Nb Se Research Articles

Nb–Se–C Thin Films Deposited by a 1064 nm Laser as Model Electrodes for Li+ and Na+ Storage

Nb–Se–C Thin Films Deposited by a 1064 nm Laser as Model Electrodes for Li⁺ and Na⁺ Storage

Effect of substitution on the superconducting phase of transition metal dichalcogenide Nb(Se_{x}S_{1-x})_{2} van der Waals layered structure

By means of first-principles cluster expansion, anisotropic superconductivity in the transition metal dichalcogenide Nb(Se_{x}S_{1-x})_{2} forming a van der Waals (vdW) layered structure is observed theoretically. We show that the Nb(Se_{0.5}S_{0.5})_{2} vdW-layered structure exhibits minimum ground-state energy. The Pnnm structure is more thermodynamically stable when compared to the 2H–NbSe_{2} and 2H–NbS_{2} structures. The characteristics of its phonon dispersions confirm its dynamical stability. According to electronic properties, i.e., electronic band structure, density of states, and Fermi surface indicate metallicity of Nb(Se_{0.5}S_{0.5})_{2}. The corresponding superconductivity is then investigated through the Eliashberg spectral function, which gives rise to a superconducting transition temperature of 14.5 K. This proposes a remarkable improvement of superconductivity in this transition metal dichalcogenide.

X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb1+xSe2 (0 < x < 0.1)

Single crystals of 2H and 3R niobium diselenide were grown by a chemical transport reaction. The current determinations by single crystals X-ray diffraction reveal a non-stoichiometric composition. The structures are built from Se—Nb—Se slabs with Nb in trigonal prismatic coordination whereas the extra or additional Nb atoms are located in the octahedral holes between the slabs giving rise to the formula 2H and 3R-Nb1+xSe2 with 0.07 < x < 0.118. In particular, vacancy and Nb-Nb interactions may play an important role on the non-stoichiometry and the stacking mode in NbSe2. By increasing the number of additional Nb atoms in the pure 2H-NbSe2, a transition 2H to 3R polytype should occur in order to minimize the Nblayer—Nbextra—Nblayer repulsions between these adjacent slabs. The theoretical study shows that both 2H and 3R-Nb1+xSe2 are thermodynamically stable in the range 0 < x < 0.1.

Crystal structure, properties and griffiths-like phase in niobium diselenide intercalated with chromium

Polycrystalline CrxNbSe2 samples intercalated with Cr atoms up to x = 0.5 have been synthesized and studied by means of X-ray diffraction, magnetization, 53Cr NMR spectra, electrical resistivity and specific heat measurements. Within the concentration range 0 < x < 0.25, the intercalated compounds are observed to exhibit the same crystal structure as the parent compound 2H–NbSe2 and a cluster-glass-like magnetic behavior; while above x = 0.25, the Cr atoms are ordered, and starting from x = 0.33 the CrxNbSe2 compounds show a soft ferromagnetic behavior with maximal Curie temperature TC ∼82 K. According to the 53Cr NMR data, the valence state of chromium atoms is close to Cr4+. A decrease in an average magnetic moment per Cr atom in saturation observed in CrxNbSe2 with increasing x above x = 0.33 is ascribed to the possible appearance of a magnetic moment on Nb atoms due to the redistribution of the electron density between the Nb–Se and Cr–Se bonds. The deviation of the reciprocal susceptibility from the Curie-Weiss law above TC implies the presence of a Griffiths-like phase in the nominally paramagnetic state, which affects the behavior of the electrical resistivity and specific heat.

High Coercivity and Magnetization in WSe2 by Codoping Co and Nb.

Introducing ferromagnetism in transition metal dichalcogenides has attracted lots of attention due to the possible applications in spintronics devices. Generally, single magnetic element doping is used to introduce magnetism. However, mostly, weak ferromagnetism is observed. In this work, codoping of two kinds of transition metals (Nb and Co) into WSe2 is used to study its magnetic properties. In detail, single crystal WSe2 is codoped with 4 at% Co and various concentrations of Nb by employing the physical ion implantation method. Raman, X-ray diffraction and X-ray photoelectron spectroscopy results reveal the effective substitutional doping of implanted elements (Co and Nb). Magnetic measurements illustrate that both un-doped and 4 at% Co doped WSe2 show weak ferromagnetism whereas magnetization is strongly enhanced when Co and Nb are codoped into WSe2 . The magnetization is comparable with a ferromagnet, which may be attributed to Co, Nb doping and defects. In addition, a large coercivity of ≈1.2 kOe is observed in the 1 at% Nb-4 at% Co codoped WSe2 sample, which may be ascribed to the combined effect of doping-induced stress, defect-dictated pinning and anisotropy of NbSe bond owing to the charge transfer between Nb and Se ions.

Signature of coexistence of superconductivity and ferromagnetism in two-dimensional NbSe2 triggered by surface molecular adsorption.

Ferromagnetism is usually deemed incompatible with superconductivity. Consequently, the coexistence of superconductivity and ferromagnetism is usually observed only in elegantly designed multi-ingredient structures in which the two competing electronic states originate from separate structural components. Here we report the use of surface molecular adsorption to induce ferromagnetism in two-dimensional superconducting NbSe2, representing the freestanding case of the coexistence of superconductivity and ferromagnetism in one two-dimensional nanomaterial. Surface-structural modulation of the ultrathin superconducting NbSe2 by polar reductive hydrazine molecules triggers a slight elongation of the covalent Nb–Se bond, which weakens the covalent interaction and enhances the ionicity of the tetravalent Nb with unpaired electrons, yielding ferromagnetic ordering. The induced ferromagnetic momentum couples with conduction electrons generating unique correlated effects of intrinsic negative magnetoresistance and the Kondo effect. We anticipate that the surface molecular adsorption will be a powerful tool to regulate spin ordering in the two-dimensional paradigm.

Enhanced upper critical fields in a new quasi-one-dimensional superconductor Nb2PdxSe5

We report the discovery of superconductivity with Tc = 5.5 K in Nb2PdxSe5, in which one-dimensional (1D) Nb–Se chains existing along the b-direction hybridize each other to form the conducting b–c* plane. The magnetic susceptibility and specific heat data in both single crystal and polycrystal show evidence of bulk superconductivity. The resistivity, Hall coefficient and magneto-resistance data all indicate the presence of an energy scale T* = ∼ 50 K, which becomes systematically lowered under hydrostatic pressure and competes with the stabilization of superconductivity. Combined with the band calculation results showing the Fermi surfaces with 1D character, we postulate that the energy scale T* is related to the formation of a density wave or a stabilization of low-dimensional electronic structure. The zero-temperature upper critical field, Hc2(0), of the single crystal is found to be 10.5, 35 and 22 T in the a′, b and c* directions, respectively. While the linearly increasing Hc2(T) for H//c* indicates the multi-band effect, Hc2(0) for H//b and c* are found to be much bigger than the Bardeen–Cooper–Schrieffer (BCS) Pauli limiting field, 1.84 Tc ∼ 9 T. The suppressed Pauli paramagnetic effect points to the possibility of enhanced spin–orbit scattering related to the low-dimensional electronic structure or the presence of heavy elements such as Pd.

Electronic properties of superconducting NbSe2 nanotubes

AbstractAtomic models of zigzag (n,0)‐ and armchair (n,n)‐like nanotubes (n = 7–58) of the superconducting NbSe2 are constructed and their electronic properties and bond indices are investigated using the tight‐binding band theory. The calculations performed show that all the considered NTs are uniformly metallic‐like. The density of states at the Fermi level contains a considerable contribution from Nb4d states, which will be significant for the superconductivity. The stability and electronic properties of the NbSe2 NTs depend on the atomic configuration and diameter of the tubes. Nb—Se covalent bonds were found as basic in NbSe2 tubes. According to our estimations, zigzag‐like configurations of the NbSe2 nanotubes are more stable.

A TEM and XRD study of the SbNbSe3 misfit layer structures

Two new misfit layer structures have been synthesized within the Sb–Nb–Se system. Powder X-ray diffraction and electron microscopy techniques (electron diffraction, HREM, XEDS) have been used to determine the nature of their structure. According to TEM and XEDS data (for more than 15 crystals studied) both phases are monolayer type, i.e. (SbSe)1+δ (NbSe2). Electron microscopy reveals a composite modulated structure that consists of the periodical intergrowth of a pseudotetragonal SbSe layer, denominated as Q, and a pseudohexagonal layer NbSe2, denominated as H. Both layers fit along b, stack along c and do not fit along a (misfit) giving rise to an incommensurate modulation along this direction. The two phases differ in the symmetry of the Q layers being in one case orthorhombic (for δ=0.17) and monoclinic in the other (for δ=0.19). After the characterization of the sample by electron microscopy the unit cells of the basic layers could be refined for both phases by powder X-ray diffraction: aQ=5.824(2)Å,bQ=5.962(5)Å,cQ=23.927(6)Å,α=90°, β=90° and γ=90° and aH=3.415(5)Å,bH=5.962(6)Å,, cH=11.962(1)Å,α=90°, β=90° and γ=90° for the orthorhombic phase; aQ=5.844(2)Å,bQ=5.981(1)Å,cQ=23.919(5)Å,α=90°, β=90° and γ=96.00(3)° and aH=3.439(1)Å,bH =5.994(2)Å,cH=11.956(3)Å,α=90°, β=90° and γ=90° for the monoclinic phase. The phase with the monoclinic Q-sublattice often appears as twinned crystals. The more abundant crystals are disordered intergrowths of both monolayer phases.

The synthesis and structural characterization of the [Nb(Se) 3S tBu] 2−, [Nb(O)(Se 2)SeH] 2−, [(Nb(O)(Se 2) 2) 2Se 4] 4− and {[Nb(Se) 3] 2O} 4− anions. New elementary selenoniobates

The reaction of NbCl 5, NaS t Bu, Se and Et 4NCl in CH 3CN in a 1:6:1:1.6 molar ratio affords crystalline (Et 4N) 2[Nb(Se) 3(S t Bu)], 1. The same salt is obtained in about 25% yield by the oxidation of [Nb(S)(S t Bu) 4] − with 1 equiv. of Se. Oxidation of 1, or its Ph 4P + analog with 1.5 equiv. of Se in the presence of water, results in the formation of the [Nb(O)(Se 2) 2SeH)] 2− anion, 2, that can be isolated as Ph 4P + or Et 4N + salts. The SeH vibration in the Et 4N + salt of 2 is found at 2526 cm −1 and the proton in the anion is easily exchanged with D 3COD. The (Et 4N) 4[(Nb(O)(Se 2) 2) 2Se 4] dimer, 3 was obtained by the oxidation of (Et 4N) 2[Nb(O)(Se 2) 2SeH)] with 2 equiv. of elemental selenium. The reaction of (Et 4N) 2[Nb(Se) 3(S t Bu)] with water in CH 3CN, led to isolation of the oxo-bridged dimer, (Et 4N) 4[(Se) 3NbONb(Se) 3], 4. The crystal structures of 1–4 have been determined. Single crystal X-ray diffraction data for 1–4 were collected on a Nicolet P3/F diffractometer using MoKα radiation and the structures solved using a combination of heavy atom Patterson techniques, direct methods and Fourier techniques. The number of parameters refined and the final R factors obtained for each structure at the conclusion of refinement are: (1) 280, 0.0463; (2), 528, 0.0941; (3), 318, 0.0886 and (4), 205, 0.0625 respectively. The anion in 1 exhibits tetrahedral geometry while the structure of the anion in 2 shows the Nb 5+ ion in a pentagonal pyramidal geometry with two η 2-Se 2 2− and the η 1-SeH − ligands in the equatorial plane and the oxo ligand in the apical position. The Nb atom is located 0.70 Å above the plane defined by the five equatorial Se atoms. In 2, the NbO, NbSeH and NbSe 2 bonds are found at 1.671(11), 2.608(4) and 2.60(1) Å, respectively. The structure of the anion in 3 shows 2 [Nb(O)(Se 2) 2] units, bridged by a tetraselenido ligand to give a dimer with pentagonal pyramidal [Nb(O)(Se 2) 2Se] subunits and the oxo ligands in an anti conformation. In 4 the μ-oxo ligand of the anion is located on a crystallographic centre of symmetry and the NbO bond, in the linear NbONb unit is 1.916(1) Å. The two Nb(Se) 3 units are oriented in a staggered fashion to give an idealised D 3d symmetry.

Raman spectra of niobium chalcogenide superconductors: NbSe 2 and SnNb 5Se 9

Micro- and FT-Raman spectra of NbSe 2 and SnNb 5Se 9 have been carried out between 5.5, 80 and 300 K. The spectrum of NbSe 2 exhibits several features not found previously. A mode at 276 cm −1 is absent below the CDW transition temperature. The spectrum of SnNb 5Se 9 presents significant differences allowing the estimation of the force constants K s, γ, and β, corresponding to the interlayer SeNbSe coupling, the Se..Se and the Nb..Se bonds. We find that K s is unchanged in the new compound (0.027 mdyn.Å −1) whereas the ratio β/γ is considerably reduced from 0.43 to 0.29. Thus the Nb..Se and Se..Se bonds are respectively longer and shorter in SnNb 5Se 9 than in NbSe 2. This suggests that the structure of SnNb 5Se 9 is more anisotropic than that of NbSe 2.

Synthesis, structure and properties of barium niobium triselenide

The compound BaNb 0.8Se 3 was prepared from BaSe, Nb, and elemental selenium in a sealed silica tube at 800°C. The structure was refined from X-ray powder diffraction data, using a Rietveld profile analysis, which yielded the following residuals: R = 9.46 %, R wp = 9.25 %, R bragg = 9.73 %. The compound has the BaVS 3 structure type (space group P6 3 mmc ) with a = 7.1216(5) and c = 5.9796(5) A ̊ . Pertinent bond distances (Å) are: NbNb = 2.990(1); BaSe = 3.562(6), 3.576(6); NbSe = 2.618(9). Magnetic susceptibility and resistivity studies showed the compound to be diamagnetic ( χ0 = −7.8 × 10 −5 emu/mol) and semiconducting with a 0.1 eV activation energy.

Synthesis and characterization of the new quaternary two-dimensional materials KCu 2Nb Q4( Q = Se, S)

Reaction of K 2Se 5 or K 2S 5 with elemental Nb, Cu, and Se or S affords KCu 2NbSe 4 or KCu 2NbS 4. The structure of KCu 2NbSe 4 has been determined by single crystal X-ray diffraction methods. KCu 2NbSe 4 crystallizes with four formula units in the space group C 16 2 v - C2 cm of the orthorhombic system in a cell of dimensions a = 5.663(2) Å, b = 18.791(5) Å, c = 7.645(3) Å at −167°C. The structure consists of two-dimensional metal-chalcogen layers separated by K + cations. The Nb atom and two independent Cu atoms are in tetrahedral sites, and there appears to be some substitutional disorder among these sites, as judged from the isotropic thermal parameters. The CuSe 4 and NbSe 4 tetrahedra share edges and corners to make up the two-dimensional layers. If we arbitrarily choose a cutoff of 4.00 Å then the K atom is coordinated by nine Se atoms. The metal tetrahedra have their expected dimensions with NbSe distances varying from 2.392(3) to 2.452(3) Å and CuSe distances from 2.427(2) to 2.440(4) Å. There are short metal-metal interactions with NbCu distances at 2.831(1) and 2.833(4) Å. From Weissenberg photography KCu 2NbS 4 is found to be isostructural: cell dimensions at room temperature are a = 5.76 Å, b = 18.46 Å, c = 7.45 Å.

Synthesis, structure, and electrical properties of Eu 0.59Nb 5Se 8

The new compound Eu 0.59Nb 5Se 8 has been prepared from a combination of the elemental powders at 1275 K. The material crystallizes with four formula units in space group C 3 2h- C2 m of the monoclinic system in a cell (at 113 K) of dimensions a = 17.95(2), b = 3.388(4), c = 19.30(2)Å, β = 107.90(2)°. The structure has been solved and refined from single-crystal X-ray data. The asymmetric unit contains two independent Eu atoms, five independent Nb atoms, and eight independent Se atoms. Each Eu site is approximately 59% occupied; the Eu atoms are in an unusual distorted cube of Se atoms with EuSe interactions ranging from 3.108(6) to 3.454(6) Å. Three of the independent Nb atoms are in trigonal prisms of Se atoms with NbSe interactions ranging from 2.548(6) to 2.617(6) Å while the other two independent Nb atoms are in unusual distorted octahedral environments with NbSe interactions ranging from 2.526(9) to 2.718(7) Å. These latter two Nb atoms show short NbNb interactions of about 2.95 Å. The material exhibits excellent metallic conductivity along the needle ( b) direction with σ 298 K of 35 × 10 3 ohm −1 cm −1 and σ 12 K ≈ 1.4 × 10 6ohm −1 cm −1.

NMR study of the phasing of charge density waves in 2HNbSe 2

The 93Nb NMR spectra are analyzed to obtain phase parameters of the charge-density-wave (CDW) state in 2HNbSe 2. It is shown that the experimental data rule out the hexagonal symmetry of the CDW state. The shapes of both the Knight shift and the electric-field-gradient distributions are consistent with the locally commensurate orthorhombic CDW state within each SeNbSe layer. The phase parameters of this state are found to be close to those obtained for the commensurate 2HTaSe 2 by Pfeiffer et al.

Studies of niobium selenido-halides: crystal structure of [Nb2Cl4(Se2)2(SMe2)4

The reactions of [NbCl3Se] with the sulphur-donor ligands dimethyl sulphide (dms), tetrahydrothiophen (tht), and 1,2-bis(phenylthio)ethane(bpte), and the reaction of [NbBr3Se] with tht under moisture- and oxygen-free conditions are described. The reactions gave insoluble products with analyses close to those expected for 1:2 (dms, tht) or 1:1 (bpte) adducts. Recrystallisation of the product from the reaction of [NbCl3Se] with dms, nominally ‘NbCl3Se·2dms’, yielded the niobium(IV) compound [(dms)2Cl2Nb(Se2)2NbCl2(dms)2] whose structure has been determined. Crystals are monoclinic, space group P21/n with a= 8.366(10), b= 13.121 (14), c= 11.204(14)Å, β= 99.53(10)°, and Z= 2. 1 855 Reflections above background were collected on a diffractometer; the structure was solved by direct methods and has been refined to R= 0.068. The structure contains centrosymmetric dimers in which each niobium atom is bonded to two chlorine atoms [Nb–Cl 2.527(5), 2.510(5)Å], four selenium atoms in two bridging Se22– groups [Nb–Se 2.638(2), 2.630(2), 2.640(2), 2.625(3)Å], and two sulphur atoms from the dms group [Nb–S 2.739(4), 2.739(4)Å].

Anisotropic superconducting properties Nb Se 3 under pressure

We have measured the resistive transition under magnetic fields for several pressures for which Nb Se 3 behaves as a bulk superconductor. The anisotropy of the critical magnetic fields is well described by an effective mass model of Josephson coupled chains. The ratios of the effective masses that we deduced in two different configurations is in good agreement with values obtained from Shubnikov de Haas oscillations and anisotropy of conductivity measurements.

Le seleniure NbSe 3: Obtention et structure

A phase with the composition NbSe 3 has been found in the Nb—Se system, having the monoclinic unit cell dimensions a = 10.006, b = 3.478, c = 15.626 A ̊ , β = 109.30°, and with the space group Pm. Three-dimensional X-ray data have been obtained by means of a NONIUS CAD 3 diffractometer using Mo radiation. The results of least-squares refinements of the NbSe 3 crystal structure are given.

Optical Properties of Single Crystals of NbSe2 and Nb1.04Se2

Niobium-selenium compounds in the composition range NbSe2–Nb1.14Se2 can form layered crystals which are readily cleaved in a direction perpendicular to the c axis. For compositions in the range NbSe2–Nb1.05Se2, two structures, a high-temperature ζ phase and a lower-temperature ε phase may be formed. Single crystals of ε-NbSe2, ε-Nb1.04Se2, ζ-NbSe2, and ζ-Nb1.04Se2 were grown by the vapor transport technique and the optical absorption spectra of thin-cleaved specimens in the visible and near infrared regions were studied. An infrared absorption spectrum to λ = 15 μ was also obtained for ε-NbSe2. Accurate values of the absorption coefficient α were measured at 10 nm intervals in the spectral region 400–620 nm. For all crystal types, α was found to have peak value ∼4.5×105 cm−1 at λ = 460 nm, decreasing steadily to ∼1.3× 105 cm−1 at λ = 620 nm. The values of α for ε-NbSe2, along with bulk reflectivities R0 measured on basal plane cleavage surfaces of opaque sections of ε-NbSe2, have been used to calculate the optical constants n and k. The most notable feature of the visible and near infrared studies is the similarity of the gross features of the absorption spectra of all four crystal types. This indicates that the absorption is governed principally by a mechanism operating largely within the Se–Nb–Se ``layer molecules'' and relatively independent of structure or the presence of interstitial excess metal. An absorption minimum centered about λ = 1.2 μ has been attributed to plasma oscillation of the conduction electrons, and calculations based on this assumption have yielded a value m*≤5.9 m for the effective mass of these electrons.