La 3d Research Articles

Resonant photoemission of LaNiO3-δthin films

Thin LaNiO3−δ films with pseudocubic (100) preferred orientation were prepared by reactive DC magnetron sputtering and annealed at ultra high vacuum above dehydration temperature. The resonant photoemission spectroscopy was used to study the surface composition and electronic structure under La 4d→4f and Ni 3p→3d photoexcitation. The resonance features observed in core level and valence band spectra under La 4d→4f transition were explained in terms of autoionization process and lanthanum–oxygen valence band states’ hybridization. No resonant features were observed in the valence band spectra under Ni 3p→3d excitation indicating that nickel species are not present at the LaNiO3−δ film surface after heat treatment. Keywords: rare earth alloys and compounds, oxide materials, resonant photoelectron spectroscopy

Surface stability of epitaxial LaNiO3-δthin films

Thin LaNiO 3−δ films with pseudocubic (100) preferred orientation were prepared by reactive DC magnetron sputtering and in situ annealed in O2 and vacuum. X-ray photoelectron spectroscopy (XPS) was used to determine the variation in composition of the films under high temperature annealing. The experimental O 1s and La 3d – Ni 2p3/2 spectra of LaNiO3−δ films was analysed in terms of O2−, O−/(OH)−, and weakly adsorbed oxygen species. It was shown that the change in the type of conductivity from metallic to semiconducting one is accompanied by a marked increase in the intensity of the lateral (~531 eV) peak of oxygen. The quantitative analyses of La 3d – Ni 2p3/2 spectra show that the Ni/La concentration ratio significantly decreases after heating above the dehydration temperature. These variations in conductivity and surface composition were attributed to the loss of lattice oxygen with subsequent adsorption of O− and (OH)− anions and weakly adsorbed oxygen species from ambient air. Keywords: rare earth alloys and compounds, oxide materials, XPS, surfaces and interfaces, LaNiO3 thin film

La2Cu0.8Co0.2O4+δ by Pechini Method

Cobalt-doped lanthanum cuprates of the type La2Cu1-xCoxO4 are Ruddlesden-Popper compounds characterized by several interesting properties. In particular, thanks to the presence of copper and cobalt, they can be utilized as catalysts in redox reactions. Moreover, from an electrical point of view, cobalt-doped lanthanum cuprates are Mixed-Electronic-Ionic-Conductors. These important characteristics could be gainfully employed in improving the Intermediate-Temperature-Solid-Oxide-Fuel-Cells electrodes performances, from both the catalytic and electrical sides. A La2.0Cu0.8Co0.2O4+δpowder was prepared by Pechini procedure. Besides the wide scan spectra, detailed spectra for the La 3d, La 4d, Cu 2p, Co 2p, O 1s and C 1s regions and related data are presented and discussed. XP analysis confirms the presence of La (III) and Cu (II); the peak shapes are consistent with a residual presence of hydroxide and carbonate species, as confirmed by the O 1s and C 1s signals. The XPS atomic composition supports this hypothesis: in fact the O/(La+Cu+Co) atomic ratio (= 1.86) is higher than the expected value (= 1.33). Also the La/(Cu+Co) atomic ratio (= 0.80) is higher than the nominal value (= 0.66) and suggests La surface segregation.

Influence of Sr and Fe Dopants on the Surface Properties of LaGaO3

LaGaO3 doped on A-site and B-site with different amounts of strontium and iron, respectively (La0.8Sr0.2GaO3-δ, La0.6Sr0.4GaO3-δ and LaGa0.8Fe0.2O3, LaGa0.4Fe0.6O3) as well as a doubly doped LaGaO3 perovskite (La0.8Sr0.2Ga0.8Fe0.2O3-δ) have been synthesized by citrate gel method. The present work focuses on the XPS analysis. In particular, the effects of dopants and dopant concentrations on the LaGaO3 surface properties have been investigated and discussed. Besides the wide scan spectrum, detailed spectra for the La 3d, Ga 2p3/2, Sr 3d, Fe 2p, O 1s and C 1s regions and related data are presented and discussed.

La0.6Sr0.4Fe0.6Co0.2Cu0.2O3-δ Powders by XPS

In recent years, the growing interest in hydrogen energy has caused particularly large demand for catalysts used in the production of hydrogen and hydrogen-containing gases by the conversion of natural hydrocarbons. Perovskite oxides having the general formula of ABO3, where B usually designates a transition metal cation surrounded by six oxygen ions in octahedral coordination, and A is a cation of rare-earth metal coordinated by 12 oxygen ions are very promising for this application. The present work focuses on the XPS analysis of a new perovskite material: La0.6Sr0.4Fe0.6Co0.2Cu0.2O3-δ powder. This powder was obtained by citrate gel process. The surface proprieties are investigated by means of XPS. Besides the wide scan spectrum, detailed spectra for the La 3d, Sr 3d, Fe 2p, Co 2p, Cu 2p, Cu LMM, O 1s and C 1s regions and related data are presented and discussed.

LaMnO3: Influence of the Addition of Ba and Sr

The present work focuses on lanthanum manganite-based materials obtained by citrate (LaMnO3 and La0.6Sr0.4MnO3) and Pechini (La0.6Ba0.4MnO3) methods. The influence of the addition of barium and strontium is studied. The surface properties of LaMnO3, La0.6Ba0.4MnO3 and La0.6Sr0.4MnO3 powders are investigated by means of XPS using a standard Al Kα source. Besides the wide scan spectra, detailed spectra for the La 3d, Mn 2p, O 1s, Sr 3d and Ba 3d regions and related data are presented and discussed.

Effect of hydrogenation on the electronic state of metallic La hydrides probed by X-ray absorption spectroscopy at the La L-edges

We have investigated the effect of hydrogenation on La 5d and 6p electronic states in metallic LaHx (x = 0.0, 2.0, 2.3, and 2.6) by X-ray absorption near edge structure at the La L-edges. As the hydrogen content x increases from 0 to 2.6, white-line intensity at the La L2,3-edges shows a remarkable increase in the range of x>2.0. This is interpreted as the increase in La 5d hole induced by interstitial H atoms on the octahedral sites. On the other hand, the shoulder structure at the La L1-edge disappears in the process of x = 0.0rightarrow;2.0, indicating that the p-d hybridization is weakened by H atoms on the tetrahedral sites. This study demonstrates that H atoms on the two interstitial H sites provide different contribution to the modification of the electronic states.

Electronic structure of lanthanum transition-metal oxyarsenides La MAsO ( M = Fe, Co, Ni) and LaFe 1− xM′ xAsO ( M′ = Co, Ni) by X-ray photoelectron and absorption spectroscopy

The electronic structures of the quaternary oxyarsenides La MAsO ( M = Fe, Co, Ni) were examined with X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Interpretation of the metal 2p 3/2 and arsenic 3d 5/2 binding energies, as well as a satellite feature in the Co 2p XPS spectrum, suggests charges that are much less extreme than expected (i.e., not M 2+ and As 3−) because of the strong covalent character within the M–As bonds. As M is varied, the differing degrees of charge transfer from M to As atoms within these bonds are manifested by shifts in the As 3d 5/2 binding energies and changes in the As K-edge intensities. This charge transfer is isolated within the [ MAs] layer and does not influence the O 1s and La 3d XPS spectra. Fitting the experimental valence band spectra of these oxyarsenides La MAsO yielded electron populations of states that support the formal charge assignment [La 3+O 2−][ M 2+As 3−]. The mixed-metal series LaFe 1− x M′ xAsO ( M′ = Co, Ni) was examined by XANES; analysis of the metal K- and L-edges, as well as of the Co 2p XPS satellite feature, revealed that no metal–metal charge transfer takes place.

Electronic structures and Hall effect in low-doped La0.9Hf0.1MnO3 epitaxial films

The electronic structures of low-doped epitaxial La0.9Hf0.1MnO3 (LHMO) thin films are investigated by x-ray photoemission spectroscopy (XPS) for the first time. XPS spectra of core levels (La 3d, Hf 4f, O 1s, Mn 2p, and Mn 3s) are taken from the cleaned LHMO film surface. Hf 4f spectrum exhibits a typical binding energy (BE=2 eV) of Hf4+. The splitting energy of La 3d core-lever spectrum agrees with the previous reports of other doped LaMnO3 and suggests a trivalent state in the LHMO film. The calculated result of LHMO nominal composition, the shape of Mn 2p spectrum, the separated BE (111.1 eV) between O 1s and Mn 2p2/3 peaks and the splitting energy (6.0 eV) of Mn 3s all reveal that the LHMO compound is in a mixed valence state of Mn2+ and Mn3+, implying an electron-doped conduction mechanism. The magnetic-field dependence of Hall resistivity further confirmed that the carriers in LHMO are electrons.

CuO/La0.6Sr0.4Co0.2Fe0.8O3-δ Powder by XPS

A CuO/La0.6Sr0.4Co0.2Fe0.8O3-δ supported system was obtained by depositing, by wet impregnation, 10 wt% CuO loading on the La0.6Sr0.4Co0.2Fe0.8O3-δ surface. The surface properties are investigated by means of XPS. Besides the wide scan spectrum, detailed spectra for the La 3d, Sr 3d, Co 2p, Fe 2p, Cu 2p, O 1s, and C 1s regions, and related data, are presented and discussed.

Electronic structure and photocatalytic water splitting of lanthanum-doped Bi 2AlNbO 7

Bi 2− x La x AlNbO 7 (0 ≤ x ≤ 0.5) photocatalysts were synthesized by the solid-state reaction method and characterized by powder X-ray diffraction (XRD), infrared (IR) spectra and ultraviolet–visible (UV–vis) spectrophotometer. The band gaps of the photocatalysts were estimated from absorption edge of diffuse reflectance spectra, which were increased by the doping of lanthanum. It was found from the electronic band structure study that orbitals of La 5d, Bi 6p and Nb 4d formed a conduction band at a more positive level than Bi 6p and Nb 4d orbitals, which results in increasing the band gap. Photocatalytic activity for water splitting of Bi 1.8La 0.2AlNbO 7 was about 2 times higher than that of nondoped Bi 2AlNbO 7. The increased photocatalytic activity of La-doped Bi 2AlNbO 7 was discussed in relation to the band structure and the strong absorption of OH groups at the surface of the catalyst.

Structural investigation of the LaAlO 3(110) surface

Chemical and structural characterizations of the LaAlO 3(110) surface was investigated by XPS, XPD, LEED and RHEED. The atomic structure of this ternary alloy, can be viewed as a superposition of LaO and AlO 2 planes in the [001] direction. For clean substrates, a c(4 × 2) surface reconstruction was pointed out by RHEED and LEED. Polar X-ray photoelectron diffraction investigations made in the [− 110] azimuthal direction, for La 3d, O 1s and Al 2score levels reveal well structured diagrams reflecting the atomic distribution of the two kinds of parallel planes in the [− 110] direction and perpendicular to the (110) surface. The main crystallographic directions give strong forward-scattering peaks at 0°, 35° and 54.7° corresponding to the [110], [111] and [112] directions respectively.

First-principles study of structural stabilities and electronic characteristics of Mg–La intermetallic compounds

A first-principles pseudopotential plane-wave method based on density functional theory is used to investigate the crystal and electronic structure of Mg–La intermetallic compounds with different types of structure. The obtained lattice constants a 0 for all intermetallics considered are very close to the corresponding experimental values. The calculation of cohesive energies indicated that the structure stability of Mg–La alloy will become higher with increasing La element independent on crystal structure type. The calculations of formation heats showed that the alloying abilities of Mg 3La, Mg 2La and MgLa were much stronger than Mg 12La and Mg 17La 2, while the Mg 17La 2 alloying ability was slightly higher than Mg 12La alloy. The energy band and the densities of states (DOS) of these intermetallic compounds revealed that the bonding occurred mainly among the valence electrons of Mg 3s, 2p and La 5d, 4f orbits, and the discrepancy in the stability of Mg–La intermetallic compounds could be attributed to the variation of bonding electron numbers at low-energy region of Fermi level.

Electronic structure of [formula omitted

The electronic band structure of LaRh 2 Si 2 has been calculated using the self-consistent full potential nonorthogonal local orbital minimum basis scheme based on the density functional theory within local density approximation. The La 5d and Rh 4d states play dominant roles near the Fermi level. The three different Fermi surfaces are made of the Rh 4d and La 5d states. Our fixed spin moment calculations of the value S ∼ 2.2 does not indicate any ferromagnetic instability of the paramagnetic ground state.

A study of transition metal K-edge x-ray absorption spectra ofLaBO3 (B = Mn, Fe,Co, Ni), La2CuO4, and SrMnO3 using partial density of states

Transition metal K-edge x-ray absorption near edge structure (XANES) studies have been carried outon LaBO3 (B = Mn, Fe,Co, Ni), La2CuO4, and SrMnO3 compounds. The theoretical spectra have been calculated using the transition metal (TM)4p density of states (DOS) obtained from full-potential LMTO density functional theory.The exchange–correlation functional used in this calculation is taken under the localdensity approximation (LDA). The comparison of experimental spectra with the calculatedones indicates that single-particle transitions under the LDA are sufficient to generateall the observed XANES, including that which has earlier been attributed tomany-body shake-up transitions and core–hole potentials. The present studyreveals that all the experimentally observed features are mainly due to distributionin TM 4p DOS influenced by hybridization with other orbitals. Specifically, forLaMnO3, the feature earlier attributed to the shake-up process is seen toarise from hybridization of Mn 4p with La 6p and O 2p orbitals; inLa2CuO4 the features attributed to the core hole potential correspond to hybridization ofCu 4p with La 6p, La 5d, and O 2p orbitals. To see the effect of inhomogeneouselectronic charge distribution and on-site Coulomb and exchange interaction(U′) on the XANES of these compounds the generalized-gradient approximation andU′ corrections are incorporated in the calculations. These corrections do not generate any newfeatures in the spectra but affect the detailed intensity and positions of some of thefeatures.

La2TeI2: A New Layered Telluride Iodide with Unusual Electrical Properties

A new layered metal-rich telluride halide, La2TeI2, has been synthesized by heating stoichiometric mixtures of LaI3, La, and Te under argon at 900 degrees C, and its structure has been refined from X-ray powder diffraction data. The compound crystallizes in the 3R-Lu2CCl2 structure type (rhombohedral space group R(-)3m with a = 4.5074(4) A, c = 32.528(2) A, and Z = 3). The crystal structure is composed of infinite layers of edge-sharing, Te-centered metal atom octahedra and iodine atoms separating these layers to form three close-packed I-Ln-Te-Ln-I slabs within the unit cell. The title compound is metallic at room temperature and exhibits an anomaly in the resistivity around 140 K which is closely related to changes in the a lattice parameter with temperature. The chemical bonding and metallic properties of La2TeI2 can be plausibly understood in terms of an ionic description (Ln3+)2Te2-(I-)2(e)2 where two electrons are delocalized in the La 5d conduction band.

Effects of strain on the electronic structures and TC’s of the La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 thin films deposited on SrTiO3

The strain effects on the electronic structures of La0.67Ca0.33MnO3 (LCMO) and La0.8Ba0.2MnO3 (LBMO) thin films have been studied by O K-edge x-ray absorption near edge structure (XANES) spectroscopy. For LCMO, the first-principles calculations reveal that the features in the XANES spectra are associated with hybridized states between O 2p and Mn minority-spin 3d t2g and eg, La 5d∕Ca 3d, and Mn 4s∕Ca 4p states. An analysis of these features shows that the tensile strain decreases substantially La–O and Ca–O hybridization and TC for LCMO. For LBMO, the small compressive strain enhances slightly La–O and Ba–O hybridization and TC.

Electronic states of LaCoO3: Co K-edge and La L-edge x-ray absorption studies

Room-temperature Co K-edge and La L-edge x-ray absorption studies have been carried out onLaCoO3. Experimental near-edge structures have been analysed by theoreticalLDA+U density of states (DOS) and multiple scattering (MS) calculations. Use of both MSand DOS calculations yields additional information about hybridization of thestates of the central atom with neighbouring atoms responsible for producingthe near-edge structures. Absorption processes at the Co K-edge, and the LaL1-,L2-,and L3-edges have been attributed to electronic transitions from Co 4p, La 6p, La 5d, and La 5d, respectively. All the pre-edge and post-edge features including the shape of the mainabsorption edge have been generated by taking the convolution of the calculatedDOS, indicating that single particle approximation is sufficient to express allexperimentally observed major structures. Two pre-edge structures observed in theCo K-edge spectrum are attributed to Co and quadrupole transitions in contrast to earlier identification of the same to Co and eg transitions. The influence of La 6p states on the Co 4p states is such that the inclusion of Laatoms in the MS calculations is necessary to generate post-edge structures in the Co K-edgespectrum. The importance of the hybridization of the O 2p state with La 6p and 5d in theL-edge absorption processes is also discussed. A 10% contribution of the quadrupolechannel has been estimated in the La L-edges.

Infrared‐Spectroscopic and Density‐Functional‐Theory Investigations of the LaCO, La2[η2(μ2‐C,O)], and c‐La2(μ‐C)(μ‐O) Molecules in Solid Argon

Reactions of laser-ablated lanthanum atoms with CO molecules in solid argon have been studied. The neutral lanthanum monocarbonyl (LaCO), produced upon sample deposition at 7 K, exhibits a C-O stretching frequency of 1772.7 cm(-1); to the best of our knowledge this is the lowest yet observed for a terminal CO in a neutral metal-carbonyl molecule (MCO, M = metal atom), implying anomalously enhanced metal-to-CO back-bonding. The infrared (IR) absorption band at 1145.9 cm(-1) is assigned to the C-O stretching mode of the side-on-bonding CO in the La2[eta2(mu2-C,O)] molecule. This CO-activated molecule undergoes an UV/Vis-photoinduced rearrangement to the CO-dissociated molecule, c-La2(mu-C)(mu-O). Density functional theory (DFT) calculations have been performed on these molecules, the results of which lend strong support to the experimental assignments of the IR spectra. LaCO is predicted to have a quartet ground state, corresponding to a linear geometry. Its formation involves La 6s-->4f promotion, which increases the strength of La-CO bonding by decreasing the sigma repulsion and, remarkably, by increasing the La 5d and 4f-->CO 2pi back-bonding. The observations schematically depict the whole process, starting with the interaction of CO with metal and ending with CO dissociation by the lanthanum dimer.

Composition dependence of local structure in lanthanoborate glasses

Lanthanoborate glasses ((La 2O 3) x (B 2O 3) 1− x glasses, where x = 0.22, 0.25, 0.28, 0.30) were prepared by rapid-quenching method using twin-roller apparatus. The composition dependence of the local structure in the lanthanoborate glasses was investigated by Raman spectroscopy, XAFS and XPS measurements. The XAFS measurements of La LIII-edge showed that the lanthanum ion is surrounded by nine oxygen atoms and that the La–O distance decreases with increasing of the La 2O 3 content, which shows that the La–O interaction becomes stronger with increasing of the La 2O 3 content. The corresponding result was obtained from XPS spectra for La 3d 3/2 and 3d 5/2. The Raman spectra showed a band at ca. 761 cm −1 which is assigned to metaborate chain structure. It is found that the band decreases with increasing of La 2O 3 content. The bands at 635, 835, and 925 cm −1 increase with increasing of La 2O 3 content, which are assigned to small anionic units, such as metaborate ring, pyroborate, and orthoborate structure, respectively, which shows that the network structure in the glasses changes to small anionic units as the La 2O 3 content increases.