NiAs Structure Research Articles

Synthesis of Ultrathin FeS Nanosheets via Chemical Vapor Deposition.

Fe-based 2D materials exhibit rich chemical compositions and structures, which may imply many unique physical properties and promising applications. However, achieving controllable preparation of ultrathin non-layered FeS crystal on SiO2/Si substrate remains a challenge. Herein, the influence of temperature and molecular sieves is reported on the synthesis of ultrathin FeS nanosheets with a thickness as low as 2.3nm by molecular sieves-assisted chemical vapor deposition (CVD). The grown FeS nanosheets exhibit a non-layered hexagonal NiAs structure and belong to the P63/mmc space group. The inverted symmetry broken structure is confirmed by the angle-resolved second harmonic generation (SHG) test. In particular, the 2D FeS nanosheets exhibit exceptional metallic behavior, with conductivity up to 1.63 × 106S m-1 at 300K for an 8nm thick sample, which is higher than that of reported 2D metallic materials. This work provides a significant contribution to the synthesis and characterization of 2D non-layered Fe-based materials.

Control of Polarity in Kagome‐NiAs Bismuthides

AbstractA 2×2×1 superstructure of the P63/mmc NiAs structure is reported in which kagome nets are stabilized in the octahedral transition metal layers of the compounds Ni0.7Pd0.2Bi, Ni0.6Pt0.4Bi, and Mn0.99Pd0.01Bi. The ordered vacancies that yield the true hexagonal kagome motif lead to filling of trigonal bipyramidal interstitial sites with the transition metal in this family of “kagome‐NiAs” type materials. Further ordering of vacancies within these interstitial layers can be compositionally driven to simultaneously yield kagome‐connected layers and a net polarization along the c axes in Ni0.9Bi and Ni0.79Pd0.08Bi, which adopt Fmm2 symmetry. The polar and non‐polar materials exhibit different electronic transport behaviour, reflecting the tuneability of both structure and properties within the NiAs‐type bismuthide materials family.

Control of Polarity in Kagome-NiAs Bismuthides.

A 2×2×1 superstructure of the P63/mmc NiAs structure is reported in which kagome nets are stabilized in the octahedral transition metal layers of the compounds Ni0.7Pd0.2Bi, Ni0.6Pt0.4Bi, and Mn0.99Pd0.01Bi. The ordered vacancies that yield the true hexagonal kagome motif lead to filling of trigonal bipyramidal interstitial sites with the transition metal in this family of "kagome-NiAs" type materials. Further ordering of vacancies within these interstitial layers can be compositionally driven to simultaneously yield kagome-connected layers and a net polarization along the c axes in Ni0.9Bi and Ni0.79Pd0.08Bi, which adopt Fmm2 symmetry. The polar and non-polar materials exhibit different electronic transport behaviour, reflecting the tuneability of both structure and properties within the NiAs-type bismuthide materials family.

Crystal and electronic structure of the ternary Zintl bismuthide BaLiBi

AbstractReported is the accurate refinement of the structure of the ternary bismuthide BaLiBi, based on single‐crystal X‐ray diffraction data. This compound crystallizes with the ZrBeSi structure type with the space group P63/mmc (no. 194), a=4.9917(6) Å, c=9.079(2) Å, V=195.92(7) Å3 with two formula units per unit cell. In addition to being a colored ternary variant of the AlB2 type, the crystal structure of BaLiBi can be also viewed as a “stuffed” variant of the NiAs structure, where the Bi atoms form a hexagonal close packing, the Ba atoms occupy the octahedral voids in this packing, and the Li atoms are located between adjacent tetrahedral voids on their common triangular faces. In the absence of direct Bi–Bi interactions, the BaLiBi crystal structure rationalized according to the notation (Ba2+)(Li+)(Bi3−), suggesting an electron‐balanced composition, i. e., a Zintl phase. In line with this notation, scalar‐relativistic first‐principle calculations with the LMTO code reveal a semiconducting ground state, with a bandgap of about 0.6 eV. Fully relativistic electronic structure calculations predict a semimetallic ground state.

Evaluation of the critical behavior near ferromagnetic to paramagnetic phase transition in CrTe1-xSex alloys: An experimental study

We used the conventional solid-state reaction method to prepare stoichiometric samples of CrTe1-xSex, where 0 ≤ x ≤ 0.10, and investigated the structural and critical behavior of the prepared samples. Room temperature powder X-ray diffraction, along with Rietveld refinement, revealed the emergence of the NiAs structure with P63/mmc (194) space group with increasing Se substitution. The high-temperature linear fit to inverse of the dc-susceptibility versus temperature for the mother sample resulted in an effective moment of 3.65μB Cr with Curie-Weiss temperature near 335K, which is slightly higher than the Tc of ∼332K obtained from the inflection point of magnetization versus temperature. Magnetization isotherms were employed to investigate the critical behavior of ferromagnetic CrTe1-xSex with 0.0 ≤ x ≤ 0.10 near their Curie temperatures (Tc). The magnetic behavior near Tc was found to follow 3D mean-field critical exponents with a second-order phase transition in all samples investigated. We fine-tuned the critical exponents (β, γ, and δ) using a combination of an iteration technique, the Kouvel-Fisher method, and modified Arrott plots. All samples follow a mean field behavior, with Tc ranging from 298 to 340K. The acquired values exhibit self-consistency, as indicated by the results from the Widom scaling relation. Furthermore, the magnetization isotherms exhibit a universal scaling behavior, providing additional credence to the calculated critical exponents.

From Layered Antiferromagnet to 3D Ferromagnet: LiMnBi-to-MnBi Magneto-Structural Transformation

The intermetallic compound LiMnBi was synthesized by the two-step solid-state reaction from the elements. A synthesis temperature of 850 K was selected based on in situ high-temperature powder X-ray diffraction data. LiMnBi crystalizes in the layered-like PbClF structure type (a = 4.3131(7) Å, c = 7.096(1) Å at 100 K, P4/nmm space group, Z = 2). The LiMnBi structure is built of alternating [MnBi] and Li layers, as determined from single-crystal X-ray diffraction data. Magnetic property measurements and solid-state 7Li nuclear magnetic resonance data collected for polycrystalline LiMnBi samples indicate the long-range antiferromagnetic ordering of the Mn sublattice at ∼340 K, with no superconductivity detected down to 5 K. LiMnBi is air- and water-sensitive. Under aerobic conditions, Li can be extracted from the LiMnBi structure to form Li2O/LiOH and MnBi (NiAs structure type, P63/mmc). The obtained MnBi polymorph was previously reported to be one of the strongest rare-earth-free ferromagnets, yet its bulk synthesis in powder form is cumbersome. The proposed magneto-structural transformation from ternary LiMnBi to ferromagnetic MnBi involves condensation of the MnBi4 tetrahedra upon Li deintercalation and is exclusive to LiMnBi. In contrast, ferromagnetic MnBi cannot be obtained from either isostructural NaMnBi and KMnBi or from the structurally related CaMn2Bi2. Such a distinctive transformation in the case of LiMnBi is presumed to be due to its fitting reactivity to yield MnBi and a favorable interlayer distance between [MnBi] layers, while the interlayer distance in NaMnBi and KMnBi structural analogues is unfavorably long. The studies of delithiation from layered-like LiMnBi under different chemical environments indicate that the yield of MnBi depends on the type of solvent used and the kinetics of the reaction. A slow rate and mild reaction media lead to a high fraction of the MnBi product. The saturation magnetization of the “as-prepared” MnBi is ∼50% of the expected value of 81.3 emu/g. Overall, this study adds a missing member to the family of ternary pnictides and illustrates how soft-chemistry methods can be used to obtain “difficult-to-synthesize” compounds.

Room temperature magnetocaloric effect in CrTe1−xSex alloys

In this work, we report room temperature magnetocaloric properties in CrTe1−xSex (0.00 ≤ x ≤ 0.10) alloys prepared by a conventional solid-state reaction. The Rietveld refinement of the XRD pattern of CrTe1−xSex showed the emerging of pure hexagonal NiAs structure with P63/mmc (194) space group with increasing Se substitution. Upon μ0H = 5 T applied magnetic field, the magnetocaloric effect analysis revealed maximum magnetic entropy change $$(\Delta {S}_{M})$$ in the range 7.9–9.2 J/kg K−1 with a relative cooling power (RCP) value of about 550–694 J/kg. The RCP values exceed the corresponding RCP value of the prototype Gd element near room temperature. The obtained results suggest that CrTe1−xSex (0.00 ≤ x ≤ 0.10) alloys is a viable candidate for a room temperature magnetic cooling application.

Insulating antiferromagnetism in VTe

We report a detailed theoretical and experimental study on the vanadium monotelluride VTe, which crystallizes in the NiAs hexagonal structure. First-principles calculations reveal a complex hierarchy of magnetic interactions and energy scales, with the ground state theoretically determined as an ($\frac{1}{2}$, 0, $\frac{1}{2}$) antiferromagnetic ordering with insulating character and a band gap of 0.5 eV. Experimental synthesis and characterization efforts find a substantially off-stoichiometric orthorhombic structure (a defect NiAs structure) with composition ${\mathrm{V}}_{0.85}\mathrm{Te}$, and an apparent N\'eel point of some 45 K. First-principles calculations find good agreement with the observed N\'eel point. We also give an extended examination of the effects of off-stoichiometry on the calculated energetics, finding significant volume-related effects. Our first-principles calculations find the stoichiometric phase VTe to have a negative vanadium defect formation energy of over 1 eV, thus explaining the formation of the off-stoichiometric phase. Finally, we provide a structural explanation for the formation of defect structures in this and numerous other NiAs-structure materials.

A First‐Principles Study of the Structural, Magnetic, Optical Properties and Doping Effect in Chromium Arsenide

Pristine and doped chromium arsenide (CrAs) in six different crystal structures is systematically studied to investigate the structural, magnetic, and optical properties for real applications by first‐principles calculations. First, it is found that the ground‐state structure is an orthorhombic MnP‐type structure with antiferromagnetic spin order. The rocksalt structure is a low‐energy metastable phase and a ferromagnetic metal with high spin polarization at the Fermi level. Second, the NiAs structure and MnP structure have a higher absorption coefficient than other structures in the infrared region and ultraviolet region, respectively. In the visible region, the wurtzite and zincblende structures are more transparent than other structures. At last, the substitution of Cr by Ti and the substitution of As by Te can lead to a phase transition in ground‐state structure and ground‐state magnetic order, respectively. These results can promote the application of the CrAs system into spintronics and optoelectronics.

Atomistic study of stability and new phase transitions of zinc selenide using FP-LMTO-PLW method

By full potential linear muffin-tin orbitals (FP-LMTO-PLW) method, we have studied the phase transitions of ZnSe under high pressures. The local density approximation (LDA) was used for the exchange and correlation energy. The most important result is the prediction of the possibility of four phase transitions from the cubic zincblende (ZnS-B3) to the B81 (NiAs) structure at 8.00 GPa, the second from B3 to the B32 (NaTi) and Bh (WC) structures at above 45 GPa for each one of them, and from B3 to B2 (CsCl) phase at around 31GPa. The first one (ZnS–NiAs) occurring at a lower pressure than the well knownZnS to NaCl transition (found here to be 11.5 GPa).

Effects of the C interstitial doping on the magnetic properties of LTP MnBi

Experimental and theoretical investigations on the structural and magnetic properties of the MnBi and MnBiC hard magnetic phases are presented. X-ray diffraction patterns showed that the highest concentration of MnBi low-temperature phase (LTP) was obtained for annealing at 400 °C for 12 h. X-ray photoemission measurements (XPS) of the core levels (Mn 2p and 3d; Bi 4d and 5p, respectively) show chemical shifts by C addition, providing evidence that the C atoms enter the MnBi structure in the vicinity of both type of metallic atoms. C atoms were assumed to occupy the 2d interstitial crystal sites of the hexagonal NiAs structure type, as suggested in earlier studies. The theoretical calculations show that C addition enhances slightly the magnetic moment of the samples compared to pristine LTP MnBi, in agreement with our magnetic measurements. Also, following both theoretical and experimental investigations, we obtained increased magnetocrystalline anisotropy energy (MAE) by adding C as interstitial dopant.

Study of structural, elastic, electronic, dynamical and optical properties of beryllium selenide (BeSe) semiconductor in zinc blend and NiAs phases

We report structural, elastic, electronic, dynamical and optical properties of beryllium selenide in zinc blend and NiAs phases. The zinc blend structure is the most stable at room temperature and equilibrium hydrostatic pressure. The HSE hybrid functional underestimates the lattice parameters compared to the GGA-PBE and LDA, but gives a fundamental gap closer to its experiment value. We derive bulk modulus, shear modulus, Young's modulus and Poisson's ratio using Voigt, Reuss and Hill approximations. Computed elastic constants in NiAs structure are predictions. Waves velocities for beryllium selenide in zinc blend (NiAs) phase along [100], [110] ([100], [110], [111]) directions are predictions. The indirect band gap Γ-X (2.852 eV) and Γ-K (0.536 eV) for BeSe in zinc blend and NiAs structures translates the semiconductor nature. Hybridization occurs between Se: p and Be: s states at the top of the valence band and traduces the covalent bonding character. The first conduction band is quite wide in both structures, where the contribution is due to Se: p, Se: s and Be: s sites. The electronic transition occurs from Se: p orbital of valence band to Be: s site of conduction band. Absorption peaks are attributed to the photo transition energies from the maximum valence band to the minimum conduction band under ultra violet light irradiation.

Insight into the phase transition, elastic and thermodynamic properties of BeS compound under high pressure and temperature from the first principle calculation

Based on the first principle plane wave pseudopotential and generalized gradient approximation (GGA) method, the phase, elastic and thermodynamic properties of BeS compound at pressure ranging from 0 to 175 GPa are calculated. After studying the energy-volume and entropy difference-pressure of the zinc blende, NiAs, NaCl and CsCl structures of BeS compound in depth, we consider the zinc blende and NiAs structures as two stable structures for BeS compound. The phonon spectra and elastic constants reveal that zinc blende and NiAs structures are also structurally and mechanically stable at ambient conditions. Furthermore, the phase transition pressure from zinc blende structure to NiAs structure is 58.615 GPa. Intriguingly, the anisotropies of the elastic and sound velocities of BeS are found to exhibit anisotropic characteristics, which is the first comprehensive study on the mechanical properties of BeS compound. Notably, we report for the first time the thermodynamic properties of NiAs structure, such as the Debye temperature, specific volume heat capacity, which shows great potential in the further study on the properties of BeS compound.

Jahn–Teller Disproportionation Induced Exfoliation of Unit‐Cell Scale ϵ‐MnO2

AbstractExfoliation of non‐layered (structurally) bulk materials at the nanoscale is challenging because of the strong chemical bonds in the lattice, as opposed to the weak van der Waals (vdW) interactions in layered materials. We propose a top‐down method to exfoliate ϵ‐MnO2 nanosheets in a family of charge‐ordered La1−xAExMnO3 (AE=Ca, Sr, Ba) perovskites, taking advantage of the Jahn–Teller disproportionation effect of Mn3+ and bond‐strength differences. ϵ‐MnO2 crystallized into a nickel arsenide (NiAs) structure, with a thickness of 0.91 nm, displays thermal metastability and superior water oxidation activity compared to other manganese oxides. The exfoliation mechanism involves a synergistic proton‐induced Mn3+ disproportionation and structural reconstruction. The synthetic method could also be potentially extended to the exfoliation of other two‐dimensional nanosheet materials with non‐layered structures.

Jahn-Teller Disproportionation Induced Exfoliation of Unit-Cell Scale ϵ-MnO2.

Exfoliation of non-layered (structurally) bulk materials at the nanoscale is challenging because of the strong chemical bonds in the lattice, as opposed to the weak van der Waals (vdW) interactions in layered materials. We propose a top-down method to exfoliate ϵ-MnO2 nanosheets in a family of charge-ordered La1-x AEx MnO3 (AE=Ca, Sr, Ba) perovskites, taking advantage of the Jahn-Teller disproportionation effect of Mn3+ and bond-strength differences. ϵ-MnO2 crystallized into a nickel arsenide (NiAs) structure, with a thickness of 0.91 nm, displays thermal metastability and superior water oxidation activity compared to other manganese oxides. The exfoliation mechanism involves a synergistic proton-induced Mn3+ disproportionation and structural reconstruction. The synthetic method could also be potentially extended to the exfoliation of other two-dimensional nanosheet materials with non-layered structures.

Room temperature magnetocaloric effect in CrTe1-xSbx alloys

We investigate the magnetocaloric effect (MCE), relative cooling power (RCP) and crystalline structure in Sb substituted CrTe1-xSbx (0 ≤ x ≤ 0.2) alloy. The Rietveld refinement of the XRD pattern of CrTe1-xSbx showed the emerging of pure hexagonal NiAs structure with P63/mmc (194) space group with increasing Sb substitution. We detect a slight increase in the basal plane a-lattice parameter, with a much larger reduction in the c-axis. Magnetic isotherms were measured in the temperature range of 50–400 K. The results revealed an increase in the maximum entropy change |SM(T,H)| with Sb-substitutions in the temperature range (~285–325 K). Moreover, The RCP values increased by about 33% with 20% Sb substitutions. These findings suggest that CrTe1-xSbx alloys can be used in room temperature magnetic cooling at fraction of the coast of pure Gd element the porotype magnetic material for magnetic refrigeration.

Solid Solutions PdTe1– xBix (x < 0.8) with the NiAs Structure in the Pd–Bi–Te System

Three solid solutions in the Pd–Bi–Te system were obtained by high-temperature ampoule synthesis from the elements at 700°C. The crystal structures of the solid solutions were determined using powder diffraction data by the full-profile Rietveld method. The solid solutions PdTe0.33Bi0.67 (a = 4.19816(8) A, c = 5.6861(1) A), PdTe0.5Bi0.5 (a = 4.18888(9) A, c = 5.6778(1) A), and PdTe0.67Bi0.33 (a = 4.17796(8) A, c = 5.6733(1) A) belong to the PdTe1 – xBix series: the structures of compounds are based on the statistical substitution of Bi atoms for Te atoms in the PdTe structure (NiAs type, hexagonal system, space group P63/mmc). The concentration limit for substitution in the PdTe1 – xBix series of solid solutions is in the 0.67 < x < 0.8 range. Band structure calculations demonstrate the metallic nature for all compounds and similar electronic structures for PdTe, PdTe1– xBix, and PdBi.

High energy product of MnBi by field annealing and Sn alloying

Permanent-magnet materials are one cornerstone of today’s technology, abundant in disk drives, motors, medical equipment, wind generators, and cars. A continuing challenge has been to reconcile high permanent-magnet performance with low raw-material costs. This work reports a Mn-Bi-Sn alloy exclusively made from inexpensive elements, exhibiting high values of Curie-temperature, magnetization, anisotropy, coercivity, and energy product. The samples are produced by field annealing of rapidly quenched Sn-containing MnBi alloys, where the improvement of the magnetic properties is caused by the substitutional occupancy of the 2c sites in the hexagonal NiAs structure by Sn. The substitution modifies the electronic structure of the compound and enhances the magnetocrystalline anisotropy, thereby improving the coercivity of the compound. The energy product reaches 114 kJ/m3 (14.3 MGOe) at room temperature and 86 kJ/m3 (10.8 MGOe) at 200 °C; this value is similar to that of the Dy-free Nd2Fe14B and exceeds that of other rare-earth-free permanent-magnet bulk alloys, as encountered in automotive applications.

Phase stability and physical properties of BAs and BP compounds: An ab-initio study

In this study, density functional theory (DFT) calculations were performed to investigate the structural stability of different crystallographic phases, the pressure-induced phase transition, the electronic and thermal properties of BAs and BP compounds. The zinc blende (B3), wurtzite (B4), NaCl (B1), CsCl (B2), NiAs (B8), crystal structures are considered. The Perdew-Wang local density approximation (LDA), the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) and the newly form (WC) of the generalized gradient approximation (GGA) that was proposed by Wu and Cohen are used to treat the exchange-correlation terms. Moreover, the modified Becke-Johnson (mBJ) scheme is also applied for the band structure calculations. At zero pressure, our findings show that the zinc blende structure is the most stable phase adopted by these compounds. The transition pressure between the various phases is calculated. The band structure calculations reveal a metallic behaviour in NaCl, CsCl and NiAs structures whereas for the other structures, a semiconducting behaviour is observed. The quasi-harmonic Debye model has been used to explore the temperature and pressure effects on the thermal properties for both the compounds.

The Ternary Bi-Mn-Sb Phase Diagram and the Crystal Structure of the Ternary \u03a4 Phase Bi0.8MnSb0.2

The phase equilibria of the ternary system Bi-Mn-Sb were experimentally investigated with focus on the possible existence of a ferromagnetic continuous solid solution between α-BiMn and MnSb at 320 °C (both B81, NiAs structure type). Isothermal sections at 320 and 600 °C were studied applying powder x-ray diffraction and scanning electron microscopy including energy dispersive x-ray spectroscopy. The corresponding isothermal sections at 320 and 600 °C were established based on these results. A complete ternary reaction scheme (Scheil diagram) and a liquidus surface projection based on primary crystallization analyses and DTA (differential thermal analysis) were constructed. Furthermore, the vertical section at 10 at.% Sb was studied by DTA. A continuous solid solution between the isostructural phases α-BiMn and MnSb, as reported in the literature, could not be confirmed. A new hexagonal P63/mmc setting of the known orthorhombic P2221 ternary phase BixMnSb1−x, based on single-crystal x-ray diffraction data, is described.