Increasing Ga Content Research Articles

Effect of cation disorder on the magnetic properties ofSr2Fe1−xGaxReO6(0<x<0.7)double perovskites

The effect of diamagnetic dilution of the Fe sublattice on the structural and magnetic properties of the double perovskite ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{Re}{\mathrm{O}}_{6}$ $(0<x<0.7)$ has been explored by means of x-ray structural analysis, magnetometry, M\"ossbauer spectroscopy, and band structure calculations. The end member of the solid solution series, ${\mathrm{Sr}}_{2}\mathrm{Ga}\mathrm{Re}{\mathrm{O}}_{6}$, was predicted to be a half-metallic ferromagnet based on ab initio band structure calculations using the WIEN2k software package. Because the Re-containing double perovskites, like the parent compound ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Re}{\mathrm{O}}_{6}$, rarely exhibit antisite disorder, the appearance of increasing amounts of antisite disorder in ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{Re}{\mathrm{O}}_{6}$ $(0<x<0.7)$ is unexpected. Although the amount of antisite disorder increases with increasing Ga content, it also depends on the sample preparation history. For $0<x<0.4$ only a tetragonal, magnetically ordered phase was detected by x-ray structural analysis as well as M\"ossbauer analysis. A phase separation into a tetragonal and a cubic phase for $x\ensuremath{\geqslant}0.4$ is detected by x-ray structural analysis accompanied by the observation of a magnetically ordered and a paramagnetic phase in the corresponding M\"ossbauer spectra. Below 20% Ga content, Ga statistically dilutes the -Fe-O-Re-O-Fe- double-exchange pathways. Phase separation begins at 20% Ga substitution; between 20% and 40% of Ga content, the paramagnetic Ga-based phase does not contain any Fe. The Fe-containing, paramagnetic cubic phases which can be detected by M\"ossbauer spectroscopy first appear for $x=0.4$.

Composition-induced structural modifications in the quaternary CuIn1−xGaxSe2thin films: bond properties versus Ga content

In this paper the dependence of structural properties of the quaternary CuIn1−xGaxSe2 films with tetragonal structure on the Ga content has been systematically investigated by Raman scattering and x-ray diffraction spectra. The shift of the dominant A1 mode, unlike the lattice constants, does not follow the linear Vegard law with increasing Ga content x, whereas exhibits approximately polynomial change from 174 cm−1 for CuInSe2 to 185 cm−1 for CuGaSe2. Such behaviour should be indicative of presence of the asymmetric distribution of Ga and In on a microscopic scale in the films, due to Ga addition. The changes in the tetragonal distortion η lead to a significant variation in the anion displacement parameter U, which should be responsible for the evolution of bond parameters and resultant Raman bands with x.

Structural, electrical, and optical properties of transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering

Transparent conductive Ga-doped zinc oxide (ZnO:Ga) films with highly (0 0 2)-preferred orientation were deposited on glass substrates by DC reactive magnetron sputtering. Variation of structural, electrical, and optical properties with Ga content was investigated. The peak position of the (0 0 2) plane is linearly shifted to the lower 2 θ value with the increase of Ga content. The lowest resistivity of the ZnO:Ga films is 3.51×10 −4 Ω cm and the average transmittance of the films is over 90% in the visible range. The optical band gap of the films is in the range of 3.58–3.74 eV. The optimum growth condition was obtained by using the Zn–Ga alloy target of 3.0 at% Ga content.

Superconductivity in gallium-substitutedBa8Si46clathrates

We report a joint experimental and theoretical investigation of superconductivity in Ga-substituted type-I silicon clathrates. We prepared samples of the general formula ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46\ensuremath{-}x}{\mathrm{Ga}}_{x}$, with different values of $x$. We show that ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{40}{\mathrm{Ga}}_{6}$ is a bulk superconductor, with an onset at ${T}_{C}\ensuremath{\approx}3.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For $x=10$ and higher, no superconductivity was observed down to $T=1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This represents a strong suppression of superconductivity with increasing Ga content, compared to ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ with ${T}_{C}\ensuremath{\approx}8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Suppression of superconductivity can be attributed primarily to a decrease in the density of states at the Fermi level, caused by a reduced integrity of the $s{p}^{3}$-hybridized networks as well as the lowering of carrier concentration. These results are corroborated by first-principles calculations, which show that Ga substitution results in a large decrease of the electronic density of states at the Fermi level, which explains the decreased superconducting critical temperature within the BCS framework. To further characterize the superconducting state, we carried out magnetic measurements showing ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{40}{\mathrm{Ga}}_{6}$ to be a type-II superconductor. The critical magnetic fields were measured to be ${H}_{C1}\ensuremath{\approx}35\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ and ${H}_{C2}\ensuremath{\approx}8.5\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. We deduce the London penetration depth $\ensuremath{\lambda}\ensuremath{\approx}3700\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and the coherence length ${\ensuremath{\xi}}_{c}\ensuremath{\approx}200\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Our estimate of the electron-phonon coupling reveals that ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{40}{\mathrm{Ga}}_{6}$ is a moderate phonon-mediated BCS superconductor.

Structural ordering and magnetic properties of arc-melted FeGa alloys

X-ray diffraction, Mössbauer spectrometry and magnetic properties have been performed on FeGa arc-melted binary alloys in order to study the compositional dependence of the structural ordering and magnetic properties of these alloys. The average magnetic hyperfine field at 300 K decreases with increasing Ga content as does the average magnetic moment per Fe atom, and a linear dependence between both quantities is found for Ga content up to 20 at%. The substitution of Fe by Ga atoms increases the lattice parameter and causes a change in the order of the crystal structure that has been modelled using a binomial distribution method. The effect of Ga atoms as near neighbours of Fe on the average magnetic hyperfine field is evaluated.

Electrical properties of homogeneous Cu(In,Ga)S 2 films with varied gallium content

Indirect synthesis via a copper-free precursor film leads not only to superior transport properties but also to an inherent grading of the Ga/(In + Ga) ratio when Cu-rich Cu(In,Ga)S 2 films have been deposited for thin film solar cells by two-stage sequential multi-source evaporation (sequence (In,Ga)–S/Cu–S). An inherent grading of the Ga/(In + Ga) ratio could be avoided by including a certain amount of Cu in the precursor. We have for the first time investigated the electrical properties and microstructure of the homogeneous Cu(In,Ga)S 2 films with changing Ga content. The carrier densities in Cu(In,Ga)S 2 films with Ga/(In + Ga) > 30% are found to be one order of magnitude larger than those in films with lower Ga content. The resistivity, its activation energy and mobility in Cu(In,Ga)S 2 films decreased with Ga/(In + Ga) > 30%. Electrical data are supplemented by micro-structural characterization using a transmission electron microscope. A reduction in grain size with increasing Ga content has been observed.

Effect of the ordering process on pseudoelasticity in Fe 3Ga single crystals

The pseudoelastic behaviour of Fe 3Ga single crystals with the D0 3 structure was investigated focusing on the annealing conditions and Ga content. In these D0 3–ordered Fe 3Ga single crystals, uncoupled and paired 1/4[1 1 1] superpartial dislocations moved, dragging the nearest-neighbour and next-nearest-neighbour anti-phase boundaries (APBs), respectively. These APBs pulled back the superpartials during unloading, resulting in pseudoelasticity, though a martensitic transformation never occurred in the crystals. The amount of strain recovery increased with increasing Ga content up to 25.4 at.% when the crystals were annealed at 853–893 K within 10 h. In particular, nearly perfect pseudoelasticity with a recoverable strain of 5% was obtained at 24.2 and 25.4 at.% Ga after an appropriate heat treatment. Isothermal annealing in the D0 3 single phase region was effective in improving pseudoelastic properties. Moreover, the metastable D0 3 phase also exhibited pseudoelasticity, even if the (α + L1 2) or (D0 3 + L1 2) dual phase was thermally stable in the equilibrium state.

Investigation of relation between Ga concentration and defect levels of Al/Cu(In,Ga)Se 2 Schottky junctions using admittance spectroscopy

Defect levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) were studied by using admittance spectroscopy with varying Ga concentration of x from 0.38 to 1.0. Two distinct peaks ( α and ζ) were detected from all of CIGS samples. Peak α could be attributed to the shallow acceptors. Peak ζ was closely correlated with the surface potential of CIGS; therefore, it may have been caused by traps near the surface of CIGS such as grain boundary defects. The intensity of peak ζ increases with increasing Ga concentration. Further, this intensity was correlated with the ideality factor of the Al/CIGS Schottky junction. We suggest that the degradation of the performance of CIGS solar cell with increasing Ga concentration may be concerned with defects corresponding to peak ζ˙

Thermal Expansion and Phase Transition Behavior of Al2-xMx(WO4)3 (M=Y, Ga and Sc) Ceramics

In order to obtain materials with a tailored thermal expansion, in particular zero-thermal-expansion between -50~100°C, the possibility of preparation, thermal expansion behavior and phase transition of Al2-xMx(WO4)3 (M: Y, Ga and Sc) have been examined. Y was not substituted for the Al site in Al2(WO4)3. Al2-xGax(WO4)3 single phase could be prepared in x range as high as 0.4. The phase transition temperature involving abrupt expansion increased with an increase of Ga content, resulting in no phase transition for Al1.6Ga0.4(WO4)3 between -50~100°C. However, zero-thermal expansion could not be obtained in Al2-xGax(WO4)3 at the temperature range. Al2-xScx(WO4)3 single phase was obtained for the whole x range. The temperature of the phase transition decreased with an increase of Sc content and the abrupt expansion due to the phase transition disappeared above -50°C for the specimen with x larger than 0.3. The thermal expansion coefficient could be controlled with Sc content and zero-thermal-expansion was observed for the specimens with x around 0.1 and 1.5.

Crystal structure and thermoelectric properties of type-I clathrate compounds in the Ba–Ga–Ge system

The crystal structure and thermoelectric properties of type-I clathrate compounds in the Ba–Ga–Ge system have been investigated as a function of Ga content. The solid solubility of Ga in the type-I clathrate compounds is determined to be X=16 when expressed with the formula of Ba8GaXGe46−X. As the Ga content increases, the crystal structure changes from a superlattice structure to the normal type-I clathrate structure with the transition occurring at X=3.5–5. The density of Ge vacancies in the type-I clathrate phase decreases as the Ga content increases. The absolute values of electrical resistivity and Seebeck coefficient increase, while that of lattice thermal conductivity decreases with the increase in the Ga content. The changes in electrical resistivity and Seebeck coefficient are explained in terms of the number of excess electrons, while the change in lattice thermal conductivity is explained in terms of the extent of the rattling motion of Ba atoms encapsulated in the cage structure.

Synthesis, cation distribution and crystal structure of the spinel oxides MnGa xFe 1−xCrO 4 by neutron diffraction

The spinel system MnGa x Fe 1− x CrO 4 ( x=0.0, 0.25, 0.50, 0.75 and 1.0) was prepared by the solid state sintering method in air at 1523 K. X-ray and neutron powder diffraction measurements were performed on the samples at 295 K for structural characterization. Rietveld refinement of the neutron diffraction data revealed that all the samples of the series possessed cubic symmetry corresponding to the space group Fd3m. The distributions of the four cations Mn, Ga, Fe and Cr over the two sublattices and other crystallographic parameters were found. The lattice constant showed a decrease with increasing Ga content in the system. The magnetic structure at room temperature was found to be ferrimagnetic in the composition range x⩽0.50.

Synthesis of poly(L-lactic-co-glycolic acid) telechelic prepolymers by direct melt polycondensation, and their characterization

A series of L-lactic/glycolic acid polymers (PLLGAc) differing in molar ratios of L-lactic (L-LAc) to glycolic (GAc) acid and of relatively low molecular weights [Mn(theor) = 4000] were synthesized by direct melt polycondensation in the presence of 1,4-butanediol and sulfuric acid as a molecular weight regulator and catalyst, respectively. The characterization experiments (FT-IR, 1H NMR) demonstrated a good correlation between the polymer compositions and the ratios of L-LAc and GAc in the feed. The DSC studies of the prepolymers indicated that the melting point as well as degree of crystallinity of PLLGAc decreased with the increase in the molar fraction of GA units in the copolymer. The glass transition temperatures of the polymers varied from 29 to 45 C degrees, depending on the polymer composition. The alkaline hydrolyzability (0.1 M NaOH, 37 C degrees) of the polymers is strongly affected by GA content in PLLGAc. With increasing GA content in the prepolymer, weight loss of a sample becomes faster.

Mössbauer effect studies of Fe100−xGax films prepared by combinatorial methods

Magnetostrictive Fe100−xGax alloysover the composition range 0<x<36 have been prepared by combinatorial sputtering methods. These films were investigated by x-ray diffractionand 57Fe Mössbauer effect spectroscopy techniques that have been adapted for the efficient study ofcombinatorial samples. X-ray diffraction measurements show the presence of a disorderedbcc phase with a lattice parameter that increases with increasing Ga content. Mössbauereffect measurements show that even for low Ga content the disordered alloysare not truly random but show some degree of short range Ga clustering. Forx>20, the Mössbauer effect results suggest the presence of short rangeD03 type order. This persists up to the maximum Ga content studied here(x = 36), although there is no evidence to support the formation of long rangeD03 structural order in any of the films prepared in this study.

Structure, magnetostriction, and magnetic properties of melt-spun Fe–Ga alloys

Six samples of Fe100−xGax (15⩽x⩽30) were prepared by melt spinning for a systemic investigation of the structure, magnetostriction, and magnetic properties of Fe–Ga alloys. A large magnetostriction (&amp;gt;750ppm) that increases with increasing Ga concentration has been observed in Fe100−xGax (15⩽x&amp;lt;25) ribbons melt spun at a wheel speed of 10m∕s. When x was equal to or greater than 25, the samples demonstrated magnetostriction of less than 200ppm. The abrupt decrease in magnetostriction at x=25 was attributed to the change in structure from disordered bcc to an ordered phase (DO3). The melt-spun Fe100−xGax (15⩽x⩽30) samples exhibited large saturation magnetization and high Curie temperature, which allowed the alloys to achieve the saturation magnetostriction under easily attainable fields. The angle dependence of the magnetostriction and the effect of the wheel speed are also discussed.

High thermoelectric performance of type-III clathrate compounds of the Ba–Ge–Ga system

The thermoelectric properties of type-III clathrate compounds, Ba 24Ga X Ge 100− X , have been investigated as a function of Ga content and temperature. The substitution of Ga atoms for Ge atoms leads to a decrease of carrier (electron) concentration. Electrical conduction is of n-type for all clathrate compounds investigated and the values of electrical resistivity and Seebeck coefficient increase with increasing Ga content and temperature. Both electronic and lattice thermal conductivities decrease with an increase in Ga content because of the decreased carrier concentration and the increased extent of the rattling motion of Ba atoms encapsulated in open dodecahedrons, respectively. A very high thermoelectric figure of merit (ZT) value of 1.25 is obtained at 670 °C when X = 15.

Structural and morphological properties of ZnO:Ga thin films

Zinc oxide (ZnO) is a promising semiconductor material with a great variety of applications. Compared to undoped ZnO, impurity-doped ZnO has a lower resistivity and better stability. With this aim, Ga has been proposed as a dopant. In this study, the structural characteristics and surface morphology of ZnO films produced by PEMOCVD at a substrate temperature of 250 °C on the c-plane (001) of sapphire were investigated. Doping was realized with 1, 3, 5 and 10 wt.% of Ga 2(AA) 3 in the precursor's mixture. At lower contents, Ga stimulates growth of (002) oriented textured films and the smallest FWHM was obtained as low as 0.17° for ZnO:Ga with 1 wt.%. A change in preferential orientation as well as surface smoothing and roughness decreasing of the films were observed with further increasing Ga content in precursor's mixture. We assume a key role of Ga and note that such a feature would be beneficial for the application of ZnO thin films for formation of abrupt junctions in p–n device structures.

Gallium-based catalysts for growth of GaN nanowires

Gallium nitride nanowires (GaN NWs) are traditionally fabricated using foreign metal catalysts, with an accompanying risk of increased contamination of the grown material and associated electrical and optical device degradation. Catalyst-free growth suffers from the inability to direct NW growth into predefined device architectures. In the present work, GaN NWs were fabricated by atmospheric CVD with a Ga-based catalyst, thus avoiding the dangers of foreign metal contamination while retaining the ability to spatially control NW growth. The effect of the total gas flow rate on wire morphology was examined and explained as a consequence of increasing Ga concentration with gas flow. Transmission electron microscopy was used to compare the crystalline structure of NWs grown from Ga(NO 3) 3-derived catalysts to that of the more conventional, Ni-catalyzed wires. Electron diffraction confirmed that both NW types exhibited the wurtzite crystal structure. The single crystal Ni-catalyzed NWs showed predominantly a [0 1 1] growth direction, whereas the Ga-based catalyst resulted in wires composed of single crystal segments with predominantly [0 0 1] or [1 1 1] growth directions.

Phase equilibria and phase transformation of Co−Ni−Ga ferromagnetic shape memory alloy system

Phase equilibria and martensitic and magnetic phase transformations of the β phase in the Co−Ni−Ga system have been investigated. It has been shown that the β phase is in equilibrium with the α-phase over a wide range compositions at 600–1100°C. The β phase exhibits both a paramagnetic-ferromagnetic transition and a thermoelastic martensitic transition from B2 to L10 structure. The Curie temperature TC increases with decreases with decreasing Ni content and with increasing Ga content. The composition region of the β phase exhibiting the thermoelatstic martensitic transformation from ferromagnetic austenite is located near the α+β two-phase region. TC and Ms of α+β two-phase alloys increase with increasing annealing temperature.

Anisotropic superconductivity in layered silicides

CaAlSi is a layered silicide with the AlB2-structure and shows superconductivity below 7.7 K. The angular dependence of the upper critical field, Hc2(θ), in CaAlSi shows a cusp-like anomaly, suggesting the presence of decoupled two-dimensional superconducting layers. Concomitant with this feature, a strong lattice modulation along the c-axis exists in this material. On the other hand, Hc2(θ) in CaGaSi is well fitted by th anisotropic Ginzburg–Landau (GL) model, and the superstructure along the c-axis is absent. In order to get some insight into the origin of the anomalous Hc2(θ) in CaAlSi, we have performed magneto-optical observations of the vortex penetration for fields applied parallel and perpendicular to the c-axis. Vortices are found to penetrate the crystal inhomogeneously when the field is applied perpendicular to the c-axis, indicating the presence of macroscopic inhomogeneities in CaAlSi. We have also grown single crystals of CaAl1−xGaxSi and studie the evolution of anomalous Hc2(θ) and the lattice modulation along the c-axis. With increasing Ga content in CaAl1−xGaxSi, the superstructure along the c-axis survives up to x ¼ 0.2, although the anomaly in Hc2(θ) is strongly suppressed. It may suggest irrelevance of the two features, althoughit seems to be closely correlated in CaAlSi and CaGaSi. Possible scenarios for the appearance of the anomalous Hc2(θ) are proposed.

Compositional variation of the phonon dispersion curves of bcc Fe-Ga alloys

Inelastic neutron scattering techniques have been used to measure the phonon dispersion curves of bcc ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}$ ($x=10.8$, 13.3, 16.0, 22.5) alloys as a function of Ga concentration. The phonon frequencies of every branch were found to decrease significantly with increasing Ga concentration. The softening was most pronounced for the ${T}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ branch and, to a lesser extent, the $L[\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}]$ branch in the vicinity of $\ensuremath{\xi}=\frac{2}{3}$. The concentration dependence of the shear elastic constant ${C}^{\ensuremath{'}}=1∕2({C}_{11}\ensuremath{-}{C}_{12})$, calculated from the slope of the ${T}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ branch, was found to agree with the results of sound velocity measurements. For the higher concentration sample measured, 22.5 at. % Ga, new branches appeared, an effect associated with the increase in the number of atoms per unit cell.