of the unit cell volume in terms of the Debye-Grüneisen equation reveals significant deviation from phonon contributions and the presence of the quadratic spin-lattice coupling below the FM TC, in the precursor SRT regime as well as below TSRT. The present results on the magnetoelastic coupling in the hexagonal MnNiGa may provide the necessary insight towards understanding the extreme sensitivity of the magnetic skyrmionic textures to external stresses.

Transport Properties of Intergrowth Structures Ba5In2Al2ZrO13 and Ba7In6Al2O19

The development of solid oxide fuel cells operating at medium temperatures (500–700 °C and even lower) requires the search for proton conductors based on complex oxides that would have a wide range of required properties. This task stimulates the search for new promising phases with proton conductivity. The new hexagonal perovskite-related compound Ba7In6Al2O19 was synthesized by the solid-state method. The phase was characterized by powder X-ray diffraction, thermogravimetric analysis, FT-IR spectroscopy, and impedance spectroscopy (in a wide range of temperatures, and partial pressures of oxygen at various atmospheric humidities). The investigated phase had a hexagonal structure with a space group of P63/mmc; the lattice parameters for Ba7In6Al2O19 are a = 5.921(2) Å, c = 37.717(4) Å. The phase is capable of reversible hydration and incorporates up to 0.15 mol H2O. IR-data confirmed that protons in the hydrated compound are presented in the form of OH–-groups. Electrical conductivity data showed that the sample exhibited dominant oxygen-ion conductivity below 500 °C in dry air and dominant proton conductivity below 600 °C in wet air.

Synthesis and Surface Chemistry of Bimetallic Chromium-Iron Carbide (CrxFe1-x)7C3 Solid Solution Nanoparticles

A series of hexagonal (CrxFe1-x)7C3 solid solution compounds were synthesized for the first time as nanomaterials using a unique amine-metal oxide composite (AMOC) method. These materials are related to stainless steel where chromium provides protection from oxidation and could be potential low-cost catalyst materials for a variety of important reactions. Thus, a simple synthesis method coupled with a detailed understanding of their surface chemistry and composition is extremely important. X-ray photoelectron spectroscopy (XPS) studies revealed that both Fe and Cr in the bulk of (CrxFe1-x)7C3 solid solutions are mostly metallic. Further, the surface of these nanomaterials maintained exposed zero valent metals (∼10 at%) after long term air exposure demonstrating the corrosion and oxidation resistant nature of this Cr-Fe-C ternary system. This suggests that these compounds have stable surface chemistry which makes them potentially good candidates for catalysts. These bimetallic carbides were subsequently tested as electrochemical catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) applications in acidic (0.1 M HClO4) and alkaline (0.1 M KOH) electrolytes. All (CrxFe1-x)7C3 solid solutions exhibit better ORR and OER activities than Cr7C3, Fe3C, and Co3O4 and both (Cr0.4Fe0.6)7C3 and (Cr0.5Fe0.4)7C3 are good OER catalysts in alkaline media demonstrating their potential for future catalysis applications.

EFFECT OF PRESSURE ON ELASTIC CONSTANTS AND RELATED PROPERTIES OF RARE-EARTH INTERMETALLIC COMPOUND TBNIAL

The Lennard-Jones potential approach is used to investigate the effect of pressure on the ultrasonic and elastic properties of the rare-earth ternary TbNiAl intermetallic compound. The second- and third-order elastic constants of TbNiAl are considered using the potential model. The pressure-dependent higher-order elastic constants are studied, and it is observed that the elastic constants of the TbNiAl compound increased monotonously with pressure. The hexagonal TbNiAl compound is mechanically stable up to the pressure 20 GPa according to the Born elastic stability criteria. The Voigt-Reuss-Hill approach is used to compute such elastic parameters as Young’s modulus, bulk modulus, Poisson’s ratio, and shear modulus in the pressure range 0-45 GPa. Hardness, melting temperature, and anisotropy are also determined for the intermetallic TbNiAl compound. The pressure-dependent velocities and attenuation of ultrasonic waves in this ternary compound are evaluated. The computation results are also satisfactory in estimating the Debye temperature and thermal conductivity Kmin under different pressure. It is observed that TbNiAl has a significant anisotropy at zero pressure, which becomes stronger as the pressure increased. This ternary compound behaves as its purest form at higher pressure and is more ductile, which is demonstrated by the minimum attenuation.

Magnetic Toroidal Moment under Partial Magnetic Order in Hexagonal Zigzag-Chain Compound Ce3TiBi5

We theoretically investigate an antiferromagnetic structure in a hexagonal zigzag-chain compound Ce$_3$TiBi$_5$, which shows a linear magnetoelectric effect in metals and an unfamiliar temperature dependence of magnetic susceptibility. We find a partial magnetic order as a coexistence of a nonmagnetic zigzag chain and staggered antiferromagnetic zigzag chains. The latter accompany in-plane magnetic toroidal dipole moments which are responsible for the observed behaviors. We show that magnetic frustration arising from competing exchange interactions plays an important role to realize such a partial magnetic order. Furthermore, we present the important spin-orbit coupling parameters to induce the physical phenomena driven by the magnetic toroidal dipole moment, such as the linear magnetoelectric effect and nonlinear transport.

Giant low-field magnetocaloric effect in hexagonal Eu3B2O6 compound

A modified solid-phase reaction method was used to synthesize the polycrystalline Eu3B2O6-as a novel cryogenic magnetocaloric material. In addition, investigations into the magnetocaloric effect (MCE), magnetic properties and crystal structure of compound have been precisely completed. The results indicate that the compound undergoes a second-order magnetic-phase transition from ferromagnetic to paramagnetic at 8 K and has a hexagonal structure that belongs to the R-3c space group. Meanwhile, the MCE was evaluated by using magnetic entropy change (-ΔSM), temperature averaged entropy change (TEC) as well as the refrigeration capacity (RC) for Eu3B2O6. Under the field change from 0 to 5 T, the maximal values of -ΔSM and TEC (5) are 38.6 and 35.7 J kg−1 K−1, respectively, while the corresponding RC reaches 435.9 J kg−1. The giant low-field MCE facilitates device miniaturization and cost reduction, making Eu3B2O6 compound a promising candidate for cryogenic magnetic refrigeration.

Effects of Ge addition on the structure, mechanical and magnetic properties of (CoCrFeNi)100-xGex high-entropy alloys

Alloying non-metallic elements can not only change the structure of alloys, but also tailor the mechanical and physical properties. The microstructure, mechanical and magnetic properties of (CoCrFeNi)100-xGex (molar ratio, x = 0 ∼ 33) high-entropy alloys (HEAs) have been systematically studied in this paper. The experimental results show that the (CoCrFeNi)100-xGex HEAs transform from a face-centered cubic (FCC) to a body-centered cubic (BCC) solid solution when 10 ≤ x ≤ 25, and a hexagonal intermetallic compound (IM) formed at x = 33. Tensile measurement revealed that the strength and plasticity of the alloy were enhanced simultaneously with Ge addition in the single FCC region (x ≤ 10), a decrease in ductility but sharp increase of strength was found in the alloys when the BCC phase starts to appear. Theoretical calculations and magnetic hysteresis loop measurements show that the alloys transform gradually from a paramagnetic to a ferromagnetic state at room temperature.

Large topological Hall effect and in situ observation of magnetic domain structures in the Mn2FeSn compound

The search for novel topological magnetic domain structures such as magnetic skyrmions, bubbles and vortices, and relative physical effect has always been the focus of spintronics devices. Here, the magnetism, transport properties and magnetic domain structures of the polycrystalline hexagonal Mn2FeSn compound are studied in detail. We find the strong texture along c-axis in this bulk compound owing to large temperature gradient between the bottom and top of the ingot during the preparation process. The resistivity resulting from topological Hall effect reaches about 5.19 μΩ cm at 50 K in the sample of compound obtained by cutting along perpendicular texture direction. Spontaneous nanoscale magnetic bubbles have directly been observed at low temperature due to the competition among the magnetocrystalline anisotropy, antiferromagnetic, and ferromagnetic exchange interactions as decreasing temperature. It is the formation of non-collinear or non-coplanar spin configuration that causes the obvious topological Hall effect. Additionally, the biskyrmion-like state can be obtained with evolution of stripe domains under an appropriate magnetic field. Based on these results, this study of Mn2FeSn magnet may be beneficial to the development for exploring novel magnetic spin textures.

Pressure-dependence of mechanical properties and thermodynamic behavior of hexagonal and orthorhombic Ti3Sn: A first-principles investigation

The influence of pressure on the mechanical, thermodynamic and electronic properties of orthorhombic and hexagonal Ti3Sn compounds was investigated by using first-principles calculations. The pressure–lattice parameters relationships of both compounds can be obtained by applying hydrostatic pressure. According to the curves of elastic constants as a function of pressure, it is revealed that high pressure improves the resistance to elastic moduli of orthorhombic Ti3Sn compound as well as bulk modulus of hexagonal phase. Both compounds are considered as ductile materials in the range of 0–100 GPa. It is demonstrated that the high pressure can effectively enhance the Debye temperature and minimum thermal conductivity of orthorhombic Ti3Sn compound, whereas hexagonal phase has the maximum values at 10 GPa. Results of electronic structures indicate that the p-d hybridization between Sn and Ti atoms of orthorhombic Ti3Sn compound is stronger than that of hexagonal phase.

Phase Equilibria of the Mg-Zn-Sm System in Mg-Rich Corner at 320 °C and 400 °C

To clarify the controversy regarding the phase equilibria in the Mg-rich corner of the Mg-Zn-Sm system, alloys annealed at 320 °C and 400 °C were employed to determine the phase constitution, composition and crystal structure by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The maximum solubility of Zn in Mg3Sm was measured to be 49.2 at.% at 400 °C. The Y phase (Mg62Zn31Sm7), only observed in the as-cast alloys, was determined to have an orthorhombic structure with lattice parameters of a = 10.20 Å, b = 11.26 Å and c = 9.64 Å by TEM. The hexagonal compound μ7, identified with lattice parameters of a = 34.62 Å and c = 8.94 Å, was detected during the transformation of the Y phase to the Z phase in the alloys located in the (Mg) + Mg3Sm + Z three-phase region. The phase equilibria (Mg) + Mg41Sm5 + Mg3Sm, Mg + Mg3Sm + Z, (Mg) + Z + liquid and Mg2Zn3 + Z + liquid at 400 °C are confirmed, and the three-phase region (Mg) + Z + MgZn exists in the Mg-Zn side at 320 °C. Subsequently, a self-consistent thermodynamic description was obtained based on the experimental data. Meanwhile, solidification simulation of Y phase formation was conducted by suppressing the stale Z phase, which can reasonably explain the as-cast microstructure of alloys in the Mg-rich corner. The thermodynamic database would be helpful for the further development of Mg-Zn-Sm alloys.