Effects Of Off-stoichiometry Research Articles

Off-stoichiometry effect on orbital order inA-site manganites probed by x-ray absorption spectroscopy

We here report on the occurrence of strong orbital order driven by off-stoichiometry in oxide manganite films. Multiple-electronic-valence Mn ions are possibly substituted into the perovskite atomic $A$ site (namely, $A$-site manganites). By combining state-of-the-art synchrotron ultrafine spectroscopies, we did provide evidence of a high ferromagnetic metallic state together with a strong orbital order of Mn ions, despite that the kinetic energy of the free charge carriers should prevent long-range orbital order. By varying the total amount of Mn ions at the perovskite $A$ site, a specific orbital order (either in-plane or out-of-plane) can be determined.

Effect of oxygen off-stoichiometry on magnetic and magneto-transport properties of nanocrystalline under-doped LCMO

Magnetic and magneto-transport investigation of nanocrystalline under-doped manganite La0.875Ca0.125MnO3+δ with δ = 0.0 (sample x1) and δ = 0.06 (sample x2) reveals that the dipolar interactions are primarily responsible for establishing ferromagnetic order in the insulating state and that the uniaxial magnetic anisotropy is stronger in x2. The reduction of magnetization and resistivity in x2 compared to that in x1 is brought about by the oxygen off-stoichiometry as it increases the concentration of Mn4+ ions. Bringing the crystallite size down to nanometer range induces an insulating-to-metal transition and a re-entrant insulating (ferromagnetic) phase at temperatures T < 30 K.

Cation off-stoichiometry leads to highp-type conductivity and enhanced transparency in Co2ZnO4and Co2NiO4thin films

We explore the effects of cation off-stoichiometry on structural, electrical, optical, and electronic properties of Co${}_{2}$ZnO${}_{4}$ normal spinel and Co${}_{2}$NiO${}_{4}$ inverse spinel using theoretic and experimental (combinatorial and conventional) techniques, both at thermodynamic equilibrium and in the metastable regime. Theory predicts that nonequilibrium substitution of divalent Zn on nominally trivalent octahedral sites increases net hole density in Co${}_{2}$ZnO${}_{4}$. Experiment confirms high conductivity and high work function in Co${}_{2}$NiO${}_{4}$ and Zn-rich Co${}_{2}$ZnO${}_{4}$ thin films grown by nonequilibrium physical vapor deposition techniques. High $p$-type conductivities of Co${}_{2}$ZnO${}_{4}$ (up to 5 S/cm) and Co${}_{2}$NiO${}_{4}$ (up to 204 S/cm) are found over a broad compositional range, they are only weakly sensitive to oxygen partial pressure and quite tolerant to a wide range of processing temperatures. In addition, off-stoichiometry caused by nonequilibrium growth decreases the optical absorption of Co${}_{2}$ZnO${}_{4}$ and Co${}_{2}$NiO${}_{4}$ thin films, although the 500-nm thin films still have rather limited transparency. All these properties as well as high work functions make Co${}_{2}$ZnO${}_{4}$ and Co${}_{2}$NiO${}_{4}$ thin films attractive for technological applications, such as hole transport layers in organic photovoltaic devices or $p$-type buffer layers in inorganic solar cells.

Off-stoichiometry effects on BiFeO 3 thin films

The effects of Bi and Fe-excess on the structure, ferroelectric, leakage current and magnetic properties of BiFeO 3 (BFO) thin films are reported. BFO with 5% excess exhibits no change in the structure with an improvement in leakage current properties in comparison to stoichiometric BFO. Raman spectroscopy of 10% Bi excess suggests a structural change from monoclinic to rhombohedral accompanied with an improvement of resistivity and ferroelectric polarization switching. A higher Fe-excess leads to the formation of pyrochlore Bi 2Fe 4O 9 and gamma-Fe 2O 3 that cause an increase in conductivity at the macroscopic scale. The results are discussed in terms of Fe and Bi-excess effects on the defect structure of BFO.

Off-Stoichiometry Effects on the Crystalline and Defect Structure of Hexagonal Manganite REMnO3 Films (RE = Y, Er, Dy)

The crystalline and defect structure of epitaxial hexagonal RExMnyO3 (RE = Er, Dy) films with varying cationic composition was investigated by X-ray diffraction and transmission electron microscopy. The films are composed of a strained layer at the interface with the substrate and of a relaxed layer on top of it. The critical thickness is of ∼10 to 25 nm. For Mn-rich films (or RE deficient), an off-stoichiometric composition maintaining the hexagonal LuMnO3-type structure is stabilized over a large range of the RE/Mn ratio (0.72−1.00), with no Mn-rich secondary phases observed. A linear dependence of the out-of-plane lattice parameter with RE/Mn is observed in this range. Out-of-phase boundary (OPB) extended defects are observed in all films and exhibit a local change in stoichiometry. Such a large solubility limit in the RE deficient region points toward the formation of vacancies on the RE site (RExMnO3−δ, with 0.72 ≤ x < 1), a phenomenon that is encountered in perovskite manganites such as LaxMnO3−δ (x < 1) and that may strongly impact the physical properties of hexagonal manganites.

Effects of off-stoichiometry and density on the magnetic and magneto-optical properties of yttrium iron garnet films by magnetron sputtering method

Yttrium iron garnet films were deposited on Si and Si/CeO2 substrates by magnetron sputtering method followed by postannealing. By varying the fabrication parameters such as sputtering atmosphere, sputtering power, and annealing atmosphere, single phase garnet films were obtained with different off-stoichiometry and film density. The dependence of cation ratio, magnetic and magneto-optical characteristics, and absorption coefficient were systemically investigated. The results reveal that a proper oxygen pressure in both sputtering and annealing process give rise to a small cation ratio of (Fe2+ or Fe4+)/Fe3+, thus is beneficial to obtain large saturation magnetization, large Faraday rotation, and small optical absorption. The sputtering power can also affect the properties of the film through changing the film density. Our results indicate that the properties of sputtering deposited yttrium iron garnet (YIG) film can be easily tuned and optimized by modifying the off-stoichiometry and density of the film, thus provides flexibility to fabricate YIG film for extensive applications.

Material characterization of superconducting T′-Nd2CuO4 films synthesized by metal organic decomposition

Recently we have achieved superconductivity in T′-RE2CuO4 (RE=Pr, Nd, Sm, Eu, and Gd), films by metal organic decomposition (MOD). In this article, we first report the cation off-stoichiometry effect, which aims at screening out the possibility of hole doping by cation deficiencies. We also investigated the structure and microstructure of superconducting T′-Nd2CuO4 films synthesized by MOD. This investigation aims at elucidating why MOD-grown Nd2CuO4 films become superconducting whereas our previous Nd2CuO4 films grown by molecular beam epitaxy were not superconducting.

Hyperfine magnetic field on iron atoms and Co–Fe disordering in Co2FeSi

The Heusler compound Co2FeSi is a prospective half-metallic material for spintronic applications. Defects and antisite disordering play a crucial role among the factors reducing spin polarization in such materials. To clarify effects of possible off-stoichiometry, a continuous series of model solid solutions Co3−xFexSi (0.6≤x≤1.4) was investigated by F57e Mössbauer spectroscopy and C59o nuclear magnetic resonance. It has been shown that the hyperfine magnetic fields on Fe can be used to monitor Co–Fe disordering in Co2FeSi-based bulk samples and thin films.

Effects of Off-Stoichiometry of LiC6 on the Lithium Diffusion Mechanism and Diffusivity by First Principles Calculations

Lithium diffusion in a stage-1 structure of LiC6 has been theoretically investigated from first principles on the basis of transition state theory. The calculated chemical diffusion coefficient of lithium atoms under the lithium excess conditions is much larger than that under the lithium deficient conditions, e.g., 2 × 10−7 cm2/s vs 1 × 10−10 cm2/s at room temperature. The calculated activation energies of the chemical diffusion coefficients under the lithium excess and deficient conditions are also different, e.g., 0.30 and 0.49 eV, respectively. The fast diffusion under the lithium excess conditions is attributed to the “interstitialcy” mechanism. These results mean that a small deviation of lithium composition from the exact composition of LiC6 causes a large difference in the lithium diffusion coefficient. The off-stoichiometry can be the major reason why the experimental diffusion coefficients and activation energies are widely scattered.

Effect of off-stoichiometry and site disorder on the properties ofNi3Al: II. Magnetics

A detailed comparison between the magnetic behaviours of the ‘as-prepared’ap-NixAl100−x alloyswith x = 74.3, 74.8, 75.1 and 76.1 at.% (that have both compositional disorder and site disorder) and ‘annealed’counterparts (that have only compositional disorder) over a wide range of temperatures and magneticfields (H) permits us to draw the following conclusions about the role ofdisorder. Regardless of the type of disorder, Curie temperature,TC, and spontaneousmagnetization at 0 K, M0, decrease in accordance with the power lawsTC(x) = tx(x−xc)τ and M0(x) = mx(x−xc)ψ as (the threshold Ni concentration below which the long-rangeferromagnetic order ceases to exist). Site disorder lowers the value ofxc by nearly 1 at.%Ni, enhances TC for a given composition (more so as ) by increasing the number of Ni nearest neighbours for a given Ni atom, and leavesM0 essentially unaltered because site disorder has essentially no effect on the density of states,N(EF), at theFermi level, EF, and the shape of the density-of-states curve nearEF (exceptfor x≈xc, where site disorder tends to primarily enhanceN(EF) and thereby stabilize long-range ferromagnetic order for Ni concentrations below thethreshold concentration, at.%, dictated by compositional disorder). At low and intermediate temperatures, spontaneous magnetization,M(T,H = 0), as well as the ‘in-field’magnetization, M(T,H), exhibit non-Fermi liquid behaviour in the samplesap-Ni74.3 andap-Ni74.8.As xc is approached from above, i.e. as the compositional disorder increases, strongerdeviations from the Fermi liquid behaviour occur and the temperature rangeover which the non-Fermi liquid behaviour persists widens. In contrast, theap-Ni75.1 and ap-Ni76.1 alloys follow the behaviour that the self-consistent spin-fluctuation theory predicts for aweak itinerant-electron ferromagnet with no disorder. Both compositional disorderand site disorder have no effect on the critical behaviour of the alloys near theferromagnetic-to-paramagnetic phase transition.

Effect of off-stoichiometry and site disorder on the properties ofNi3Al: I. Electrical and magneto-transport

Electrical resistivity, ρ(T), and longitudinal magnetoresistance, , of ‘as-prepared’ NixAl100−x alloyswith x = 74.3, 74.8, 75.1 and76.1 at.% and ‘annealed’ Ni75.1Al24.9 alloy, measured over wide ranges of temperature and external magnetic field(H), are discussed in the light of existing theoretical models.ρ(T) exhibits a non-Fermi liquid (NFL) behaviour at low temperatures in the range1.7 K≤T≤Tx (where Tx decreases from 25 to 21 K as the Ni concentrationx increases from 74.3 to 75.1 at.%) in the alloys withx<76.1 at.%. Compositional disorder (particularly under-stoichiometry) gives rise tostronger deviations from the Fermi liquid behaviour and widens the temperaturerange over which the NFL behaviour persists whereas site disorder makes theNFL behaviour more prominent, particularly in the stoichiometric composition(Ni3Al), and stabilizes the NFL behaviour in any given compositionover a much wider temperature range. The main contributions toρ(T) and arise from the scattering of conduction electrons from the ‘unconventional’ spinwaves (exchange-enhanced non-propagating spin-density fluctuations) at lowtemperatures (intermediate temperatures and temperatures close to the Curie point,TC). The self-consistent spin fluctuation theory correctly predicts thatH leaves the functionaldependence of ρ on temperature unaltered and quantitatively describes the suppressionof the spin-wave and spin-density fluctuation contributions toρ by H in different temperature regimes.

Effect of off stoichiometry on Raman scattering from epitaxial and polycrystalline HfNx (0.85≤x≤1.50) grown on MgO(001)

Group-IV HfNx transition-metal nitride layers, with 0.85≤x≤1.50, are grown on MgO(001) and analyzed by Raman spectroscopy. Samples with 0.85≤x≤1.20 are single-phase epitaxial NaCl-structure HfNx(001), while layers with higher N concentrations are two-phase mixtures containing N-rich inclusions. All samples exhibit both first- and second-order Raman scattering. The presence of first-order scattering indicates that Oh symmetry, a characteristic of the NaCl crystal structure, is broken even in stoichiometric HfN. As x decreases in understoichiometric HfNx samples, corresponding to a decrease in the number of valence electrons per unit cell from nine (x=1) to eight (x=0.75), the positions of the acoustic lines shift to higher frequencies. This provides an indirect probe of phonon anomalies in acoustic mode dispersion curves and signifies an increase in N vacancy concentration. The persistence of strong first-order acoustic scattering from overstoichiometric (x&amp;gt;1) samples indicates that adding more N does not substantially reduce the average defect density around Hf sites. The absence of a frequency shift in the acoustic modes of overstoichiometric HfNx, with 1.17≤x≤1.27, also shows that the local order around Hf sites does not change significantly, while the observed shift of the first-order optical modes to higher frequency with increasing x is characteristic of increasing lattice disorder adjacent to N sites. The Raman results, together with the nature of the variation in the HfNx lattice parameter, the electron carrier concentration, and the superconducting transition temperature as a function of x, can be explained by the presence of N vacancies for x&amp;lt;1, both N and Hf vacancies in stoichiometric HfN(001), and the combination of N vacancies and NHf antisite defects for x&amp;gt;1.

Effect of off-stoichiometry on the mobility of point-like defects and damping in binary Cu–Al martensites

An attempt is undertaken to define a factor, apart from temperature, which allows for the mobility of quenched-in defects in a Cu–Al system of martensitic alloys. Experiments were performed with single-and polycrystalline alloys with nominal Al content ranging from 10 to 13.5 wt.%. An acoustic technique, operating for temperatures ranging from ambient down to 10 K, was applied to detect the existence of mobile quenched-in defects. No mobility of quenched-in defects was detected for either single-or polycrystalline alloys with Al contents close to and below the Cu 3Al stoichiometric one (Cu–12.4 wt.% Al), corresponding to perfect DO 3 ordering. For the alloy Cu–13 wt.% Al, mobility of quenched-in defects was detected starting from temperature of around 70 K. It is suggested that the off-stoichiometry of the alloys is accommodated through anti-site defect formation on the Al-deficient side and through vacancy formation on the Al-rich side of the phase diagram. The composition dependence of damping of Cu–Al martensites exhibits a sharp maximum around the stoichiometric composition Cu 3Al (Cu–12.4 wt.% Al), which is presumably formed by variations of ordered domain size and point defect concentration on both sides of the stoichiometry.

Synthesis and characterization of CuFeO 2+ δ delafossite powders

Effects of oxygen off-stoichiometry have been investigated on CuFeO 2+ δ delafossite-type powders prepared by solid state reaction. Materials annealed at different temperatures under air atmosphere were compared. Thermogravimetric analysis coupled with X-ray structural analysis showed that a powdered CuFeO 2 can be oxidized up to CuFeO 2+ δ with δ = 0.18. Electrical measurements confirm the existence of oxidized CuFeO 2+ δ phases. Particular attention is paid to the correlation between oxidation process and phase stabilities in the CuFeO 2 and copper spinel phase system.

Analysis of Cu(InGa)Se2Alloy Film Optical Properties and the Effect of Cu Off-Stoichiometry

AbstractVariable angle spectroscopic ellipsometry has been used to characterize Cu(InGa)Se2 thin films as a function of relative Ga content and to study the effects of Cu off-stoichiometry. Uniform Cu(InGa)Se2films were deposited on Mo-coated soda lime glass substrates by elemental evaporation with a wide range of relative Cu and Ga concentrations. Optical constants of Cu(InGa)Se2were determined over the energy range of 0.75–C4.6 eV for films with 0 ≤ Ga/(In+Ga) ≤ 1 and used to determine electronic transition energies. Further, the changes in the optical constants and electronic transitions as a function of Cu off-stoichiometry were determined in Cu-In-Ga-Se films with Cu atomic concentration varying from 10 to 25 % and Ga/(In+Ga) = 0.3. Films with Cu in the range 16–24 % are expected to contain 2 phases so an effective medium approximation is used to model the data. This enables the relative volume fractions of the two phases, and hence composition, to be determined. Two distinctive features are observed in the optical spectra as the Cu concentration decreases. First, the fundamental bandgaps are shifted to higher energies. Second, the critical point features at higher energies become broader suggesting degradation of the crystalline quality of the material.

Concentration of point defects in binary NiAl

A mean-field-type model was established to describe the concentration of point defects in binary NiAl by taking into account the effects of crystal expansion and off-stoichiometry. Different from some recent studies, this analysis shows that constitutional vacancies are present only in Al-rich NiAl at T=0 K but that the vacancy concentration decreases with increasing temperature until diffusion is activated and thereafter becomes nearly independent of temperature. Such vacancy formation behaviour in Al-rich NiAl belongs to the category of metastable constitutional type according to a method of classifying vacancy formation in B2 crystals proposed in this letter.

Investigation on Additional Hardening Mechanisms in Off-Stoichiometric Al-Rich Ni3Al Alloys in Terms of Anti-Site Defect Configuration

ABSTRACTOne of the unsettled issues of the mechanical properties of L12-Ni3Al compound is the effect of off-stoichiometry on the 0.2% flow stress at low temperature region. 0.2% flow stress at 77 K increases as deviating from the stoichiometric composition to both Ni-rich and Al-rich sides, but in Al-rich side the increasing rate is higher. According to the XRD results for the binary Ni3Al alloys, however, the composition dependence of the lattice constant is the same in both sides of the stoichiometric composition, which implies that the magnitude of lattice strain induced at off-stoichiometric composition is symmetrical. And it is not rational to associate the strengthening mechanism to a sole effect of isotropic strain due to an atomic size difference. In this study, attempts are made to explain the additional strengthening in Al-rich alloys in terms of the tetragonal distortion hardening mechanism, which arises from anisotropic atomic configuration around an anti-site Al atom.

Effect of Al-rich off-stoichiometry on superplastic flow stress in fine-grained Ni 3Al alloys

Effect of Al-rich off-stoichiometry on superplastic flow stress in fine-grained Ni 3Al alloys

Effect of off-stoichiometry on the transport properties of the Heusler-typeFe2VAlcompound

We report on the effect of off-stoichiometry on the temperature dependence of electrical resistivity, the Seebeck coefficient, and the Hall coefficient in the Heusler-type ${\mathrm{Fe}}_{2}\mathrm{VAl}$ compound. While the stoichiometric ${\mathrm{Fe}}_{2}\mathrm{VAl}$ exhibits a semiconductorlike resistivity behavior, a small deviation of the Al content from stoichiometry causes a significant decrease in the low-temperature resistivity and a large enhancement in the Seebeck coefficient. Substantial enhancements for the Seebeck coefficient are in reasonable accord with changes in the Hall coefficient and can be explained on the basis of the electronic structure, where the Fermi level shifts slightly from the center of a pseudogap due to off stoichiometry.

Order-disorder phase transformations and specific heat of nonstoichiometric vanadium carbide

AbstractA study has been made of the order-disorder phase transformations in the homogeneity region of the VCy nonstoichiometric cubic vanadium carbide (0.66<y<0.88). It has been established that an ordered V6C5 phase with monoclinic (space group C2/m) or trigonal (P31) symmetry, and a cubic (space group P4332) ordered V8C7 phase can form in the VCy carbide below 1450 K, depending on the actual composition. The effect of off-stoichiometry and structural vacancy ordering on the specific heat of the VCy carbide has been investigated. The temperatures and heats of the reversible order-disorder equilibrium transitions have been determined. The ordering in the VCy carbide is shown to be a first-order phase transition. An equilibrium diagram of the V-C system taking into account ordering in the nonstoichiometric vanadium carbide has been constructed.