Ga Substitution Research Articles

Effects of Ga substitution on the structural and magnetic properties of half metallic Fe2MnSi Heusler compound

The so-called half-metallic magnets have been proposed as good candidates for spintronic applications due to the feature of exhibiting a hundred percent spin polarization at the Fermi level. Such materials follow the Slater-Pauling rule, which relates the magnetic moment with the valence electrons in the system. In this paper, we study the bulk polycrystalline half-metallic Fe2MnSi Heusler compound replacing Si by Ga to determine how the Ga addition changes the magnetic, the structural, and the half-metal properties of this compound. The material does not follow the Slater-Pauling rule, probably due to a minor structural disorder degree in the system, but a linear dependence on the magnetic transition temperature with the valence electron number points to the half-metallic behavior of this compound.

Influence of Ga Substitution on the Nature of Glasses in Zr69.5Al7.5-xGaxCu12Ni11 and Ce75Al25-xGax Metallic Glass Compositions

ABSTRACTIn this presentation, results of our recent investigations on the role of Ga on Al site in Zr69.5Al7.5-xGaxCu12Ni11 and Ce75Al25-xGax metallic glass compositions will be discussed. Ga like Al is normally expected to be in trivalent state. However, it may go in monovalent state depending on other alloying elements. The rapidly solidified melt spun ribbons of above two alloys gave rise to two important conclusions. The Zr69.5Al7.5-xGaxCu12Ni11 system displayed metallic glass formation in the range of x=0 to 7.5. In this process, we have come out with a new composition of glass without Al corresponding to x=7.5. In contrast to the above, for Ce-Al(Ga) system, we have observed phase separation in glass after dilute substitution of Ga. It seems that such a phase separation in this system cannot be understood in terms of summation of enthalpy of mixing of the various possible binaries in this system. The substitution of Ga in different valence states might have created chemical pressure leading to creation of two types of distinct major clusters. The phase separation may be due to this. This has also given rise to excursion of Ce 4f-states of the alloy. This and aforesaid ‘chemical pressure’ will be corroborated based on results of binary Ce-Al system under pressure by other investigators.

Theoretical insight into a novel zinc di-corrole dye with excellent photoelectronic properties for solar cells

A new zinc di-corrole dye has been designed by substitution of Ga with Zn in a Ga di-corrole dye. Its optical and electronic properties were studied by extensive DFT calculations.

Carrier concentration dependent optical and electrical properties of Ga doped ZnO hexagonal nanocrystals.

Colloidal trivalent gallium (Ga) doped zinc oxide (ZnO) hexagonal nanocrystals have been prepared to introduce more carrier concentration into the wide band gap of ZnO. The dopant (Ga) modifies the morphology and size of ZnO nanocrystals. Low content of Ga enhances the optical band gap of ZnO due to excess carrier concentration in the conduction band of ZnO. The interaction among free carriers arising from higher concentration of Ga gives rise to narrowing of the band gap. Surface plasmon resonance absorption appears in the infrared region due to excessive carrier concentration. A broad emission band consists of blue, yellow and green colors associated with different native defects of ZnO. Intrinsic defects and extrinsic dopant Ga control the defect related emission spectrum in the visible region. Replacement of Zn by Ga induces a room temperature metallic state in a degenerate semiconductor. Cationic disorder leads to metal-semiconductor transition at low temperature strongly dependent on the concentration of Ga. Pure semiconducting behavior up to about 80 K is observed for the highest amount of Ga. Temperature dependent metal-semiconductor transition has been interpreted by localization of charge carriers due to disorder arising from random Ga substitution.

Aluminum and Gallium Substitution into 0.5Li2MnO3·0.5Li(Ni0.375Mn0.375Co0.25)O2 Layered Composite and the Voltage Fade Effect

Lithium- and manganese-rich layered composite cathodes in the general form (Li2MnO3•LiMO2; M = transition metals), suffer from voltage profile suppression during cycling in Li-ion cells leading to overall gradual energy losses in the system. The suppression in cathode voltage which is called ‘voltage fade’ is a general phenomenon for these class of materials which needs to be understood and mitigated for enabling this chemistry in advanced Li-ion cells. Synthetic manipulation of the composition in 0.5Li2MnO3•0.5LiNi0.375Mn0.375Co0.25O2 (= Li1.2Ni0.15Mn0.55Co0.1O2 in layered notation) cathode material via aluminum and gallium substitution (max. 10%) via sol-gel reaction was completed and the voltage fade percentage was measured following a prescribed electrochemical testing protocol. While the specific capacities (∼ 250 mAh/g) are unaffected by the Al and Ga substitution (up to 5%), ∼3 to 4% of the impedance-corrected average voltage is still lost (first 25 cycles) which is comparable to what the pristine material experiences. This result suggests that the path of structural change that occurs in the material during Li cycling is unaltered by synthetic manipulation of octahedral and/or tetrahedral sites though the use of Al and Ga non-redox active cations.

Identification of the rate-determining step in oxygen transport through La(1−x)SrxFe(1−y)GayO3−δ perovskite membranes

La(1−x)SrxFe(1−y)GayO3−δ perovskites are some of the most promising materials for oxygen transport membranes due to their high oxygen semi-permeation and good chemical stability at high temperature under a wide range of oxygen partial pressures (from 0.21 to 10−19atm.). This paper is focused on a better understanding of the influence of the Ga and Sr substitution ratios in the La(1−x)SrxFe(1−y)GayO3−δ perovskite series on oxygen semi-permeation performances, oxygen diffusion and surface exchange coefficients. This study identifies the optimal compositions of the La(1−x)SrxFe(1−y)GayO3−δ perovskite series. In addition, this study shows that the rate-determining step of oxygen transport through the membrane depends strongly on the Ga and Sr substitution ratios.

Broadly tunable emission from Mn-doped zinc gallogermanate phosphors through composition modification

Nowadays, one-phase and full-color phosphors have gained increasing interest. Here, we show that Mn-activated zinc gallogermanate, Zn1+xGa2-2xGexO4: Mn, x = 0 ~1, phosphors exhibit a broadly tunable luminescence from green to deep red with the substitution of Ga3+ by Ge4+. The green and deep red emissions are attributed to Mn2+ and Mn4+ occupying the tetrahedrally coordinated Zn2+ and octahedrally coordinated Ga3+ sites, respectively. The origin of the tunable luminescence is discussed. The present phosphors have potential uses in field emission displays and in vivo bio-imaging.

Electronic structure and magnetic properties of quaternary Heusler alloy Co2CrGa1−Ge (x=0−1)

Abstract The electronic structure of Co-based quaternary Heusler compounds Co 2 CrGa 1− x Ge x ( x =0.00, 0.25, 0.50, 0.75, 1.00) are calculated by first-principles density functional theory. The substitution of Ga by Ge leads to increase in the number of valence electrons. With increasing concentration of Ge, lattice constant decreases linearly whereas bulk modulus and total magnetic moment increases. This shows that the magnetic properties of the compound are dependent on electron concentration of main group element. The calculations show that the alloys with x =0.00, 0.25, 0.50 are not true half-metallic materials whereas alloy with x =0.75, 1.00 exhibit 100% spin polarization at the Fermi level. It shows that the Fermi level can be shifted within the energy-gap to achieve 100% spin polarization. The effect of volumetric and tetragonal strain on magnetic properties is also studied.

Electric transport through perovskite La(Fe,Ga)O3 nanowires formed by electrospinning

Orthoferrite perovskite LaFeO3 has been of great interest with its versatility for potential applications in electromagnetic, catalytic, and sensing applications. Nanowires have shown great potential as active building blocks for assembling a variety of nanoscale devices, nevertheless far less is known about the characteristics of LaFeO3 nanowires. Here, we show that LaFeO3 nanowires with Ga doping can be produced by electrospinning, yielding the formation of specific nanostructure of fine crystallite array arranged along nanowire framework. The structure and chemical state analyses were performed to reveal the nanostructural difference induced by Ga substitution. Furthermore, the La(Fe,Ga)O3 nanowire devices were built and showed the temperature and field dependence on electric transport following the Poole-Frenkel conduction model. The dramatic conductance lowering by Ga doping was observed and explained by the change in trap energy for carrier hopping.

Anisotropic polaron localization and spontaneous symmetry breaking: Comparison of cation-site acceptors in GaN and ZnO

The behavior of cation substitutional hole doping in GaN and ZnO is investigated using hybrid density functional calculations. Our results reveal that Mg substitution for Ga $({\mathrm{Mg}}_{\mathrm{Ga}})$ in GaN can assume three different configurations. Two of the configurations are characterized by the formation of defect-bound small polaron (i.e., a large structural distortion accompanied by hole localization on one of the neighboring N atoms). The third one has a relatively small but significant distortion that is characterized by highly anisotropic polaron localization. In this third configuration, ${\mathrm{Mg}}_{\mathrm{Ga}}$ exhibits both effective-mass-like and noneffective-mass-like characters. In contrast, a similar defect in ZnO, ${\mathrm{Li}}_{\mathrm{Zn}}$, cannot sustain the anisotropic polaron in the hybrid functional calculation, but undergoes spontaneous breaking of a mirror symmetry through a mechanism driven by the hole localization. Finally, using ${\mathrm{Na}}_{\mathrm{Zn}}$ in ZnO as an example, we show that the deep acceptor levels of the small-polaron defects could be made shallower by applying compressive strain to the material.

Computational prediction of a global planar penta-coordinate carbon structure CAl4Ga+

Ever since the proposal of the exotic concept “planar tetra-coordinate carbon” (ptC), continuous endeavors have been hotly devoted to the design and characterization of molecules with planar tetra- or hyper-coordinate atomic centers. In sharp contrast to the big ptC family, examples with the planar penta-coordinate carbon (ppC) have remained very scarce, only including CAl5+, CAl4Be, CAl3Be2, CAl2Be32 and CBe5E (E=Al, Ga). To enrich the member of ppC, in this work, we report our global isomeric characterization of CAl4X+ (X=Al, Ga, In and Tl) at the CCSD(T)/def2-TZVPP//B3LYP/def2-TZVP level with zero-point energy correction, among which, to our knowledge, systems with X=Ga, In and Tl have not been considered previously. For CAl5+, besides confirming previous computational study that ppC is the ground structure, we find a new isomer (4th) among the former seven species here. Promisingly, by means of the structural and orbital analysis, CAl4Ga+ is a novel ppC structure though the penta-coordinate ligands of C are not average due to the Al→Ga substitution. For the heavier CAl4X+ (X=In and Tl), there does not exist a ppC structure and the global isomer can formally be considered as an X+ adsorbed to the neutral Td-like CAl4. In the order of X=Al, Ga, In and Tl, the preference of ppC over the Td-C decreases for CAl4X+, which could be interpreted by the decreased covalent ability. The newly disclosed CAl4Ga+ awaits future mass spectrometry characterization.

Study of lattice deformation and atomic bond length for AlxGa1−xN epi-layers with synchrotron radiation X-ray absorption spectroscopy

The lattice deformation in wurtzite AlxGa1−xN epi-layers (0.19 ≤ x ≤ 0.58) grown on sapphire substrates by metal organic chemical vapor deposition has been studied with synchrotron radiation X-ray absorption spectroscopy and X-ray diffraction. The result reveals that Al substitution for Ga in AlGaN epi-layers induces a significant contraction in crystal lattice and c/a ratio (c and a are the lattice constants for AlGaN epi-layer, respectively), while the substitution only results in a slight expansion in internal parameter u, which is used to describe the lattice deformation of wurtzite structure. This fact suggests that u is not sensitive to Al substitution for Ga. A detailed analysis of extended X-ray absorption fine structure of Ga K-edge spectra show that the Ga–N bond length has only a weak composition dependence, while the Ga–Ga (Al) bond lengths are strongly dependent on the Al composition.

Heteroepitaxy of Ga2(1‑x)In2xO3 layers by MOVPE with two different oxygen sources

Ga2(1‑x)In2xO3 epitaxial layers have been grown on (0001) Al2O3 substrates by metal organic vapour phase epitaxy (MOVPE). The process parameters were optimized and the effects related to the use of two alternative oxygenation sources like O2 and H2O were studied. Different In content x [x = In/(In + Ga)] were investigated in order to determine the In solubility limit in β‐Ga2O3. By using pure O2 during the growth, the In amount detected in the layers increased linearly with the flux of In precursor injected into the reactor, but the X‑ray diffraction patterns showed the presence of In2O3, in addition to β‐Ga2O3, for every In content. The second phase formation was confirmed by a double step feature placed at the absorption edge in optical transmission spectra, compatible with the presence of the two materials. With H2O as an oxygenation source, no In2O3 phase was observed up to x∼0.25. The growth of the mixed Ga2(1‐x)In2xO3 phase was confirmed by XRD spectra, wherein the β‐Ga2O3‐related peaks shifted at lower 2θ angles as expected from the substitution of Ga3+ ions with bigger In3+ ones. The variation of bandgap was confirmed by optical transmittance measurements, which showed a red shift of the absorption edge corresponding to In incorporation.

Influence of composition and heat treatment on damping and magnetostrictive properties of Fe–18%(Ga + Al) alloys

The structure, magnetostriction and damping properties of Fe82Ga(18−x)Alx (x=0, 5, 8, 12) alloys were analyzed. The anelastic response of Fe–18(Ga+Al) alloys was studied as a function of temperature (from 0 to 600°C), frequency (from 0.01 to 200Hz) and amplitude (from 0.0004% to 0.2%) of forced vibrations. The origin of the relatively high damping capacity of Fe–Ga–Al alloy at room temperature was determined by applying a magnetic field and different heat treatment regimes. The substitution of Ga by Al in Fe–18% Ga alloys was found to decrease magnetostriction and damping. The heat treatment of alloys influences the damping capacity of alloys more than variations of their chemical compositions. Thermally activated frequency and temperature-dependent anelastic effects in Fe–Ga–Al alloys were analyzed and the corresponding activation parameters for relaxation processes were evaluated. Internal friction effects caused by structural transformations were recorded and were found to be consistent with the A2→D03→L12 reaction. The physical mechanisms for all anelastic effects are discussed.

Ag3Ga3SiSe8: a new infrared nonlinear optical material with a chalcopyrite structure

An infrared (IR) nonlinear optical (NLO) material, Ag3Ga3SiSe8, has been synthesized for the first time. It crystallizes in the tetragonal chalcopyrite AgGaSe2 structure type and the substitution of Ga3+ with Si4+ generates an equal amount of vacancies at the Ag+ position. Ag3Ga3SiSe8 is a congruent melting compound with a melting temperature of 784 °C. The second-harmonic generation (SHG) effect of Ag3Ga3SiSe8 is similar to that of AgGaSe2, as confirmed by the first-principles studies. Moreover, the diffuse reflectance spectra reveal that the energy band gap of Ag3Ga3SiSe is 2.30 eV, 0.44 eV larger compared with AgGaSe2 (1.86 eV). The birefringence of Ag3Ga3SiSe8 is predicted to be above 0.1 at the wavelength of 1 μm, significantly larger than that of AgGaSe2 (Δn ~ 0.02). Our results indicate that Ag3Ga3SiSe8 may possess improved optical properties compared with AgGaSe2 and has a promising application as a good IR NLO material.

Crystal structure and microwave dielectric properties of CaTiO3–La[Ga(1−δ)Alδ]O3 ceramics system

The crystal structure and microwave dielectric properties of 0.64CaTiO3–0.36La[Ga(1−δ)Alδ]O3 specimens have been investigated in this work. When δ≤0.5, the crystal structure of 0.64CaTiO3–0.36La[Ga(1−δ)Alδ]O3 was orthorhombic, while it transformed into the tetragonal structure when δ>0.5. When δ=0.5, specimens exhibited good microwave dielectric properties with ϵr=45.54, Q×f=GHz) and τf=−1.7ppm/°C. The 50 at% substitution of Ga3+ by Al3+ in 0.64CaTiO3–0.36LaGaO3 benefited the Q×f value, the results showed that all samples of xCaTiO3–(1−x)La[Ga0.5Al0.5]O3 (x=0.62, 0.63, 0.64, 0.65, 0.66) exhibited ρr≥96.8%, Q×f≥39,000GHz, 44≤ϵr≤48 and −8ppm/°C≤τf≤+6ppm/°C.

Electronic and optical properties of Ga-doped ZnO

This study adopted density functional theory and the Hubbard-U method in an investigation of the electronic and optical properties of pure ZnO and Ga-doped ZnO. The difference in the lattice constant between calculated results and experimental measurements is less than 1%, while the calculated band gap of pure ZnO is in excellent agreement with experimental values. Three structures, including the substitution of Ga for Zn (Gas(Zn)), interstitial Ga in an octahedron (Gai(oct)), and interstitial Ga in a tetrahedron (Gai(tet)), were considered. Calculations related to formation energy revealed that Gas(Zn) forms more easily than Gai(oct) and Gai(tet). All three of the Ga defect models resulted in an upward shift in the Fermi level into the conduction band, resulting in n-type conductive characteristics and expansion of the optical band gap beyond that of pure ZnO. In the Gas(Zn) model, the average transmittance levels in the visible and UV regions were 90.5% and 77.8%, respectively, which are higher than those obtained using the pure ZnO model. However, in both the Gai(oct) and Gai(tet) models, an increase in effective mass resulted in a decrease in carrier mobility, thereby reducing electrical conductivity. In addition, interestitial Ga atoms within the ZnO crystal reduced transmittance significantly.

Magnetoelectric Polarization of Paramagnetic HoAl<sub>3-X</sub>Ga<sub>X</sub>(BO<sub>3</sub>)<sub>4</sub> Single Crystals

HoAl3-xGax(BO3)4 (0 ≤ х ≤ 3) single crystals were grown from Bi2Mo3O12Li2MoO4-based fluxes. Structural inversion twinning depending on crystal composition and heat treatment conditions is revealed. It is established that upon substitution of Ga3+ ions for Al3+ the magnetic characteristics of the crystals do not significantly change, while the magnetoelectric polarization decreases with increasing gallium content. The measured value and sign of polarization charges depend on a ratio between the left and right isomers.

Thermoelectric properties of Cu2Ga4Te7 based compounds with Zn substitution for Cu and Ga

We present two series of Cu2Ga4Te7‐based compounds with Zn substituting for Cu (Cu‐poor) and Ga (Ga‐poor), and have observed enhanced thermoelectric performance. Generally, the Seebeck coefficient (α) and lattice thermal conductivity (κL) increase and electrical conductivity (σ) decreases with the Zn content increasing. The variation in electrical properties might be mainly due to the formation of antisite defects ( and ), the acceptors. The defects increase the local density of states (DOS) near the Fermi level and effective mass, leading to the increase in α, while on the other hand, they widen the bandgap (Eg) leading to the degradation of the electrical conductivity. The gradual enhancement in κL is attributed to the reduced phonon scattering resulting from the decrease in vacancy concentration. As a result, we attained the maximum ZT value of 0.46 for Cu‐CGT and 0.47 for Ga‐CGT at ∼770 K with a proper Zn content, which is about 20–22% higher than that of intrinsic Cu2Ga4Te7.

Reduction of critical field for magnetic and orbital-ordering phase transition in impurity-substituted Nd0.45Sr0.55MnO3 crystal

We have investigated the Mn-site substitution effect in Nd0.45Sr0.55MnO3 single crystal, which has an A-type layered antiferromagnetic (A-AFM) phase with the 3\documentclass[12pt]{minimal}\begin{document}$d_{x^2-y^2}$\end{document}dx2−y2-type orbital-order. Substitution of Fe or Ga for Mn-site suppresses both the A-AFM order and competing ferromagnetic (FM) correlation, whereas Cr substitution suppresses only the A-AFM order but reactivates the underlying FM correlation via double-exchange mechanism along the AFM coupled c-direction. In Nd0.45Sr0.55Mn0.95Cr0.05O3, the A-AFM state with the orbital-order is changed into the orbital-disordered three-dimensional FM metallic state by applying magnetic field of μ0H = 12 T, which is much smaller than that of the parent compound Nd0.45Sr0.55MnO3.