Lanthanum Substitution Research Articles

Methane Activation On Oxyhalogenated Rare-Earth Systems : Effect Of Pr3+ Substitution In Laocl Host Structure On Co2 FORMATION

Catalytic activity of LnOX (Ln = Sm, La or Nd; X = Cl or Br) and Ln1-xLmxOX (Ln = La and Lm = Pr, 0 ≤ x ≤ 1) catalysts has been studied using a continuous flow reactor system. Methane oxidation using LnOX results in an increase of CO2 selectivity in the order Nd > La > Pr > Sm, while using Ln1-xLmxOX systems CO2 selectivity is enhanced by increasing partial lanthanum substitution with praseodymium. A decrease in CO and C2 hydrocarbons selectivities is also observed at isoconversion. The different compounds structures studied do not change with time under reaction conditions within the temperature range at which the catalysts are stable.

Electronic Structure of Pristine and Solute‐Incorporated SrTiO3: III, Perfect‐Crystal, Grain‐Boundary Geometry, and Acceptor Doping

Electronic structure is investigated for donor‐impurity‐incorporated perfect‐crystal and 36.8° symmetric tilt Sigma5 (310) grain‐boundary geometries of SrTiO3. The relaxed model of the atomic structure of the grain boundary used in the present investigations is the same as that used in Part II, as derived by Ravikumar et al. using lattice‐statics simulations based on pair‐potential calculations. As in Part II, the methodology of one‐electron first‐principle cluster calculations, which is discussed in Part I, is extended to clusters with single‐donor impurity substitutions at the central titanium and strontium sites. The effects of niobium substitution at a titanium site and lanthanum substitution at a strontium site in the bulk and at the grain‐boundary core have been investigated by determining the aspects of the electronic structure discussed for the acceptors. The influence of grain‐boundary crystallography on donor impurity incorporation has been evaluated in terms of variations in densities of states, spatial charge densities, and charge populations at the grain boundary. As in Parts I and II, no additional local lattice relaxations around the impurity are considered for the impurity‐incorporated clusters. Donor compensation mechanisms reported in the literature are discussed in connection with the electrical activity of the material. The calculations reveal that, in perfect‐crystal geometry, although lanthanum exhibits almost the expected donor behavior at the strontium site, niobium does not exhibit very good donor behavior at the titanium site. Moreover, a decrease in the donor behavior of these impurities is observed in the grain‐boundary geometry. Such variations in the electronic behavior of donors are due to the crystallographic variations at the grain boundary and are likely to decrease the grain‐boundary‐charge and associated space‐charge effects in the presence of donors as compared with the increase of such effects in the presence of acceptors. Consequently, a decrease in space‐charge‐induced segregation of donors at grain boundaries and the effects thereof can be expected.

Synthesis and surface characterization of nanometric La 1− xK xMnO 3+ δ particles

The use of a precursor-based synthetic method, namely, the freeze-drying of acetic acid solutions, allows the preparation of single-phased perovskites in the series La 1− x K x MnO 3+ δ ( x≤0.15) at temperatures as low as 873 K. This soft treatment of the precursors yields products with high specific surface areas (20–26 m 2 g −1), constituted by uniform nanometric particles (30–50 nm), with their surface enriched in K and Mn. Samples prepared at 1073 K present lower specific surface areas (7–14 m 2 g −1), and show no K- and Mn-enrichment for x≤0.15. However, in samples with x=0.20 and 0.25, the presence of a secondary phase, K 2Mn 4O 8, is detected by X-ray diffraction, as well as a surface enrichment in K and Mn. These perovskite materials were found to be active for the catalytic combustion of ethane at low temperatures (573–648 K). As a general trend, the substitution of lanthanum by potassium decreases the areal rate, and for the more substituted samples ( x>0.10), it increases the selectivity to ethene.

Czochralski growth of RE 3Ga 5SiO 14 (RE=La, Pr, Nd) single crystals for the analysis of the influence of rare earth substitution on piezoelectricity

Pr 3Ga 5SiO 14 and Nd 3Ga 5SiO 14 single crystals with constant diameter of 22 mm and lengths up to 145 mm have been grown by the Czochralski method. The phase identification, site occupancy of cations and axial lattice parameter distribution were determined by X-ray analysis. The transmission spectra within the 340–3200 nm wavelength region were measured. The centre of interest are the piezoelectric properties of (2 1̄ 0) and (0 1 0) plates in comparison with former grown La 3Ga 5SiO 14 crystals in order to find out the influence of the rare earth substitution of La 3+ by Pr 3+ and Nd 3+ on the piezoelectric strain constant d 11. A decrease of | d 11| with increasing atomic number was found giving the hint to the substitution of lanthanum by further elements with larger atomic radii.

F -Electron delocalization/localization and the abrupt disappearance of uranium magnetic ordering with dilution alloying

We have applied the model and technique previously applied to the change of Curie temperature with pressure for correlated-electron uranium systems to predict the change in Curie temperature and ordered moment with dilution alloying. The theory is remarkably successful in its predictions, and this success has important implications for the overall understanding of magnetic ordering in correlated-electron systems including heavy fermion systems. For US, the dilution alloying behavior found experimentally is dramatic. In UxLa1−xS, the magnetic ordering abruptly disappears at about 55% uranium. Our ab initio-based theory quantitatively predicts this abrupt disappearance while also quantitatively predicting the monotonic decrease of Curie temperature with pressure for undiluted US. In addition, in agreement with experiment, the theory predicts the correct trend for the magnetic ordering to disappear with dilution, while at the same time absolutely and quantitatively predicting the nonmonotonic variation of Curie temperature with pressure, for USe and UTe. The ab initio-based model gives absolute material-specific predictions using input from the local density approximation paramagnetic uranium f-electron-projected density of states plus ab initio calculated values of the correlation energy U. The key physics of the model is the recognition and quantification of the concept that the f spectral density in the vicinity of a specific uranium nucleus (so-to-speak in the muffin-tin sphere) can either be in a stable f3 configuration for a long enough period of time that, through coupling to other such stable f3 sites, it can magnetically order, or can be in a situation such that the configuration fluctuates rapidly between f3 and f2, and for purposes of magnetic ordering acts like a hole in the f-electron lattice in the same way that substitution of lanthanum for uranium creates a hole. Both pressure and dilution alloying, by causing an increase in f-delocalization, increase the fraction of uranium sites in the rapidly fluctuating, magnetically ineffective, condition. For dilution alloying, at a certain point this increase in fluctuations causes the stable f3 component to fall abruptly below a critical value necessary to sustain any magnetic ordering, and hence brings about a catastrophic collapse in magnetic ordering.

Coexistence of Relaxor and Normal Ferroelectric Phases in Morphotropic Phase Boundary Compositions of Lanthanum‐Modified Lead Zirconate Titanate

Hot‐stage transmission electron microscopy studies of lanthanum‐modified lead zirconate titanate (Pb1‐xLax‐(Zr0.53Ti0.47)1‐x/4O3 (PLZT 100x/53/47) have been performed for compositions close to the morphotropic phase boundary (MPB). These studies have revealed the coexistence of relaxor and normal ferroelectric phases. Lanthanum substitution disrupts long‐range ferroelectric order, resulting in short‐range polar ordering. Investigations also have been performed using dielectric spectroscopy, electrically induced polarization, and strain methods. Three phases have been found for 10/53/47: a low‐temperature tetragonal phase, a high‐temperature cubic phase, and an intermediate‐temperature pseudocubic phase that exhibits relaxor ferroelectric behavior. Induced strain studies of 10/53/47 show a gradual change from butterfly‐shaped to slim‐quadratic loops with increasing temperature. In addition, the remanent strain decreases with increasing temperature, and reversible strain shows a peak at 80°C that corresponds to the appearance of intermediate phase.

123 + CuO → 124 conversion slowed by lanthanum substitution

The rate of formation of YBa 2Cu 4O 8 (124) by solid state reaction of YBa 2Cu 3O 7−δ (123) with CuO is dramatically slowed by substitution of La 3+ at Ba 2+ sites. This is not an ion size effect, since substitution of Sr 2+ for Ba 2+ does not reduce the 123 + CuO → 124 conversion rate. The reaction kinetics have been studied as a function of La concentration and oxygen pressure. Higher oxygen pressure enhances the formation rate of 124. This is expected because conversion to 124 involves oxygen uptake since 123 is oxygen deficient at the reaction temperature. The reverse reaction 124 → 123 + CuO has been studied by thermogravimetry; the rate of this decomposition reaction remains unaffected by La substitution.

High Energy Ferrite Magnets

Lanthanum and zinc substitution in M-type hexaferrite was studied in detail in order to increase the saturation magnetization. For the composition Sr 0.7 La 0.3 Fe 11.7 Zn 0.3 O 19, the saturation magnetization was increased by more than 4% and the uniaxial anisotropy constant (K 1 ) was almost the same as that of non-substituted Ba-ferrite. Furthermore, high energy ferrite magnets having more than 40kJ/m 3 (5MGOe) were obtained for the first time by the new process for high orientation, high density and submicron-sized grains.

Possibilities of NOx and CHx determination using galvanic cells with perovskite-electrodes on YSZ

Perovskite-type mixed oxides with the formula La1−xAxMe1−yByO3±δ (x=0.01...0.2; y=0.01...0.5; A=Ca, Sr; Me=Cr, Mn, Fe, Co; B=Mg, Ga) were investigated with the aim to use the oxides as electrode materials for galvanic cells with Y2O3-stabilized ZrO2 solid electrolytes (YSZ). The catalytic activity of the oxides for the oxidation, reduction and decomposition of gas components was varied by changing the transition metal and by partial substitution of lanthanum and the metal.

On the crystal structure of the “1201” superconductors Hg 0.5Bi 0.5Sr 2− xLa xCuO 5−δ

Two compositions in the “1201” system Hg 0.5Bi 0.5Sr 2− xLa x CuO 5−δ, x = 0 and x = 0.5, have been studied by neutron diffraction, electron microscopy and X-ray absorption spectroscopy. Neutron diffraction gives evidence for a high disordering of the oxygen located in the Hg/Bi plane which is fully oxygenated. For the lanthanum-free sample, a slight oxygen deficiency is observed with oxygen vacancies (0.08 per cell) located in the basal plane of the copper octahedra. This feature is connected to the appearance of local superstructures observed by electron diffraction and high-resolution microscopy which tends to disappear after oxygen annealing and are enlarged after argon annealing. XANES spectroscopy at Bi L 3 edge shows a valence state close to +5 for bismuth in the lanthanum-free sample which is slightly reduced when lanthanum substitutes for strontium. In the same way copper K edge spectra show that lanthanum substitution leads to a decrease of the copper valency. The connection with superconductivity is discussed.

Effect of the rare earth substitution on the optical phonons of LaRBaCuO 5 (R=Nd and Eu) oxides

In this work we present, on the one hand, the effect in the Raman spectra is produced by the partial substitution of lanthanum by Nd or Eu in LaRBaCuO 5. Because the atomic masses and ionic radii of these ions are quite similar the frequencies of the phonons principally related to them are not expected to vary significantly. In fact only small changes are observed in this direction. Nevertheless, substantial changes are observed in the intensities of the low frequency modes. The frequencies of the oxygen vibrations are also influenced by the distortion produced by the rare earth substitution. On the other hand, we report the IR reflectivity spectra of polycrystalline samples of pure and mixed compounds. The lowest frequency modes present small damping which permits us to observe variations in their frequencies corresponding to the different compounds. The effect of the quality of the pressed powder on the IR reflectivity spectra is discussed.

Electrical properties of BaSnO3 in substitution of antimony for tin and lanthanum for barium

Polycrystalline materials of BaSn1−xSbxO3−δ and Ba1−yLaySnO3−δ were prepared. Substitutional solubilities of antimony for tin and lanthanum for barium, respectively, in BaSnO3 were obtained to be x=0.18 for BaSn1−xSbxO3−δ and y<0.052 for Ba1-yLaySnO3−δ. The X-ray photoemission spectroscopy measurements showed the valence of antimony and tin is mixed in our samples of BaSn1−xSbxO3−δ. At lower temperature, magnetic susceptibilities of BaSn1−xSbxO3−δ and Ba1−yLaySnO3−δ satisfy the Curie law, indicating the existence of non-interacting localized electrons at the Sn4+ site, and forming a Sn4++e− state in these systems. By substitution of antimony and lanthanum in BaSnO3, the conductive properties are semiconductor-like. To explain this conductive behaviour, three types of mechanism were taken into consideration.

The thermal expansion and electrical resistivity of the pseudobinary compound Ce 1- xLa xAl 3

Temperature dependence of the thermal expansion coefficient α and the electrical resistivity ρ of Ce 1- x La x Al 3 compounds has been measured for various lanthanum concentrations x. It is found that the temperature dependence of α and ρ is influenced largely by substitution of lanthanum. The results are briefly discussed in connection with the Kondo effect and crystalline electric field splitting of the 4f level of the Ce 3+ ion.

Effect of lanthanum substitution at the barium site on the mechanical behaviour in YBCO superconductor

The lanthanum substituted superconducting oxide system YBa 2−xLa xCu 3O 7−δ is prepared by ceramic method. The compacted and sintered pellets containing different percentages of La (x = 0.0→0.5) are indented by a 120° diamond cone. The effect of La substitution on superconducting and bulk mechanical properties as well as the deformation behavior of the material are investigated.

Magnetostriction of LaCô 13-based alloys

The magnetostriction, Curie temperature, phase composition and crystal lattice parameters in La 1−xR xCo 13 (RPr, Sm, Tb) and LaCo 11M 2 (M  Fe, Ni, Al, Si) were measured. The introduction of a lanthanum excess relative to the LaCo 13 stoichiometry was found to cause an increase in the Curie temperature by more than 50 K, which indicates the presence of a homogeneity region in LaCo 13 alloy. The room temperature saturation magnetostriction is ( λ | - λ ⊥) 8=100 × 10 −6 and weakly changes in the homogeneity region. Substitution of lanthanum by praseodymium and terbium allows one to increase the magnetostriction slightly, but this is accompanied by a decrease in magnetostriction susceptibility dλ dH . Introduction of samarium, iron, nickel, silicon and aluminium leads to a decrease in magnetostriction.

Effects of substitution in perovskites La 2− xSr xNiO 4−λ on their catalytic action for the nitric oxide+carbon monoxide reaction

The catalytic activity of the perovskite series La 2− x Sr x NiO 4−γ ( x = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) has been studied for the reduction of nitric oxide by carbon monoxide in equimolecular mixture. The conversion ( X%) of reactants approaches the value X NO/ X CO = 2 at low temperatures and low degree of substitution of lanthanum by strontium but tends to unity at high temperatures and high degree of substitution. This behaviour is due to the change of the rate determining step of the reaction which at low temperature is the oxygen formation through decomposition of nitric oxide while at high temperatures it is the reaction between the adsorbed oxygen species and carbon monoxide. The apparent activation energies for the nitric oxide elimination drop with the process of substitution from x = 0.00 to x = 1.00 and increase thereafter. The controlling factor of the catalytic activity of the series seems to be the strength of adsorption of oxygen on the perovskite surface. Thus it is shown that the sum of the enthalpies of the metal-oxygen bonds in the material is closely related to the temperature where reaction switches mechanism. Besides, the differential mean electronegativity of the solids is related both to the reaction rate as well as to the apparent activation energy for nitric oxide reduction.

Substitution effects on mixed valence in ternary intermetallics

We report here the changes in the europium valence in Eu 1−xLn xPd 2Si 2 (LnLa, Y, Yb; 0< x-<0.95) as studied by L III absorption edge spectroscopy. On the basis of the simple unit-cell volume considerations and from the fact that the unit-cell volume of EuPd 2Si 2 is intermediate between that of LaPd 2Si 2 and Y(Yb)Pd 2Si 2, one expects the europium valence to vary in both cases as a function of x, with the variation taking place in the opposite directions in the two pseudoternaries Eu 1−x La xPd 2Si 2 and Eu 1−xY(Yb) xPd 2Si 2. However, we find that the valence does not change as a consequence of lanthanum substitution and increases with the addition of yttrium (ytterbium). These results are discussed in terms of the local environment of europium atoms in these systems.

Dielectric properties of (Ba, Sr)O-(Sm, La)2O3-TiO2 ceramics at microwave frequencies

The dielectric properties of (Ba, Sr)O-(Sm, La)2O3-TiO2 material at microwave frequencies were investigated. By varying the amount of strontium from 0–25 mol% in the 0.15(Ba1−xSrx)O-0.15Sm2O3-0.7TiO2 composition, it was possible to adjust the frequency temperature coefficient, τf, from −13 p.p.m. °C−1 to + 30 p.p.m. °C−1. When 7 mol% Sr was substituted for barium, τf=0 p.p.m. °C−1 was obtained. TiO2 with rutile phase (τf≈400 p.p.m. °C−1) acted as a dominant element in τf variation of the 0.15(Ba1−xSrx)O-0.15(Sm1−yLay)2O3-0.7TiO2 (0⩽x⩽0.25, 0⩽y⩽0.6) system. Additionally, increasing the quantity of lanthanum substitution for samarium had a greater positive effect on τf than strontium substitution for barium. When 60 mol% La was substituted for samarium with 7mol% Sr substitution barium, τf of the system reached 95 p.p.m. °C−1. The effect on microwave dielectric characteristics of the 0.15(Ba0.93Sr0.07)O-0.15Sm2O3-0.7TiO2 (BSST) ceramics by varying the calcination and/or sintering conditions or doping additives, were studied. The added SnO2 acted as a firing agent to lower the sintering temperature, and the dielectricQ(Qd) value was improved by properly adding CdO. With 1 wt% CdO addition, the highestQd value of the BSST resonator, after calcination at 1100 °C/2 h and sintering at 1370 °C/4 h, reached 4180 at 4 GHz with a small τf of −4 p.p.m. °C−1 and an ɛr of 80.7 was obtained.

Anomalous transverse relaxation of Cu NQR in YBa 2Cu 3O 7-δ

To obtain information on the anomalous peal of Cu NQR transverse relaxation rate found at 35 K in the air annealed YBa 2Cu 3O 7−δ, the carrier concentration was changed by oxygen anneal and Lanthanum substitution for Barium. Carrier injection by the oxygen-anneal has been found to suppress the peak at 35 K, instead, to produce another small peak at 50 K. Five percent substitution of Lanthanum for Barium with the oxygen-anneal, which could reduce the carrier concentration, has reproduced an overall feature found in the air-annealed sample including the peak at 35 K. These observations suggest that the carrier concentration would concern with the anomalous peak, but dynamics of oxygen vacancy in the Cu(1) chain would not do.

Magnetic studies on (U, La)O 2+ x solid solutions : II. Magnetic susceptibilities of solid solutions with high lanthanum and oxygen concentrations

Magnetic susceptibilities of fluorite-type La y U 1− y O 2+ x solid solutions with various x and y values were measured from 1.8 K to room temperature. With increasing lanthanum and oxygen concentrations, the effective magnetic moment of uranium decreased, and the uranium ions were found to be oxidized from +4 to +5 to +6. The effect of lanthanum substitution for uranium and oxygen nonstoichiometry on the variation of effective magnetic moment and the Néel temperature was discussed.