Effects Of Off-stoichiometry Research Articles

Effects of off-stoichiometry and Ti doping on thermoelectric performance of Fe2VAl Heusler compound

Improvement of p-type thermoelectric performance for Heusler-type Fe2V1.08−yTiyAl0.92 alloys with y = 0–0.34 has been investigated, focusing on the valence electron concentration (VEC) effect for tuning the Fermi level across the center of the pseudogap via Ti doping, combined with a constructive modification of the electronic structure around the pseudogap through the off-stoichiometric V/Al composition change. The Seebeck coefficient changes its sign from negative to positive at around VEC = 6.0 due to Ti doping, and the peak value reaches approximately 120 μV/K at 350 K for y = 0.22. As a result of a drastic reduction in the electrical resistivity at 300–600 K, the power factor enhances up to 3.7 × 10−3 W/mK2 at 300 K for y = 0.30. The thermal conductivity increases with Ti doping to 15.5 W/mK at 350 K for y = 0.16, because of an increased stability of the L21 structure for VEC closer to 6.0, but then decreases to approximately 12 W/mK for y = 0.34. While the dimensionless figure of merit ZT = 0.13 is achieved at 500 K for y = 0.34, the codoping of Ta (Fe2V0.69Ti0.34Ta0.05Al0.92) leads to a further enhancement of up to ZT = 0.18 at 500 K.

Microstructures and tensile properties of off-stoichiometric Ni3Al–Ni3V pseudo-binary alloys

Abstract

Effect of off-stoichiometry on properties of tin selenide crystals

The tin selenide crystals with different proportions of Sn and Se were grown by a direct vapour-transport technique. The layer by layer growth of crystals from the vapour phase was promoted by screw dislocation mechanism. The powder X-ray diffraction (XRD) shows good crystallinity of grown compound. The XRD patterns of grown compounds are well-indexed to orthorhombic structure. In the off-stoichiometric compound, evidence of $$\hbox {SnSe}_{{2}}$$ secondary phase is observed due to excess of selenium. The morphological investigations were carried out using a Carl Zeiss optical microscope. The electron diffraction was also recorded from tiny flakes using a transmission electron microscope. The electrical resistivity both parallel and perpendicular to the c-axis was measured in the temperature range of 303–490 K and activation energy was also calculated using Arrhenius relation. The electrical study depicts the extrinsic semiconducting nature of grown compositions.

Biocompatible off-stoichiometric copper indium sulfide quantum dots with tunable near-infrared emission via aqueous based synthesis.

The present study reports an aqueous synthesis approach towards off-stoichiometric copper indium sulfide quantum dots with emissions in the near-infrared spectral range. The photoluminescence properties of the dots, and in particular the appearance of dual emission at high Cu deficiency, were studied with temperature-dependent steady-state and transient photoluminescence spectroscopy.

A Critical Study of the Elastic Properties and Stability of Heusler Compounds: Phase Change and Tetragonal <i>X<sub>2</sub>YZ</i> Compounds

In the present work, the elastic constants and derived properties of tetragonal and cubic Heusler compounds were calculated using the high accuracy of the full-potential linearized augmented plane wave (FPLAPW). To find the criteria required for an accurate calculation, the consequences of increasing the numbers of $k$-points and plane waves on the convergence of the calculated elastic constants were explored. Once accurate elastic constants were calculated, elastic anisotropies, sound velocities, Debye temperatures, malleability, and other measurable physical properties were determined for the studied systems. The elastic properties suggested metallic bonding with intermediate malleability, between brittle and ductile, for the studied Heusler compounds. To address the effect of off-stoichiometry on the mechanical properties, the virtual crystal approximation (VCA) was used to calculate the elastic constants. The results indicated that an extreme correlation exists between the anisotropy ratio and the stoichiometry of the Heusler compounds, especially in the case of Ni$_{2}$MnGa.

Tailoring the Oxygen Evolution Activity and Stability Using Defect Chemistry

Improving the activity of catalysts for the oxygen evolution reaction (OER) requires a detailed understanding of the surface chemistry and structure to deduce structure-function relationships (descriptors) for fundamental insight. We chose epitaxial (100)-oriented La0.6Sr0.4Mn1−δO3 (LSMO) thin films as a model system with high electrochemical activity comparable to (110)-oriented IrO2 to investigate the effect of Mn off-stoichiometry on both catalytic activity and stability. Extensive structural characterization was performed by microscopic and spectroscopic methods before and after electrochemical characterization using rotating ring-disk studies. Stoichiometric LSMO had the highest activity, while both Mn deficiency and excess reduced the catalytic activity. Furthermore, all samples preserved the crystal structure up to the very surface. Mn excess improved the long-term activity, and we hypothesize that excess Mn stabilizes the surface chemistry during catalysis. Our data show that the defect chemistry should be considered when designing catalysts with enhanced activity and rugged stability.

Impact of stoichiometry ofYb2Ti2O7on its physical properties

A series of Yb2Ti2O7 doped samples demonstrates the effects of off-stoichiometry on Yb2Ti2O7's structure, properties, and magnetic ground state via x-ray diffraction, specific heat, and magnetization measurements. A stoichiometric single crystal of Yb2Ti2O7 grown by the traveling solvent floating zone technique (solvent = 30 wt% rutile TiO2 and 70 wt% Yb2Ti2O7) is characterized and evaluated in light of this series. Our data shows that upon positive x doping, the cubic lattice parameter a increases and the Curie-Weiss temperature decreases. Heat capacity measurements of stoichiometric Yb2Ti2O7 samples exhibit a sharp, first-order peak at T = 268(4) mK that is suppressed in magnitude and temperature in samples doped off ideal stoichiometry. The full entropy recovered per Yb ion is 5.7 J/K ~ Rln2. Our work establishes the effects of doping on Yb2Ti2O7's physical properties, which provides further evidence indicating that previous crystals grown by the traditional floating zone method are doped off ideal stoichiometry. Additionally, we present how to grow high-quality colorless single crystals of Yb2Ti2O7 by the traveling solvent floating zone growth method.

The off-stoichiometry effect on the optical properties of water-soluble copper indium zinc sulfide quantum dots

CuInZnS quantum dots (CIZS QDs) were prepared via reflux method in aqueous solution using CuCl2·2H2O, InCl3·4H2O, Zn(OAc)2·2H2O and Na2S·9H2O as raw materials, l-glutathione (GSH) and sodiumcitrate (SC) as stabilizing agents, respectively. The effects of off-stoichiometry (Cu/In and Zn/Cu ratios) on the crystal structure and morphology were systematically studied by means of X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM), and the relative optical properties were also investigated by absorption and fluorescence spectra. The as-prepared water-dispersible CIZS QDs were around 3–4nm and possessed the tetragonal chalcopyrite crystal structure. The photoluminescence (PL) intensity of QDs was significantly increased with decreasing the Cu/In ratio. Compared with the Cu/In ratio variation, changing Zn/Cu ratio was an effective strategy to realize a more uniform irradiation and a wide range of emission wavelength tunability.

Effect of cation off-stoichiometry on optical absorption in epitaxial LaFeO3 films.

We have investigated the effect of La and Fe deficiency on the c-axis lattice parameter and optical absorption in La1-xFe1-yO3-δ films deposited by oxide MBE. La deficient films display minimal changes in optical and structural properties compared with stoichiometric LaFeO3, even with relatively large cation vacancy concentrations (∼22%). Fe deficient films show a linear increase in c-axis parameter with increasing Fe vacancy concentration. The optical absorption coefficient is reduced for Fe deficient films over the spectral range from 1.2 to 5.0 eV. This leads to a relatively simple and effective method to quantify the approximate Fe deficiency in LaFeO3. Minimal changes in optical and structural properties are observed when cation deficient films are exposed to strongly oxidizing (ozone annealing) conditions. This indicates a strong accompaniment of charge balancing oxygen vacancies to maintain a nominal Fe3+ valence state, acting to stabilize the band structure even in the presence of significant cation off-stoichiometry.

Experimental investigation of off-stoichiometry and 3d transition metal (Mn, Ni, Cu)-substitution in single-crystalline FePt thin films

In L10 (fct)-FePt thin films, both tuning Fe and Pt concentrations and substitution with third-metal were studied for magnetic characteristic optimization. We investigated single-crystalline FePt-X (X = Mn, Ni, Cu) thin films grown epitaxially on MgO(001) substrates at a substrate temperature of 350 °C by changing Fe, Pt, and X contents, and explored the effects of off-stoichiometry and 3d-metal-substitution. The magnetic moment per atom (m) of FePt-X films as a function of the effective number of valence electrons (neff) in 3d metal sites follows the Slater-Pauling-type trend, by which m decreases by the neff deviation from neff = 8, independently of the X metal and the Pt concentration. The magnetic anisotropy (Ku) exhibits neff dependence similar to m. This trend was almost independent of the Pt concentration after compensation using the theoretical prediction on the relation between Ku and Fe/Pt concentrations. Such a trend has been proved for stoichiometric FePt-X films, but it was clarified as robust against off-stoichiometry. The compensated Ku (Kucomp) of FePt-Mn and FePt-Cu followed a similar trend to that predicted by the rigid-band model, although the Kucomp of the FePt-Mn thin films dropped more rapidly than the rigid band calculation. However, it followed the recent first-principles calculation.

Effects of stoichiometry and substitution in quasi-one-dimensional iron chalcogenideBaFe2S3

The effects of off-stoichiometry and elemental substitution on electronic properties of iron-based ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ are investigated. Resistivity and magnetization are revealed to be quite sensitive to the stoichiometry of Fe atoms, and 10% deficiency at Fe sites reduces the antiferromagnetic transition temperature by 40 K. The antiferromagnetic transition temperature decreases even faster and collapses to zero with hole doping through 10% K substitution at the Ba site, while the antiferromagnetic ordering phase remains with electron doping through 20% Co substitution at the Fe site. Such electron-hole asymmetry is opposite to two-dimensional iron-based superconductors, and can be explained on the basis of both itinerant and localized electronic pictures.

Effect of Off-Stoichiometry on the Thermoelectric Properties of Heusler-Type Fe2VAl Sintered Alloys

Heusler-type Fe2V1−xAl1+x sintered alloys with micrometer-sized grains were fabricated by the powder metallurgical process using mechanical alloying and pulse-current sintering. Both positive (∼90 μV/K) and negative (∼−140 μV/K) Seebeck coefficients were obtained for the composition ranges of x > 0 and x < 0, respectively, resulting from a Fermi level shift caused by the change in the valence electron concentration. The electrical resistivity was reduced by the carrier doping effect, especially at lower temperatures, resulting in an increased thermoelectric power factor of 2.8 mW/m-K2 for the p-type alloy with x = 0.06 and 5.0 mW/m-K2 for the n-type alloy with x = −0.06. In addition, the lattice thermal conductivity decreased with |x| because of phonon scattering at crystal lattice defects induced by the off-stoichiometry. Consequently, the thermoelectric figure of merit, ZT, was enhanced and reached 0.07 for p-type alloys with 0.06 < x < 0.15 and 0.18 for n-type alloys with −0.15 < x < −0.10 around 500 K. The ZT value was especially enhanced at higher temperatures by the off-stoichiometric composition control, which could extend the range of heat source temperatures for thermoelectric power generation applications using this alloy.

Effects of cation stoichiometry on electronic and structural properties of LaNiO3

LaNiO3 films with varying La:Ni ratios were deposited onto SrTiO3 (001) substrates via molecular beam epitaxy to elucidate the effects of cation off-stoichiometry. The physical properties of La-deficient films are found to differ substantially from those of Ni-deficient films, with La-deficient films exhibiting lower electrical resistivities and smaller c-axis parameters than Ni-deficient films. No evidence of secondary phases is observed; however, transmission electron microscopy reveals an abundance of defects, the nature of which differs in lanthanum- and nickel-deficient films. This work illustrates the nontrivial role that cation stoichiometry can play on the functional properties of complex oxides.

Effects of the Cu off-stoichiometry on transport properties of wide gap p-type semiconductor, layered oxysulfide LaCuSO

Layered oxysulfide LaCu1−xSO (x = 0–0.03) was prepared to elucidate the effect of Cu off-stoichiometry on their electrical and thermal transport properties. Electrical resistivity drastically decreases down from ∼105 Ω·cm to ∼10−1 Ω·cm as a result of Cu deficiency (x = 0.01) at 300 K. Thermal conductivity of the samples at 300 K, which is dominated by lattice components, is estimated to be 2.3(3) Wm−1K−1. Stoichiometric LaCuSO has an optical band gap of 3.1 eV, while broad optical absorption at photon energies of approximately 2.1 eV was observed for Cu-deficient samples. Density functional theory calculation suggests that these broad absorption structures probably originate from the in-gap states generated by the sulfur vacancies created to compensate the charge imbalance due to Cu off-stoichiometry. These results clearly demonstrate that Cu deficiency plays a crucial role in determining the electrical transport properties of Cu-based p-type transparent semiconductors.

Effect of oxygen off-stoichiometry on coupled structural and magnetic phase-transitions in La0.15Sr0.85MnO3−δ (δ=0.02, 0.14)

Effect of oxygen off-stoichiometry on the structural and magnetic properties of La0.15Sr0.85MnO3−δ (δ=0.02, 0.14) has been studied employing low-temperature x-ray diffraction, calorimetry, electrical-transport and magnetization. The nearly stoichiometric composition under goes first order phase-transition from cubic to tetragonal P4/mmm at 238K accompanied by magnetic and electrical phase-transitions. On the contrary the off-stoichiometric composition does not show any transition and remains cubic Pm-3m down to 80K. Off-stoichiometry induced oxygen-vacancy seems to suppress the cooperative JT distortion and possibly stabilize a G-type antiferromagnetic phase with localized canted spins. Thus we demonstrate that controlling oxygen stoichiometry is a way of structure/property engineering in electron-doped-like manganites.

Stoichiometry of LaAlO3 films grown on SrTiO3 by pulsed laser deposition

We have studied the stoichiometry of epitaxial LaAlO3 thin films on SrTiO3 substrate grown by pulsed laser deposition as a function of laser energy density and oxygen pressure during the film growth. Both x-ray diffraction (θ-2θ scan and reciprocal space mapping) and transmission electron microscopy (geometric phase analysis) revealed a change of lattice constant in the film with the distance from the substrate. Combined with composition analysis using x-ray fluorescence we found that the nominal unit-cell volume expanded when the LaAlO3 film was La-rich, but remained near the bulk value when the film was La-poor or stoichiometric. La excess was found in all the films deposited in oxygen pressures lower than 10−2 Torr. We conclude that the discussion of LaAlO3/SrTiO3 interfacial properties should include the effects of cation off-stoichiometry in the LaAlO3 films when the deposition is conducted under low oxygen pressures.

Effect of off-stoichiometry at the Fe-site in FeSe0.5Te0.5 Superconductor

Abstract Excess iron in Fe 1+x Se superconductor plays a crucial role in the observation of superconductivity in this material. We have investigated the effect of off-stoichiometry at the Fe site in Fe 1+x Se 0.5 Te 0.5 ( x = -0.01, 0.01 and 0.03) superconductor through magnetization, electrical transport and specific heat measurements. Our results indicate an improvement in the superconducting properties in Fe-deficient compound compared to the stoichiometric or Fe-rich compounds, in contrast to what is observed in the parent compound, FeSe.

The (100) surface of the Al13Co4 quasicrystalline approximant

ABSTRACTThe structure of the (100) surface of the orthorhombic Al13Co4quasicrystalline approximant is described based on a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) study. We discuss the interplay between the surface structure and the cluster substructure of this approximant as well as possible off-stoichiometry effects that could explain the discrepancies found in the literature on possible surface models.

Effects of off-stoichiometry on the spin polarization at the Co2MnβGe0.38/MgO interfaces: X-ray magnetic circular dichroism study

We have studied the electronic and magnetic states of Mn and Co atoms at the Co${}_{2}$Mn${}_{\ensuremath{\beta}}$Ge${}_{0}.38$ (CMG)/MgO ($\ensuremath{\beta}=0.67$--1.8) interfaces, which constitute CMG/MgO/CMG magnetic tunnel junctions, by means of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroisms (XMCDs) at the Mn and Co ${L}_{2},3$ edges and compared the deduced Co and Mn magnetic moments by the density-functional calculations with the coherent potential approximation (CPA). As the Mn content $\ensuremath{\beta}$ increased, the intensity of Mn ${L}_{2},3$ XMCD decreased, in agreement with the result of the CPA calculation, which showed the resultant reduction of charges and spin moment on Mn atoms because of the delocalization of Mn 3$d$ electrons with increasing $\ensuremath{\beta}$. The Co spin magnetic moment for all the samples increased with decreasing $\ensuremath{\beta}$, again consistent with the present calculation and the previous density-functional calculation by Picozzi et al. that the Co${}_{\mathrm{Mn}}$ antisite has a larger magnetic moment than that of Co atoms at the regular sites and reduces the spin polarization at the Fermi level and hence the tunnel magnetoresistance ratio.

Effects of site disorder, off-stoichiometry and epitaxial strain on the optical properties of magnetoelectric gallium ferrite

We present a combined experimental–theoretical study demonstrating the role of site disorder, off-stoichiometry and strain on the optical properties of magnetoelectric gallium ferrite. Optical properties (bandgap, refractive indices and dielectric constants) were experimentally obtained by performing ellipsometric studies over the energy range 0.8–4.2 eV on pulsed laser deposited epitaxial thin films of stoichiometric gallium ferrite with b-axis orientation and the data were compared with theoretical results. Calculations on the ground state structure show that the optical activity in GaFeO3 arises primarily from O 2p–Fe 3d transitions. Further, inclusion of site disorder and epitaxial strain in the ground state structure significantly improves the agreement between the theory and the room temperature experimental data substantiating the presence of site disorder in the experimentally derived strained GaFeO3 films at room temperature. We attribute the modification of the ground state optical behavior upon inclusion of site disorder to the corresponding changes in the electronic band structure, especially in Fe 3d states leading to a lowered bandgap of the material.