Ta Impurities Research Articles

Electric-field gradients at the 181Ta impurity site in Yb, Y, and Dy sesquioxides.

The perturbed-angular-correlation (PAC) technique has been applied to the study of the internal electric-field gradients (EFG) in Yb, Y, and Dy sesquioxides. The $^{181}\mathrm{Ta}$ PAC probes were introduced in the oxide lattices by means of $^{181}\mathrm{Hf}$ implantation, which decays to the Ta impurity by \ensuremath{\beta} emission. The tracer $^{181}\mathrm{Ta}$ was used and the results were compared to those obtained in bixbyite structure compounds using $^{111}\mathrm{Cd}$ as a probe. The measured EFG are essentially independent from the probe, reflecting the structure characteristics of both crystalline sites in these compounds. In the case of $^{181}\mathrm{Hf}^{4+}$ and $^{181}\mathrm{Ta}^{5+}$, charge compensation (the cations are 3+) seems to be accomplished through a uniform distribution of the probe's extra electrons in the neighboring oxygen anions. Comparisons of the results to point-charge model predictions are also discussed.

Strain field due to substitutional transition-metal impurities in bcc metals: Application to dilute vanadium alloys.

The formalism for the generalized Kanzaki lattice static method is developed for a metallic crystal with bcc symmetry. The interactions up to second-nearest neighbors are included in the derivation of the dynamical matrix, impurity-induced force, and the atomic displacements. The formalism is applied to calculate the strain field due to 3d, 4d, and 5d substitutional transition-metal impurities (Ti, Cr, Mn, Fe, Nb, Mo, Ta, and W) in the vanadium metal, the only bcc systems in which the study of electric field gradients has been performed. The effective ion-ion interaction potential for the transition metals, proposed by Wills and Harrison, which includes properly the d-band effects, is used in the numerical calculations. In all the systems the atomic displacements are calculated up to 21 nearest neighbors and these show oscillatory behavior. The maximum displacement (or strain field) is caused by a Ta impurity and is about 3.5% of the first-nearest-neighbor distance. The strain field is found to depend, both in strength and range, on the excess ion-ion interaction potential due to the impurity. The atomic displacements exhibit the same trend, as shown by x-ray studies of the fractional change in the lattice parameter.

Hyperfine interactions at Ta impurities in cobalt and cobalt-hafnium intermetallic compounds.

The magnetic and electric hyperfine interaction of $^{181}\mathrm{Ta}$ impurities on substitutional sites of cobalt metal has been investigated by \ensuremath{\gamma}\ensuremath{\gamma} perturbed angular correlations (PAC) in samples prepared by melting and ion implantation of $^{181}\mathrm{Hf}$. In molten samples the saturation value of the magnetic hyperfine field of Ta is ${\mathit{H}}_{\mathit{H}\mathit{F}}$(Ta:Co(hcp); 9 K)=-40.7(6) T for hexagonal Co and \ensuremath{\Vert}${\mathit{H}}_{\mathit{H}\mathit{F}}$${(}^{181}$Ta:Co(fcc); 9 K)\ensuremath{\Vert}=36.3(5) T for cubic Co. In ion-implanted samples radiation damage causes a slight reduction of the hyperfine field. The magnetic hyperfine field of $^{181}\mathrm{Ta}$ has also been determined in Fe: \ensuremath{\Vert}${\mathit{H}}_{\mathit{H}\mathit{F}}$${(}^{181}$Ta:Fe; 9 K)\ensuremath{\Vert}=61.7(9) T for a polycrystalline sample prepared by $^{181}\mathrm{Hf}$ implantation. The temperature dependence of ${\mathit{H}}_{\mathit{H}\mathit{F}}$ in Co has been measured between 9 and 1410 K. ${\mathit{H}}_{\mathit{H}\mathit{F}}$(T) in the hexagonal phase (T700 K) suggests a Curie temperature for hcp Co of ${\mathit{T}}_{\mathit{C}}$(hcp)=975(50) K. In the cubic phase, ${\mathit{H}}_{\mathit{H}\mathit{F}}$(T) of $^{181}\mathrm{Ta}$ is close to the magnetization curve of the host with ${\mathit{T}}_{\mathit{C}}$(fcc)=1398(5) K, in contrast to the anomalous temperature dependence of ${\mathit{H}}_{\mathit{H}\mathit{F}}$ of most other impurities in Co investigated up to now. The electric-field gradient of $^{181}\mathrm{Ta}$ on substitutional sites of hexagonal Co is \ensuremath{\Vert}${\mathit{V}}_{\mathit{z}\mathit{z}}$${(}^{181}$Ta:Co; 9 K)\ensuremath{\Vert}=1.23(4)\ifmmode\times\else\texttimes\fi{}${10}^{17}$ V/${\mathrm{cm}}^{2}$.The PAC spectra reflect the transformation of the solid solution of 0.2 at. % of Hf in elemental Co into the Co-Hf intermetallic compounds ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ and ${\mathrm{Hf}}_{6}$${\mathrm{Co}}_{23}$ in the temperature range 900\ensuremath{\le}T\ensuremath{\le}1320 K. Upon cooling from 1400 K, elemental Co starts to transform into ${\mathrm{Hf}}_{6}$${\mathrm{Co}}_{23}$ at about 1320 K and into ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ at about 1220 K. At 1100 K the evolution of the phase equilibrium between the solid solution and ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ was observed to occur with a half-life of ${\mathit{T}}_{1/2}$=5.5(4) h. The temperature dependence of the hyperfine-interaction parameters suggests magnetic-ordering temperatures of 450 K and 750 K for ${\mathrm{Hf}}_{2}$${\mathrm{Co}}_{7}$ and ${\mathrm{Hf}}_{6}$${\mathrm{Co}}_{23}$, respectively.

Reply to "Comment on 'Size effects and charge-density-wave pinning in Nb1-xTixSe3: Evidence for weak pinning by a nonisoelectronic impurity' "

We review our investigations of charge-density-wave pinning in ${\mathrm{NbSe}}_{3}$. We show that (1) the measured ${\mathit{E}}_{\mathit{T}}$-${\mathit{r}}_{\mathit{R}}$ relations for both Ta and Ti impurities are consistent with weak pinning; (2) the pinning strength of Ti impurities is roughly 40 times larger than that of Ta impurities; and (3) the size dependence of charge-density-wave (CDW) properties and direct measurements of CDW correlation lengths are consistent with a size-dependent crossover in the weak-pinning dimensionality.

Determination of Ta impurities in Nb samples by X-rays in neutron activation analysis

Ta impurities in Nb samples have been determined by X-rays in 14 MeV neutron activation analysis. For X-ray detection, (HP) Ge detector with Be-window is used. The sensitivity of the determination has been found to be about 30 μg (in Nb).

Lattice position of Hf and Ta in LiNbO3: An extended x-ray-absorption fine-structure study.

The lattice position of Hf and Ta impurities in ${\mathrm{LiNbO}}_{3}$ single crystals has been investigated using fluorescence extended x-ray-absorption fine-structure (EXAFS) spectroscopy. It has been concluded that Hf is in the lithium octahedron and Ta ions are in the niobium one. Both impurities lie very close to the center of the oxygen octahedron.

IMPURITY ADSORPTION INDUCED SURFACE CHARGE-DENSITY OSCILLATION AND INDIRECT ATOMIC INTERACTIONS

Neon field ion image spots of Ta impurities, deposited on the Ir (100) surface, show a halo-ring structure. This image structure is most probably produced by the impurity adsorption induced oscillatory electronic charge-density distribution at the surface. The oscillatory electronic charge-density modulation is the cause of electronic indirect interactions of atoms in alloys and on metal surfaces. Manifestations of these interactions can be found in the compositional variation in the near surface layers of alloys in surface segregation, and in the pair-interaction of adsorbed atoms.

Dihydrogen chemisorption on a contaminated W substrate

An investigation of the indirect interaction between two hydrogen atoms adsorbed on a contaminated transition metal W(001) surface is presented in terms of the tight-binding Anderson-Newns model. The impurity effects on the charge transfer and the indirect interaction are discussed. It is found that when two adatoms are adsorbed on the contaminated surface, the impurity effects on the next-nearest-neighbour atom cannot be neglected. In addition, the introduction of an impurity atom does not affect the structure of the adsorbed layer, but can weaken the indirect interaction between the H adatoms. Thus, when tungsten is taken as catalyst, the introduction of a Re or Ta impurity atom will impel the dissociation process of H 2.

Magnetization and electron-spin-resonance studies of doped NbSe3.

We have performed dc-magnetization and electron-spin-resonance (ESR) measurements on undoped, Ta-doped, and V-doped ${\mathrm{NbSe}}_{3}$, at temperatures ranging from 1.7 to 200 K. The magnetization measurements show that although Ta impurities are nonmagnetic, V impurities are weakly magnetic. In the undoped and Ta-doped samples, the resonance involves unpaired conduction electrons; the ESR spin susceptibility, g factor, and linewidth exhibit maxima near the lower charge-density-wave (CDW) transition. We suggest that this is the result of changes in the non-CDW band structure with induced gap formation. The resonance in the V-doped sample is quite different, mixing the properties of conduction-electron and local-moment resonances.

Quantum mechanism of hydrogen diffusion in tantalum

Abstract The temperature dependence of the diffusion coefficient of H impurities in Ta is calculated with high accuracy. Phonon-assisted tunneling between the more stable tetrahedral sites, and also between octahedral sites, is considered. Comparison of the theoretical curves with high (90 K

Atomic Displacements Around Nitrogen Interstitial Impurities in Ta and Nb

ABSTRACTThe core as well as the long range elastic displacements around N octahedral interstitial atoms in Ta and Nb are modelled using the Green Function – Kanzaki force method. The theoretical calculations are compared with experimental attenuation factor and diffuse scattering data, reported in the literature, for both N in Nb and N in Ta [1,2,3]. It is shown that a third neighbor radial Kanzaki force model is needed to explain the experimental findings, and the long range elastic displacement field is non-spherical.

Tungsten-series impurity diffusion in single-crystal tungsten

The temperature dependences of the volume-diffusion coefficients of Ta, Re, Os, Ir, and W have been measured for tungsten single crystals by the sectioning method. The analysis of our data and results obtained by other authors shows that the Arrhenius dependence of self-diffusion coefficients possesses no curvature. The physically lucid and simple Lazarus-Le Claire model for the vacancy mechanism semiquantitatively describes our experimental $Q(z)$ dependences for impurities (standing on the right-hand side of W) in the tungsten series. For Ta the activation energy $Q$ is less than or equal to the $Q$ for self-diffusion. This behavior is very similar to what is observed for the diffusion of Co in Ni. This indicates that either little or no positive excess charge is localized on the Ta impurity in W.

Impurity effect on the Fröhlich conductivity in NbSe3

The effect of Ta and Ti impurities on the anomalous transport properties (non-Ohmic dc conductivity and excess microwave absorption) of Nb${\mathrm{Se}}_{3}$ has been studied. Data from 22 samples show that the characteristic field ${E}_{0}$ and the threshold field ${E}_{T}$ are very sensitive to doping. Both ${E}_{0}$ and ${E}_{T}$ obey a ${c}^{2}$ behavior where $c$ is the Ta concentration. The anomalous microwave response is also suppressed by the presence of impurities. As in the dc case Ti is more effective than Ta in suppressing the anomalous behavior. These impurity studies lend strong support to the Fr\"ohlich model in which the dc non-Ohmicity is ascribed to a sliding mode of the charge-density-wave condensate and the excess microwave absorption is ascribed to resonant oscillations of the condensate about pinning centers. The presence of impurities suppresses both the sliding and oscillatory modes. We also briefly describe the effect of the dynamic response of the massive condensate to very short pulses and its effect on non-Ohmic measurements.

The magnetic hyperfine field at181Ta impurities in the 4f-ferromagnets Ho and Er

The magnetic hyperfine fieldHhf at181Ta impurities in the ferromagnetic Rare Earth metals Ho and Er has been determined by time differential perturbed angular correlation measurements at 4.2 K. The results |Hhf(TaHo)|=101(8)kG |Hhf(TaEr)|= 94(8)kG together with the previously determined values ofHhf(TaGd) andHhf(TaDy) show that the magnetic hyperfine field at Ta impurities in the Rare Earth metals is predominantly due to the conduction electron polarization of the hosts.

Temperature dependence of the electric field gradient at Ta impurities in the heavy-rare-earth metals Gd, Dy, Ho, and Er

The time-differential perturbed-angular-correlation (TDPAC) technique has been used to study the temperature dependences of the electric field gradient (EFG) at /sup 181/Ta impurities in the heavy-rare-earth (RE) metals Gd, Dy, Ho, and Er. At room temperature the ratio ..cap alpha.. equivalent vertical-bar V/sub z/z/ (1-..gamma../sub infinity/) V/sup 1at//sub z/z vertical-bar of the measured EFG V/sub z/z and the calculated ionic EFG (1-..gamma../sub infinity/) V/sup 1at//sub z/z decreases linearly with increasing rare-earth atomic number. A linear but much stronger decrease of ..cap alpha.. has previously been reported for the impurity /sup 111/Cd. A simple model is proposed which explains the linear decrease of ..cap alpha.. and the different slopes for /sup 181/Ta and /sup 111/Cd in terms of the lanthanide contraction. This model assumes the conduction-electron contribution to the EFG to be mainly determined by the number of electrons in the Wigner-Seitz cell of the impurity. In all rare-earth hosts the EFG decreases with increasing temperature. This decrease, which is slightly stronger for Gd than for Er, is better described by a linear function of temperature than by a T/sup 3/2/ behavior, observed in many other impurity-host systems. The temperature dependence of the EFG is much stronger than expected from themore » lattice expansion. The difference between the temperature dependence of the measured EFG and of the calculated lattice EFG decreases across the rare-earth series. This can be attributed to a decrease of the amplitudes of the lattice vibrations between Gd and Er.« less

Strength, symmetry and distribution of electric quadrupole interactions at181Ta impurities in hafnium-zirconium alloys

The nuclear quadrupole interaction at substitutional Ta atoms in the alloy system HfxZr1-x has been investigated over the entire range of composition by time differential perturbed angular correlations (TDPAC). Source configurations investigated include oriented and randomised single crystals as well as textured, cold-worked and random polycrystals, all at room temperature. For both pure Zr and pure Hf, a pure electric quadrupole interaction is found of magnitude nu Q=eQVzz/h=303.9(15) MHz and 311.01(11) MHz respectively, which exhibits no electric field gradient (EFG) asymmetry and negligible damping due to finite distribution of the EFG strength. As increasing amounts of impurity are added at either end of the phase diagram, a two-component TDPAC pattern becomes evident: in addition to the undamped pure-metal spin rotation, a second component, exhibiting both damping and asymmetry, is seen at a somewhat higher frequency than that of either pure metal. The fraction of this second component, its frequency, the degree of asymmetry, and the distribution width of the EFG all increase monotonically with increasing impurity concentration.

Anelastic piezoresistance associated with oxygen impurities in Ta

Abstract Measurements of piezoresistance associated with oxygen impurities in Ta are reported. By using an electrical bridge and a phase sensitive detector. a substantial increase in sensitivity is achieved over the pioneering work by Berry and Orehotsky (1964). It is estimated that an oxygen concentration in Ta of 30 atomic parts per million could be detected.

Temperature dependence of the electric quadrupole interaction at scandium impurities in titanium

Perturbed angular correlation measurements of the electric field gradient at 44Sc impurities in Ti metal show a weak dependence on temperature similar to that previously observed in Ta impurities but much weaker than that at Cd impurities in Ti.

Study of the electric quadrupole interaction at Ta impurities in rare earth metal hosts

The time differential perturbed angular correlation technique has been used to measure the electric fieldgradient (EFG) at the site of181Ta impurities in the heavy Rare Earth metals Gd, Tb, Dy, Ho and Er at room temperature. It is found that the ratio α ≡ ¦V zz eff /V zz lat ¦ between the measured EFGV zz eff and the lattice EFGV zz lat , which is known from lattice sum calculations, is in the order of α≃300, suggesting that an important contribution to the EFG is due to electrons localized at the impurity. The ratio α is not constant throughout the Rare Earth series. It decreases from Gd to Tb and increases between Tb and Er. This behaviour is compared to the results of a previous investigation with the impurity Cd in the same hosts.

Temperature Dependence of the Impurity Electric Quadrupole Interaction: Cadmium and Tantalum in Titanium

AbstractThe electric quadrupole interaction at Cd and Ta impurity nuclei implanted in trace amounts in a titanium host lattice has been measured as a function of temperature by means of the time‐differential perturbed angular correlation technique. The results show a striking difference in the temperature dependences for the two impurity species. This implies that, in addition to the previously observed temperature dependence arising from lattice expansion and conduction electron redistribution of the host, there exist significant local temperature dependent effects associated with the specific characteristics of the impurity.