Increasing Ga Content Research Articles

Fe–Fe bond length and exchange interaction in Y 2Fe 17− xGa x ( x=0–8) compounds

The Ga-occupancy factors and the Fe–Fe bond lengths resulting from the Rietveld refinement of X-ray diffraction data show a critical point at a Ga content between x=5 and 6. The Fe–Fe exchange-interaction constant J FeFe derived from the Curie temperature increases linearly with increasing Ga concentration when x⩽4, remains almost unchanged for x=4–6, then increases sharply when x>6. The variation of the Fe–Fe exchange-interaction constant may be qualitatively explained by the variation of the Ga preferential occupancy and the Fe–Fe bond lengths.

Local coordination of Ga impurity in hydrogenated amorphous germanium studied by extended x-ray absorption fine-structure spectroscopy

The local structure of Ga-doped a-Ge:H films has been investigated by extended x-ray absorption fine-structure (EXAFS) fluorescence for impurity concentrations ranging from 1.5×1018 atoms cm−3 to 4.5×1020 atoms cm−3. The mean-coordination number of Ga atoms changes from around 4 (1.5×1018–1.5×1019 cm−3) to below 3 (1.5×1020–4.5×1020 cm−3) with rising concentration. The change from fourfold to threefold coordination occurs in a rather narrow impurity concentration range. The variance of the distance distribution function decreases with increasing Ga content, suggesting that well-ordered sites are present at high-impurity concentration. From EXAFS phase analysis the first Ga–Ge shell distance has been found to be 0.03 Å larger in the amorphous network than in Ga-doped crystalline Ge.

Synthesis of Garnet/Perovskite-Based Ceramic for the Immobilization of Pu-Residue Wastes

AbstractThe use of garnet/perovskite-based ceramic, with formula type (Y, Gd,..)3(AI, Ga,..)5O1212/(Y, Gd,..)(A1, Ga,..)O3, was tested for immobilizing plutonium residue wastes. Pu residue wastes originate from nuclear weapons production and can contain more than 50% of impurities including such elements as Am, Al, Mg, Ga, Fe, K, La, Na, Mo, Nd, Si, Ta, Ce, Ba, B, W, Zn, Zr, C and Cl. While for some of these residues, direct conversion to typical glass or ceramic forms may be difficult, ceramic forms based on durable actinide host-phases are preferred for Pu, Am and other actinides immobilization. Garnet/perovskite crystalline host-phases are chemically and mechanically durable and desirable for the incorporation of Pu and most of the impurity elements in the Pu residue wastes in the lattices of host-phases in the form of solid solutions. Experiments on the synthesis of garnet/perovskite ceramic samples were carried out using melting in air at temperatures from 1300°C (for samples doped with 10 wt.% Pu residue waste simulant) to 2000°C (for samples doped with 10 wt.% Ce or U). Samples were studied by XRD, SEM and cathodoluminescence techniques. It was found that the garnet phase can incorporate upto 6 wt.% Ce and up to 4.0-5.5 wt.% U, which is correlated with the increase of Ga content and decrease of Al content in the melt. In one of the features of the melt, the perovskite phase formation substitutes for the formation of garnet. The capacity of the perovskite lattice to accommodate Ce and U is higher than the capacity of garnet, reaching about 8 and 7 wt.%, respectively. It was shown that cathodoluminescence can be effectively used to determine the valence state of Ce and U, an important step to optimize the starting precursor preparation. Incase of U4+in the melt, the charge-compensating elements (Sn2+, Ca2+...) are needed to successfully incorporate U in the garnet lattice.

Thermal vaporization and deposition of gallium oxide in hydrogen

The thermodynamics of gallium oxide vaporization and deposition in Ar–6% H 2 at elevated temperatures are described. It is shown that Ga 2O 3 vaporizes in H 2 as Ga 2O(g) at elevated temperatures. During thermal processing the Ga 2O(g) moves to cooler zones of the furnace, back reacts with H 2(g) and H 2O(g) and condenses out as Ga(l) and Ga 2O 3(s). Upon removal from the furnace, the exposed Ga forms a ubiquitous surface oxide of Ga 2O 3. X-ray photoelectron spectroscopy (XPS) was used to examine heat treated Ga 2O 3 powders and vaporization products deposited onto SiO 2 and Cu substrates. In agreement with the thermodynamic predictions, these data demonstrate that the deposition product contained Ga 2O 3 and metallic Ga. Analysis of the XPS spectra also revealed an intermediate oxidation state for Ga. The precise bonding of this state could not be demonstrated conclusively, but it is suggested that it may be solid Ga 2O. For coherent product deposition on Cu the metallic Ga concentration increases and the Ga 2O 3 concentration decreases with sputtering depth, suggesting the metallic Ga in the outermost layers of the deposit is readily oxidized during air exposure.

Development in the short- and medium-range structure in amorphous alloys

Detailed atomic structure analysis has been made for a series of the amorphous alloys by utilizing a combination of both neutron and x-ray diffraction techniques. The structure factors and associated with the atomic environment around Mg and Ga atoms can be deduced from the two sets of the total S(Q) obtained from neutron and x-ray diffractions. A prepeak is found to grow at Å in but not in , when Ga concentration exceeds 15 at.%. Its appearance can be interpreted as the formation of the medium-range structure characteristic of the (110) lattice planes composed of all Ga atoms in the crystalline compound. The coordination number and atomic distance of the Mg-Mg, Mg-Ga and Mg-Ca atomic pairs can be deduced by decomposing the first main peak of the RDF(r) obtained by Fourier transforming for all amorphous alloys studied. We showed that the -type short-range structure develops and the Mg-Ga atomic distance is shortened to the level of 2.9 Å for samples with . This is attributed to the formation of the bonding state between Mg and Ga atoms. It is also found that the -type short-range structure, where each Ca atom is fully surrounded by Mg atoms, persists up to x = 10. However, the Ga-Ca atomic pair gradually grows with increasing Ga concentration above x = 15.

Electronic structure and electron transport properties of amorphous alloys

The resistivity at 300 K of the amorphous alloy is only but it increases to when x is increased to 30 in the amorphous alloys. We have studied in this experiment a change in the electronic states immediately below the Fermi level as a function of Ga concentration for amorphous alloys by means of photoemission spectroscopy (XPS and UPS), soft x-ray spectroscopy (SXS) and low temperature specific heat measurements. We reveal that the narrow Mg-Ga bonding states are formed below the Fermi level as a result of the hybridization of Mg-3p and Ga-4p states and that this narrow band gives rise to the pseudogap across the Fermi level. Combining the atomic structure data deduced in the preceding paper, we are led to conclude that the -like short-range order developed with increasing Ga concentration above x = 20 is responsible for causing the narrow p-type bonding states just below the Fermi level and that the simultaneous occurrence of a reduction in the density of states at the Fermi level and an increase in the effective mass results in a sharp increase in resistivity in this system.

Structure and magnetic properties of Nd 2Co 17− xGa x compounds studied by magnetic measurements and neutron diffraction

The structural properties of Nd 2Co 17− x Ga x compounds ( x=5, 6 and 7) were studied by means of neutron diffraction. The refinement results of the neutron powder diffraction patterns show that the Ga atoms completely avoid the 9d site in the rhombohedral Th 2Zn 17 structure. Furthermore, the Ga atoms increasingly occupy the 6c and 18f sites at the cost of the 18h site that shows a decreasing Ga occupancy in this concentration range. At 4.2 K, the easy magnetisation direction is perpendicular to the c-axis for x=5 and 6, but parallel to the c-axis for x=7. From magnetic measurements and neutron diffraction it is derived that the Co and Nd moments at 4.2 K strongly decrease with increasing Ga concentration. There is also a strong decrease of the Curie temperature with Ga increasing concentration.

Magnetic and crystallographic properties of Ce 2Co 17− xGa x compounds

The structural and magnetic properties of Ce 2Co 17− x Ga x compounds for Ga concentrations up to x=5 were studied by means of X-ray diffraction and magnetic measurements. The easy magnetisation direction is parallel to the c axis in all compounds. The Co moments and the Curie temperatures decrease strongly with increasing Ga concentration. From magnetic measurements made on magnetically aligned powders we have determined the concentration dependence of the anisotropy constant. The values of K 1 increase up to x=3 and then strongly decrease with increasing x. This behaviour is discussed in terms of preferential Ga substitution into the available crystallographic sites, confirming earlier conclusion that Co atoms at the 18h sites contribute negatively to the Co sublattice anisotropy in R 2Co 17− x Ga x compounds.

Optical band gap and optical constants in a-Ga xTe 100− x thin films

The optical band gap and optical constants for a-Ga x Te 100− x thin films have been measured as a function of photon energy. The optical band gap is found to increase with the increase in Ga content in the a-Ga x Te 100− x system. The spectral dependence of optical constants shows that the refractive index ( n) decreases while the extinction coefficient ( k) increases with photon energy.

Luminescence of rare‐earth‐activated Ga‐containing oxides by low‐energy electron excitation

Abstract— It was previously reported that the thermoluminescence intensity increases with an increase in Ga content in Y3(Al1−xGax)5O12:Tb3+ and LaAl1−xGaxO3:Tm3+. This fact suggests that electrical conductivity may be also increased with Ga content, because traps responsible for thermoluminescence can supply free electrons. In this work a correlation is found between the Ga content and electrical conductivity of the above phosphors, which was measured by means of the threshold anode voltage of vacuum‐fluorescent displays (VFDs) using these phosphors, where the anode current or luminance builds up. In a series of Y3(Al1−xGax)5O12:Tb3+, the maximum efficiency is obtained at the composition x = 0.4; it is about 50% higher than the efficiency of ZnGa2O4:Mn2+. Incorporation of In also decreases the threshold voltage, but it decreases the efficiency as well. The traps supplying carriers can be ascribed to oxygen vacancies and/or Ga+.

Measurement of activity of gallium in liquid Ga‐Sb‐Ge alloys by EMF method with zirconia as solid electrolyte

AbstractThe emf of galvanic cells with zirconia solid electrolytes was measured from 975 to 1242 K on binary Ga‐Ge alloys and from 1058 to 1236 K on ternary Ga‐Sb‐Ge alloys by use of Ga‐Ga2O3 as a reference electrode. From the emf values the activity of gallium in the liquid alloys is determined. At 1073 K the activity of gallium in the Ga‐Ge alloys and in the three pseudobinary alloys Ga‐(SbyGe1‐y) where y=0.25, 0.50 and 0.75 shows small negative deviations from ideality. Isoactivity curves in the ternary Ga‐Sb‐Ge alloys were derived by combining the activity data of Ga‐Sb, Ga‐Ge and the ternary alloys. The shape of the curves at 1073 K is simple in the Ga‐poor region, but shows a very small wavy form with increasing Ga concentration. Isovalues curves in the excess Gibbs energies of mixing in the ternary alloys are derived from Darken's method, and the results are compared with the model calculation proposed by Chou based on the experimental data in three binary systems. The shape of isovalue curves in ΔGxs is similar to those obtained from Chou's method, but the agreement between them in the values is not good.

X-ray diffraction, magnetization and nuclear magnetic resonance study of

A combined x-ray diffraction, magnetization and nuclear magnetic resonance ( and ) study of the system is presented. It is found that the substitution of larger Ga atoms for Fe causes a transformation from the hexagonal to rhombohedral structure and an overall average lattice expansion of per formula unit per Ga atom. Measurements on magnetically aligned powders yielded values for the anisotropy field and the average moment per Fe atom. Consistent with a change from planar to uniaxial anisotropy, the anisotropy field decreases with increasing Ga content and extrapolates to zero for . The average moment per Fe atom as well as the hyperfine field also decrease with Ga content. Using a calculated value of for the Y moment, values for the hyperfine coupling constants of and were obtained for the on-site and transferred contributions, respectively. The experimental results are discussed in terms of a model for the substitution of Ga for Fe in this system, and its relationship to the enhanced Curie temperature.

Magnetic properties of ErFeGa compounds with the 2:17-type structure

An investigation of the structure and the magnetic anisotropy of Er2Fe17−xGax compounds has been performed by means of X-ray diffraction and magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase, having the hexagonal Th2Ni17-type structure for x ≤ 3 and the rhombohedral Th2Zn17-type structure for x ≥ 4. Substitution of Ga for Fe in Er2Fe17 leads to an increase of the unit-cell volume which is linear with the Ga content. The saturation magnetization at 1.5 K decreases linearly with increasing Ga concentration. The Curie temperature is found first to increase, then to decrease with increasing Ga concentration, having a maximum value of 526 K at about x = 3. The Er2Fe17−xGax compounds with x = 2, 3 and 4 exhibit a spin-reorientation transition with the spin-reorientation temperature being 136, 140 and 102 K, respectively. X-ray diffraction measurements on magnetically aligned Er2Fe17−xGax powders show that the compounds with x ≤ 7 have easy-plane type of magnetic anisotropy, whereas the compound with x = 8 exhibits easy-axis type of magnetic anisotropy at room temperature. The easy-magnetization direction of the sample of x = 8 is perpendicular to the c-axis at low temperature. Present result demonstrates that in Er2Fe17−xGax the substitution of high concentration of Ga leads to a change in the second-order crystal field parameter A20 from negative to positive.

Effects of Ga substitution on structure and magnetocrystalline anisotropy of Tm2Fe17

A detailed investigation of the structure and magnetic properties of Tm2Fe17−xGax (x=0, 1, 2, 3, 4, 5, 6, 7, and 8) compounds has been performed by means of x-ray-diffraction, neutron-diffraction, magnetization, and ac-susceptibility measurements. Crystal-structure studies have shown that the prepared samples are single phase with the hexagonal Th2Ni17 for x⩽3 and the rhombohedral Th2Zn17 structure for x⩾5. In Tm2Fe13Ga4 the Th2Zn17 structure coexists with the Th2Ni17 structure. Substitution of Ga for Fe in Tm2Fe17 leads to an increase of the unit-cell volume, which is linear with the Ga concentration. In Tm2Fe17−xGax, the saturation magnetization at 1.5 K decreases linearly with increasing Ga content with a rate of 2.3 μB per substituted Ga atom. The Curie temperature is found first to increase with increasing Ga content, going through a maximum value of 485 K at about x=3, then to decrease. Between x=6 and 7, a minimum value of TC is reached and for higher x values TC increases again. X-ray-diffraction measurements on magnetically aligned Tm2Fe17−xGax powders show that the compounds with x⩽6 have an easy-plane type of magnetic anisotropy, whereas the compounds with x⩾7 exhibit easy c-axis anisotropy at room temperature. All Tm2Fe17−xGax compounds exhibit a spin-reorientation transition at low temperature, except for the sample with x=6, which shows an easy-magnetization direction perpendicular to the c axis in the temperature range from 5 to 300 K. For x⩽5, the spin-reorientation temperature is found first to increase with x and then to decrease, having a maximum value of 211 K at about x=3. In the samples with x⩾7, an easy-plane anisotropy was observed at low temperature, but an easy-axis preference of the magnetization at room temperature was observed. The results obtained for Tm2Fe17−xGax indicate that the mutually competing Tm- and Fe-sublattice anisotropies both change their sign with increasing Ga concentration.

Structure, exchange interactions and magnetic anisotropy of ( x = 0-8) compounds

The structural and magnetic characteristics of -based ternaries, in which Co is partly substituted by Ga atoms, have been investigated by means of x-ray powder diffraction (XRPD) and magnetic measurements. XRPD patterns show that all samples are single phase with the rhombohedral -type structure except the sample with x = 8 which contains a small amount of hcp Co and rare-earth-rich phase as impurity phases. The Ga substitution for Co in these compounds leads to a monotonic increase in unit cell volume and a monotonic decrease in Curie temperature. The saturation magnetization is found to approximately linearly decrease with increasing Ga concentration. For compounds with x = 4, 5 and 6, compensation points are observed in the corresponding temperature dependence of the magnetization. The values of compensation temperature are found to shift to higher temperature with increasing Ga content. The intersublattice-molecular-field coefficients, , have been determined on the basis of magnetization curves measured at the compensation temperature. The exchange-coupling constants have been calculated by analysing the field dependence of the magnetization and by using the method based on magnetic-ordering temperatures. It has been found that the Ga substitution for Co in these compounds has little influence on Er-Co exchange interaction. XRPD patterns on magnetically aligned samples with show that the easy magnetization direction at room temperature corresponds to the c-axis.

Influence of Ga on the Fe anisotropy in

First- and second-order anisotropy constants, and , have been determined on aligned samples of Fe (x = 0 to 7.5) compounds by measuring magnetization and torque curves at temperatures between 4.2 and 300 K. Low-field ac susceptibility has been studied from 80 K to room temperatures. It has been found that a partial substitution of Ga for Fe in initially decreases the absolute value of anisotropy constants thus indicating a weakening of the easy plane character of the Fe anisotropy. The Fe anisotropy becomes uniaxial for a Ga concentration slightly above 6.5 atoms . A strengthening of the uniaxial anisotropy with further increase of Ga concentration is indicated by an increase in , determined by torque measurements at room temperatures. The results show the important changes which Fe anisotropy undergoes when Fe atoms are partially substituted by Ga in compounds. They can be used to separate the anisotropy contributions from the Fe sublattice and the R sublattice in compounds with magnetic R and thus determine the influence of Ga on the magnetic anisotropy in these materials.

Phase-modulation-induced two-wave mixing in a temporal-nonlocal medium

First-and second-order anisotropy constants, K-1 and K-2, have been determined on aligned samples of Y2Fe17-xGax (x = 0 to 7.5) compounds by measuring magnetization and torque curves at temperatures between 4.2 and 300 K. Low-held ac susceptibility has been studied from 80 K to room temperatures. It has been found that a partial substitution of Ga for Fe in Y2Fe17 initially decreases the absolute value of anisotropy constants thus indicating a weakening of the easy plane character of the Fe anisotropy. The Fe anisotropy becomes uniaxial for a Ga concentration slightly above 6.5 atoms fu(-1). A strengthening of the uniaxial anisotropy with further increase of Ga concentration is indicated by an increase in K-1, determined by torque measurements at room temperatures. The results show the important changes which Fe anisotropy undergoes when Fe atoms are partially substituted by Ga in R2Fe17 compounds. They can be used to separate the anisotropy contributions from the Fe sublattice and the R sublattice in R2Fe17-xGax compounds with magnetic R and thus determine the influence of Ga on the magnetic anisotropy in these materials.

Raman study of phonons and crystal-field excitations in NdBa 2Cu 3- xGa xO 7− δ

We have performed Raman measurements on a series of NdBa 2Cu 3− x Ga x O 7− δ ceramics (0< x<0.08) in order to monitor the influence of Ga doping on the properties of these materials. While the O(4) apex-oxygen phonon exhibits a considerable softening of (14±2) cm −1 with increasing Ga content between x=0.00 and 0.08, the O(2)+O(3) in-phase phonon frequency is much less influenced. At low temperature, the O(2)–O(3) out-of-phase vibration is split into two peaks by interaction with a Nd 3+ crystal-field (CF) excitation of similar energy. Using a two-level model, we derive from the spectra the energies of the uncoupled phonon and the CF excitation, as well as the coupling constant of the CF-excitation–phonon interaction. The coupling constant and the phonon frequency are almost independent of Ga doping, while the energy of the CF excitation is (5.6±1.6) cm −1 lower in NdBa 2Cu 2.92Ga 0.08O 7− δ than in NdBa 2Cu 3O 7− δ . This decrease is attributed to changes in the positions and charges of the oxygen ions in the CuO 2 planes. Using structural data obtained by neutron diffraction, we perform a point-charge-model calculation to separate structural and charge-transfer contributions to the observed change in the CF level energy. We find that the negative oxygen charge in the CuO 2 planes is increased by about 1% in NdBa 2Cu 2.92Ga 0.08O 7− δ as compared to the Ga-free sample.

Structural and magnetic properties of HoErFe 17− xGa x compounds

The structural and magnetic properties of HoErFe 17− x Ga x ( x=0–7) were studied by means of X-ray diffraction and magnetization measurements. Powder X-ray diffraction studies show that the compounds transform from the hexagonal Th 2Ni 17 structure ( x⩽4) to rhombohedral Th 2Zn 17 structure ( x⩾5) with increasing Ga concentration. The substitution of Ga for Fe leads to an increase in lattice constants a, c and unit-cell volume v. The Curie temperature is found to first increase and then decrease with increasing Ga concentration, attaining a maximum value of 535 K at about x=4, which is 207 K higher than that of HoErFe 17. Both iron sublattice anisotropy and rare-earth sublattice anisotropy are found to be modified by the introduction of the Ga atoms and have been confirmed through X-ray diffractograms of magnetically aligned samples.

Measurement of activity of gallium in liquid Ga–Sb–Sn alloys by emf method with zirconia as solid electrolyte

The emf of galvanic cells with zirconia solid electrolytes was measured to determine the activity of gallium in liquid Ga–Sb–Sn alloys in the 1050–1150 K temperature range along three lines of constant antimony-to-tin mole ratio. A mixture of Ga and Ga 2O 3 was used as a reference electrode. The activity curves of Ga show negative to positive deviations from the ideal behavior with increasing Sn concentrations in the lower concentration range of Ga; whereas in the concentration range X Ga≥0.6, the activity of Ga in the ternary alloys shows positive deviations from the ideal behavior. Isoactivity curves at 1073 K, in the ternary Ga–Sb–Sn alloys, were derived by combining the activity data for Ga–Sb and Ga–Sn alloys. The shape of the curves is simple in the Ga-poor region, but shows a wavy form with increasing Ga concentration. Isovalue curves in the excess free energies of mixing in the ternary alloys are derived by the Darken method. The results are in agreement with values derived by using the Chou method on the Sb-poor side, but the agreement is not good in the Sb-rich region.