Nb Compounds Research Articles

Ab initio study of Nb 2SC and Nb 2S 2C: Differences in coupling between the S and Nb–C layers

Using ab initio calculations, we have studied the structurally related compounds Nb 2SC and Nb 2S 2C. In Nb 2S 2C (space group R 3 ¯ m , prototype Bi 2Te 3), S atoms are nearest neighbours, while in Nb 2SC (space group P6 3/ mmc, prototype Cr 2AlC) this is not the case. The calculated equilibrium volume for these two phases deviates by 1.6–3.7% to previously-published experimental data and the bulk modulus-to- c 44 ratios obtained are 1.5 and 5.9, respectively. These results indicate a resemblance of Nb 2S 2C to hexagonal BN and graphite. Furthermore, we have demonstrated that the uniform compression method is adequate for estimating the elastic properties of Nb 2SC, a so-called MAX phase. It is our ambition that these calculations will stimulate further experimental research on these compounds.

Structure and magnetic properties of Er3(Fe, Co, M)29 compounds (M=Cr, V, Ti, Mn, Ga, Nb)

A series of Er３(Fe, Co, M)２９ (M=Cr, V, Ti, Mn, Ga, Nb) compounds has been synthesized and their structure and magnetic properties have been investigates by means of x-ray diffraction and magnetic measurements. It is found that the Fe-based Er３(Fe,M)２９ compounds crystallize in the Th2Ni17-type structure with disordered substitution of the dumbbell Fe-Fe, instead of Nd3(Fe,Ti)２９-type structure, so its chemical formula can also be expressed as Er2－n(Fe,M)17+2n (n=0.2). Substitution of M for Fe in the Er３Fe29 compound leads to an increase in the Curie temperature. With the substitution of Co for Fe in the Er３(Fe,M)２９ (M=Cr, V) compounds, a new phase was found which crystallizes in monoclinic symmetry. The anisotropy of the Er３Fe19.5Co6V3.5 compound at room temperature is of easy-axis type and a spin reorientation transition from the easy-axis type to the easy-plane type anisotropy occurs when temperature decreases to 162K. A first order magnetization process (FOMP) was observed in the HMD magnetization curve of Er３Fe19.5Co6V3.5 at 5K, the critical field of the FOMP was derived to be 3.8T.

Characterization and dielectric properties of Sr4La2Ti4M6O30(M=Nb, Ta) ceramics

Sr4La2Ti4M6O30 (M=Nb or Ta) compounds were synthesized by the conventional powder compaction and high temperature solid state reaction technique. The dielectric properties of the sintered samples were measured and they were further characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. Sr4La2Ti4M6O30 compounds are paraelectric phases adopting filled tetragonal tungsten–bronze (TB) structure at room temperature. Sr4La2Ti4Nb6O30 and Sr4La2Ti4Ta6O30 show high dielectric constants of 387 and 166 together with low dielectric loss of 0.0084 and 0.0042 at 1 MHz, respectively.

Two new lithium uranyl tungstates Li 2(UO 2)(WO 4) 2 and Li 2(UO 2) 4(WO 4) 4O with framework based on the uranophane sheet anion topology

Two new lithium uranyl tangstates Li 2(UO 2)(WO 4) 2 and Li 2(UO 2) 4(WO 4) 4O have been prepared by high-temperature solid state reactions of Li 2CO 3, U 3O 8 and WO 3. For each compound, the crystal structure was determined by single crystal X-ray diffraction data, using a Brucker diffractometer, equipped with a SMART CCD detector and Mo Kα radiation. The crystal structures were solved at room temperature by direct methods followed by Fourier difference techniques, and refined by a least square procedure on the basis of F 2 for all independent reflections, to R 1=0.035 for 65 refined parameters and 807 reflections with I⩾2 σ( I) for Li 2(UO 2)(WO 4) 2 and to R 1=0.051 for 153 refined parameters and 1766 reflections with I⩾2 σ( I) for Li 2(UO 2) 4(WO 4) 4O. The crystal structure of Li 2(UO 2)(WO 4) 2 is formed by perovskite sheets of WO 6 octahedra, one octahedron thickness, connected together by (UO 5) ∞ infinite chains, and creating tunnels parallel to the c-axis. The lithium atoms are localized in the tunnels. The structure can be deduced from that of U MO 5 ( M=Mo, V, Nb) compounds by the replacement of half U atoms by Li. The crystal structure of Li 2(UO 2) 4(WO 4) 4O consists of UO 7 pentagonal bipyramids, UO 6 tetragonal bipyramids and WO 6 distorted octahedra linked together to form a three-dimensional framework creating paralleled channels filled with lithium cations. The structure can also be described by the stacking of layers with the uranophane sheet anion topology similar to those obtained in U MO 5 ( M=Mo, V, Nb, Sb) compounds with an ordered population of pentagons by U and Li and of squares by U and W. The measured conductivities are comparable to those of the better Li + ion conductor solid electrolytes such as Lisicon or Li- β-alumina. Crystallographic data: Li 2(UO 2)(WO 4) 2, orthorhombic symmetry, space group Pbcn and unit cell parameters a=7.9372(15) Å, b=12.786(2) Å, c=7.4249(14) Å, ρ cal=6.87(2) g/cm 3, ρ mes=6.89(1) g/cm 3 and Z=4. Li 2(UO 2) 4(WO 4) 4O, monoclinic symmetry, space group C2/ c and unit cell parameters a=14.019(4) Å, b=6.3116(17) Å, c=22.296(6) Å, β=98.86(3)°, ρ cal=7.16(2) g/cm 3, ρ mes=7.25(3) g/cm 3 and Z=4.

Synthesis, properties and structure determination of [formula omitted] from neutron and synchrotron X-ray powder diffraction data

The blue diamagnetic compound Nb 2 O 3 ( SO 4) 2· 1 4 H 2 O was synthesized by reaction of niobium metal and boiling sulfuric acid. The unit cell was deduced from electron diffraction patterns and the crystal structure was solved from X-ray powder diffraction data and refined from combined X-ray synchrotron and neutron powder diffraction data. Nb 2 O 3 ( SO 4) 2· 1 4 H 2 O crystallizes in the space group R-3 (148) with the cell parameters a=10.384(2) Å and c=26.55(2) Å in the hexagonal setting. The structure features covalently bonded sulfate tetrahedra and niobate octahedra.

Martensite crystallography and elastic-stiffness coefficients

We considered the effect of the elastic-stiffness coefficients on the martensite crystallographic features, especially the habit plane. We approached the problem with an elastic-energy-minimization inclusion model. As shown previously, this model agrees exactly with the WLR-BM invariant-plane-strain theories when elastic stiffness is neglected and strain energy equals zero. As a first example, we considered the large-strain Fe-31Ni (fcc to bcc) transformation. Second, we considered a small-strain example: In-23T1 (fcc to fct). Finally, we considered the A15 compound Nb 3 Sn (bcc to bct), which shows soft-mode behavior during cooling, where the C'=(C 11 -C 12 )/2 elastic stiffness essentially vanishes, leading to a near-zero shear modulus. All three cases yielded the same result: the habit plane shows no response to elastic softening or stiffening.

Nb(V) compounds as epoxides carboxylation catalysts: the role of the solvent

The synthesis of organic cyclic carbonates from epoxides and CO2 is described. The role of the solvent is discussed, together with that of Nb(V) catalysts (Nb2O5 and NbCl5). The reaction is very selective and affords the conversion of epoxides into the relevant carbonates in high yield. © 2003 Elsevier Science B.V. All rights reserved.

Structure and composition of Laves phases in binary Cr–Nb, Cr–Zr and ternary Cr–(Nb, Zr) alloys

The Laves phase structure in three Cr-based two-phase alloys, Cr–12Nb, Cr–8Zr and Cr–2Nb–1Zr was characterized using transmission electron microscopy. Whereas in the as-cast condition, Cr 2Nb exhibits the twinned C15 structure, Cr 2Zr even after hot extrusion and annealing shows a heavily faulted C36 structure (or a higher order polytype such as a 6H structure). The ternary Cr 2(Zr,Nb) compound exhibits the C36 structure in the as-cast condition but reverts to the C15 structure after annealing, the structural transformation being accompanied by a composition change. Solid-state precipitates of the Laves phase Cr 2(Nb, Zr) in the ternary alloy that result in the matrix after annealing are always associated with dislocations. These dislocations either link to the interdendritic precipitates or link the solid-state precipitates to each other. Since the solubility of Zr is low in the matrix, we propose that pipe diffusion along these dislocations serves as the principal mechanism to redistribute the Zr from the interdendritic precipitates to the solid-state precipitates.

Hydrogen in Nb(V 1− yCr y) 2 Laves-phase compounds: neutron diffraction and nuclear magnetic resonance studies

In order to study the positions and mobility of hydrogen in ternary Laves-phase compounds Nb(V 1− y Cr y ) 2, we have performed neutron-diffraction measurements in NbVCr and NbVCrD 0.38 at T=295 K and nuclear magnetic resonance measurements of the 1H, 2D and 51V spin-lattice relaxation rates T 1 −1 in NbVCrH 0.3, NbVCrD 0.38 and NbV 1.4Cr 0.6H 0.6 over the temperature range 11–424 K. It is found that the host-metal lattice of both NbVCr and NbVCrD 0.38 has C15-type structure (space group Fd 3m ), and D atoms in NbVCrD 0.38 occupy tetrahedral g sites coordinated by two Nb atoms and two V(Cr) atoms. For all the samples studied, the temperature dependence of the motional contribution to T 1 −1 shows two peaks. This feature indicates a coexistence of two hydrogen jump processes with well-separated characteristic frequencies. The parameters of H(D) motion derived from the relaxation rate data for Nb(V 1− y Cr y ) 2–H(D) are compared with those for the related Laves-phase system TaV 2–H(D).

Magnetic properties and phase evolution of nanocomposite Nd 10Fe bal.MCB 10.5 (M=Ti, Nb, V, Cr and Mo) melt-spun ribbons

The magnetic properties and the phase evolution of boron enriched Nd 10Fe bal.MCB 10.5 (M=Ti, Nb, V, Cr and Mo) melt-spun ribbons have been investigated. For M=Ti, V, Cr and Mo. The main phases in the ribbons are Nd 2Fe 14B, Fe 3B and α-Fe, while the metastable Fe 3B phase is suppressed for the alloy ribbons with M=Nb. Among them, the alloy ribbons with M=Ti and Nb exhibit the best magnetic properties and the highest coercivity: B r=9.2 kG, i H c=12.1 kOe and ( BH) max=16.2 MGOe and B r=8.5 kG, i H c=15 kOe and ( BH) max=14.5 MGOe, respectively. The high remanence of the M=Ti compound mainly results from the exchange coupling effect between the magnetically soft Fe 3B phase and the hard Nd 2Fe 14B phase. The high coercivity of the M=Nb compound is due to the existence of a large volume fraction of the Nd 2Fe 14B phase in the matrix.

Effect of High Valence Metal Doping on Thermoelectric Properties of [Ca2CoO3-δ]0.62CoO2

ABSTRACTWe report the synthesis and thermoelectric properties of high valence metal-doped [Ca2Co1-xMxO3-δ]0.62CoO2 (M = Re, Mo or Nb) compounds. Approximately 10% M-doping for Co sites in block layer was found to be effective for dramatic suppression of oxygen nonstoichiometries. Seebeck coefficients were systematically enhanced with M-doping levels (x), and however, electrical resistivities also increased with x for conventionally sintered bulk samples. On the other hand, magnetic alignment and/or densification were remarkably effective for reduction of the resistivity. In the case of Mo-doped (x = 0.1) sample, a highly c-axis aligned and densified sample showed 1–3 times larger power factor at 1100K (5.3 μ WK−2cm−1) than the non-doped one. Moreover, these high valence metal doped Ca349 compounds exhibited the oxygen nonstoichiometry suppressed dramatically.

A new practical superconductor: rapidly heated and quenched Nb 3Ga wire

A15 compound Nb 3Ga is a superconductor with high T c above 20 K and high H c2 above 30 T at 4.2 K. However, high J c has not yet been reported for any Nb 3Ga specimens fabricated through various processes. Recently we were successful in fabricating a Nb 3Ga wire with high J c through an interesting combination of a Nb/NbGa 3 microcomposite precursor wire and a rapidly heating and quenching (RHQ) process. Disordered Nb 3Ga near stoichiometry was formed directly through the RHQ treatment. Post-annealing at 600–750 °C was very effective to improve the superconducting properties of Nb 3Ga wire with improvement in long range ordering. Obtained T c, H c2 at 4.2 K, and J c in high fields are much higher than those of the commercialized Nb 3Sn wire. We also succeeded in fabrication of a Nb 3Ga multifilamentary wire with high J c through the new process. Therefore the Nb 3Ga wire is very promising as a new practical superconductor.

Soft modes and superconductivity in the layered hexagonal carbidesV2CAs,Nb2CAs,andNb2CS

The electronic structure, selected phonon frequencies, and electron phonon coupling of the hexagonal layered compounds Nb 2 CS, Nb 2 CAs, and V 2 CAs are elucidated using density functional calculations. All materials are good three-dimensional metals with moderate values of the density of states, N(E F ). The electronic structure of Nb 2 CS, the only superconductor within this family of materials, consists of well-hybridized Nb d-S p-C p derived bands characteristic around the Fermi energy. The S is more ionic in this compound than the As in the related arsenides Nb 2 CAs or V 2 CAs. This results in a relative softness for phonon modes involving S. Rigid muffin tin approximation calculations show moderate electron phonon couplings consistent with low-temperature superconductivity. The key difference from the carbo-arsenides is the presence of soft moderately coupled S modes.

Displacive precursor phenomena in Ti–22Al–27Nb intermetallic compound prior to diffusional transformation

A systematic TEM investigation was carried out on the anomalous diffuse diffraction and contrast effects in B2 phase of a Ti–22Al–27Nb compound, the decomposition of which at high temperature into O or/and DO 19 phase has so far been believed to be a purely diffusional process. The extinction rule of the diffuse scattering in B2 indicates that all these effects arise from transverse <110><1 1 ̄ 0> displacive waves. Furthermore, evidence for a martensitic transformation in thin film state of the B2 phase was also found. All these facts strongly suggest that the B2 phase of Ti–Al–Nb possesses displacive instability. As a consequence, the decomposition of the B2 phase into O/DO 19 is a diffusion-coupled displacive transformation. The diffusional–displacive nature of the transformation in this system not only naturally explains the observed orientation relationships among B2, O and DO 19 phases, but also provides new insight into the mechanical behavior of B2+O/DO 19 duplex Ti–Al–Nb compounds, such as the large primary creep and superplasticity.

Solubility and Raman spectra of Nb(V) in LiF–NaF–KF–Na 2O melts

The solubility of Nb(V) as a function of O 2− composition in the ternary eutectic LiF–NaF–KF (FLiNaK) melt at 700°C has been determined. Raman spectra at different oxide concentrations and temperatures were recorded. Based on chemical analysis of melt samples and bands in the spectra suggestions related to the Nb–O–F complex formation are given. The addition of Na 2O to FLiNaK containing 0.22 mole kg −1 K 2NbF 7, show that at: n O/ n Nb<2; there is no precipitation of Nb(V) compounds. The Raman spectra of the melt indicated the presence of monomeric NbF 7 2− at low oxide concentrations, which reacted with oxide to form monomeric NbOF 5 2− when r was increasing. Further additions of Na 2O formed the NbO 2F 4 3− species having a maximum concentration at n O/ n Nb=2; when 2< n O/ n Nb<3, a solid of the type AlkNbO 3 was formed. At n O/ n Nb=3, a minimum in the Nb(V) and O 2− solubilities were observed, but the structure of the species in the melt were uncertain. At higher oxide content, 3< n O/ n Nb<4, the AlkNbO 3(s) previously formed was dissolved, possibly with the formation of both corner and edge sharing distorted NbO 6 octahedra. The solubility of Nb 2O 5 in FLiNaK was measured as a function of temperature (550–800°C), and the dissolution mechanism seems to be given by the reaction: Alk ++Nb 2O 5(s)+4F −=NbO 2F 4 3−+AlkNbO 3(s). Δ H 0 sol for the reaction (based on concentrations in mole kg −1) was found to be ≈90 kJ mol −1 K −1.

Crystal Structures from a Building Set: The First Boridecarbides of Niobium

They exist! Boridecarbides of transition metals. A whole series of Nb compounds was prepared from syntheses in Cu/Al melts with Nb3B3C, Nb4B3C2, Nb7B4C4, and Nb7B6C3. Their crystal structures can be explained easily with a simple building-set principle based on the binary components NbC, Nb2B3, and Nb3B4.

Magnetic and structural properties of Nd 2Fe 17− xMx ( M=Cu, Cr, V, Nb, and Zr) compounds

We report the structural and magnetic properties of Nd 2Fe 17− x M x ( M=Cu, Cr, V, Nb and Zr), where x=0, 0.5, 1, 1.5, 2, 2.5, 3, and 4. X-ray diffraction shows that these compounds form the rhombohedral Th 2Zn 17 structure with a small amount of α-Fe for some of the samples with small x values. For some of these compounds the rhombohedral structure cannot be formed at large values of x. X-ray diffraction on magnetically aligned samples show that these samples have in-plane anisotropy. The unit cell volume is found to increase with x for M=V, Nb, and Zr and decrease with x for M=Cu and Cr. The saturation magnetization decreases with x for all M atoms both at 25 and 300 K with the decrease ranging from 5 to 15% for x=1. The Curie temperature increases with x for small values of x with the increase ranging from 10 to 20% for x=1.

Electric Field Gradient at Nb Site in the Intermetallic Compounds Nb3X (X = AI, In, Si, Ge, Sn) Measured by PAC

The electric field gradient (efg) at the Nb site in the intermetallic compounds Nb3X (X = Al, In, Si, Ge, Sn) was measured by the Perturbed Angular Correlation (PAC) method using the well-known gamma-gamma cascade of 133 -482 keV in 181Ta from the β- decay of 181Hf, substituting approximately 0.1 atom percent of Nb. The PAC results show that Vzz drops by nearly 40% when X changes from Al to In, and by about 25% when X changes from Si to Ge and Sn. This behavior is most probably related to the change in the degree of sp hybridization in these compounds. The Vzz values of the studied compounds do not follow the well known universal correlation for the efg's in non-cubic metals but the observed trend is well reproduced by results of ab-initio electronic structure calculations. In the case of Nb3 Al a linear temperature dependence of the quadrupole frequencies was observed in the temperature range of 6.5 to 1210 K.

Preparation of the mixed sulfide Nb2Mo3S10 catalyst from the mixed oxide precursor

Mixed sulfide Nb2Mo3S10 was obtained by reaction of H2S with the mixed oxide Nb2Mo3O14 at 873 K. High‐resolution electron microscopy and X‐ray diffraction show the formation of homogeneous lamellar mixed sulfide. Due to the increased lability of oxygen, the sulfidation of the mixed oxide is easier compared to that of Nb2O15 oxide. As follows from the characterization of transient solid products, the transformation from the oxide to the sulfide includes the intermediate formation of a partially reduced “bronze” compound. For the thiophene hydrodesulfurization model reaction, the catalytic properties of Nb2Mo3S10 mixed sulfide are intermediate between those of the individual MS2 sulfides (M = Mo, Nb), but closer to those of the Nb compound.

Computational condensed matter physics

In the high pressure laboratory at BARC, we are pursuing a program to study the behaviour of materials under static and dynamic pressures. Theoretical component has been an integral part for guiding and interpreting new experiments. The initial phase of such efforts was devoted to the development of equation of state models at arbitrary temperatures and matter densities. With the advent of diamond anvil cell device and the simultaneous provision for laser heating of the compressed samples for static high pressure studies, and with the improvements of the diagnostic techniques in dynamic shock methods, the focus of our studies switched over to the predictions and interpretations of phase transitions. Often these efforts have led to intense experimental studies and sometimes helped in resolving the controversies in data. We adopted the first principles electronic structure calculations for high pressure studies. Our work on the electronic topological transition in zinc led to many experimental and theoretical investigations. The results of electronic structure changes in similar metal cadmium shall be compared with existing understanding in Zn under pressure. Our studies on Nb and other compounds like intermetallics and borocarbides have revealed interesting electronic structure changes under pressure. However, the electronic structure based investigations of structural stabilities at high pressures involve tedious trial and error effort, which is avoided in theab initio molecular dynamics simulations. The current status of our efforts in the use of this technique is illustrated with the example of quasicrystal based clusters.