La 3d Research Articles

Electronic structure of LaTe and CeTe

We report a comparative study of the electronic structure of the compounds LaTe and CeTe, both of which crystallize in the rock salt structure. LaTe is a paramagnetic metal while CeTe is known to exhibit anomalous Kondo-like transport behaviour and undergoes a transition to a complex magnetically ordered state at low temperature (TN=2.2K). We carry out hard X-ray photoelectron spectroscopy (HAXPES) of the core-levels and valence band of LaTe and CeTe at T=20K, in order to characterize their intrinsic electronic structure, and to address the role of Kondo effect on the electronic structure of CeTe. The bulk sensitive core level HAXPES spectra show evidence of screened features in the La 3d and Ce 3d states mixed with plasmon features. From a careful analysis of the Te, La and Ce derived core levels, we separate out the respective origins of the satellites and show that CeTe indeed exhibits definitive but weak f0 and f2 satellites due to Kondo screening, in addition to the main f1 peak. The comparison of the valence band spectra of CeTe obtained using HAXPES and soft X-ray PES clearly identifies the Ce 4f derived features. Resonant photoelectron spectrosocopy across the Ce 3d−4f threshold confirms the Ce 4f1 final state at the Fermi level, corresponding to the tail of the Kondo resonance feature which occurs above the Fermi level, while the Ce 4f0 final state feature is observed at a binding energy of 2.4eV. The 4f0 and 4f1 final states show giant resonances compared to the off-resonant spectra. However, in contrast to typical Kondo systems, the tail of the Ce 4f1 Kondo resonance at the Fermi level is relatively suppressed compared to the Ce 4f0 feature, which exhibits an unusually strong resonance enhancement. The results are indicative of a weakened Kondo effect which favours the magnetically ordered ground state in CeTe.

Atomic origin of 3d94 f 1 configuration in La3+ solids

We have studied the excited electronic structure of LaBr3(Ce) scintillator by soft x-ray spectroscopy such as x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES). The La 3d XAS and XES spectra of LaBr3(Ce) are compared with those of other La3+ solids (LaF3, La2O3, and La metal). From this comparison, it turns out that the La 3d XAS and XES spectra from all the La3+ solids considered here appear at almost the same energy, even though the corresponding binding energies of the 3d core holes determined by XPS (x-ray photoelectron spectroscopy) are very different. As a result, we argue that the atomic nature of the configuration created by x-ray absorption process in La3+ solids is maintained via the localized 4 f 1 state, which screens the 3d core holes differently from one La3+ solid to another.

Magnetic properties and magnetocaloric effect in La0.7Sr0.3−xBixMnO3 manganites

Structural properties, magnetization and magnetocaloric effect measurements in the compounds based on lanthanum–strontium–bismuth manganites La0.7Sr0.3−xBixMnO3 (LSBiMO) with 0.05⩽x⩽0.30 have been performed. The valence states of elements entering into the composition of investigated LSBiMO samples were analyzed for x=0.15 and 0.3 using the HR XPS spectra of Bi 4f, La 3d, Mn 3s and Sr 3d. According to X-ray powder diffraction data the crystalline structure of the samples is perovskite-like with a small rhombohedral distortion. The a and c lattice parameters and unit cell volume V are established to increase with increasing Bi content. All the manganites show ferromagnetic-like ordering with second order phase transition to paramagnetic state. The Curie temperature and magnetization decrease at substitution of Sr2+ for Bi3+ ions. The magnetic entropy changes were calculated using the field dependence of isothermal magnetization in the terms of the thermodynamic Maxwell relation. The maximum magnetic entropy change value was shown to be 0.94Jkg−1K−1 for x=0.05 for a field change of 1T.

Synthesis, Crystal and Electronic Structure, and Optical Property of the Quaternary Selenide: La3Sb0.33SiSe7

AbstractThe quaternary selenide, La3Sb0.33SiSe7, was prepared from stoichiometric mixtures of elements by solid‐state reactions at 1223 K in an evacuated silica tube. La3Sb0.33SiSe7 crystallizes in the chiral P63 space group and belongs to the Ce3Al1.67S7 structure type. Its structure features one‐dimensional chains of face‐sharing SbSe6 octahedra running parallel to the c direction surrounded by the discrete SiSe4 tetrahedra and La cations. UV/Vis/NIR diffuse reflectance spectroscopy study shows its optical gap of around 1.75 eV. DFT study indicates an indirect bandgap with an electronic transfer excitation from Se 4p to La 5d orbital electrons.

A Series of Noncentrosymmetric Antimony Sulfides Ln8Sb2S15 (Ln = La, Pr, Nd) – Syntheses, Crystal and Electronic Structures, and NLO Properties

AbstractA series of noncentrosymmetric sulfides Ln8Sb2S15 (Ln = La, Pr, Nd) were prepared from stoichiometric mixtures of the elements at 1223 K in an evacuated silica tube. The compounds Ln8Sb2S15 with Ln = La and Nd are isostructural to Pr8Sb2S15 and crystallize in the tetragonal noncentrosymmetric space group I41cd. Their structure contains discrete [SbS3]3– trigonal pyramids separated by Ln3+ cations and S2– anions. La8Sb2S15 shows second harmonic generation with intensities 1.2 times that of the commercially used IR NLO (nonlinear optics) material AgGaS2 (at 2.05 μm laser). It exhibits excellent thermal stability up to 663 °C. Studies with UV/Vis–NIR diffuse reflectance spectroscopy show that La8Sb2S15 has an optical gap of around 2.3 eV, and a DFT study indicates a direct band gap with an electronic transfer excitation of S 3p electrons to a La 5d orbital.

Single Crystal Perovskites Analyzed Using X-ray Photoelectron Spectroscopy: 4. (LaAlO3)0.3(Sr2TaAlO6)0.7(001)

X-ray photoelectron spectroscopy (XPS) was used to analyze a commercially available (LaAlO3)0.3(Sr2TaAlO6)0.7 (001) bulk single crystal. XP spectra were obtained using incident monochromatic Al Kα radiation at 0.83401 nm. A survey spectrum together with La 3d, O 1s, C 1s, Sr 3p, Ta 4d, La 4p, Sr 3d, Al 2s, La 4d, Al 2p, Ta 5p, La 5s, Ta 4f, O 2s, Sr 4p and La 5p core level spectra and the valence band are presented. The spectra indicate the principle core level photoelectron and Auger electron signals and show only minor carbon contamination. Making use of the O 1s, Sr 3d, La 4d, Al 2p Ta 4f lines and neglecting the components related to surface contaminants, XPS quantitative analysis reveals an altered stoichiometry of the air-exposed crystal surface of La0.34Sr0.88Al0.82Ta0.43O3.

Single Crystal Perovskites Analyzed Using X-ray Photoelectron Spectroscopy: 3. LaAlO3(001)

X-ray photoelectron spectroscopy (XPS) was used to analyze a commercially available LaAlO3(001) bulk single crystal. XP spectra were obtained using incident monochromatic Al Kα radiation at 0.83401 nm. A survey spectrum together with La 3d, O 1s, C 1s, La 4p, Al 2s, La 4d, Al 2p, La 5s, O 2s and La 5p core level spectra and the valence band are presented. The spectra indicate the principle core level photoelectron and Auger electron signals and show only minor carbon contamination. Making use of the O 1s, La 4d, Al 2p lines and neglecting the components related to surface contaminants, XPS quantitative analysis reveals an altered stoichiometry of the air-exposed crystal surface of LaAl1.03O2.23.

X-ray photoelectron spectroscopy study of high-k CeO2/La2O3 stacked dielectrics

This work presents a detailed study on the chemical composition and bond structures of CeO2/La2O3 stacked gate dielectrics based on x-ray photoelectron spectroscopy (XPS) measurements at different depths. The chemical bonding structures in the interfacial layers were revealed by Gaussian decompositions of Ce 3d, La 3d, Si 2s, and O 1s photoemission spectra at different depths. We found that La atoms can diffuse into the CeO2 layer and a cerium-lanthanum complex oxide was formed in between the CeO2 and La2O3 films. Ce3+ and Ce4+ states always coexist in the as-deposited CeO2 film. Quantitative analyses were also conducted. The amount of CeO2 phase decreases by about 8% as approaching the CeO2/La2O3 interface. In addition, as compared with the single layer La2O3 sample, the CeO2/La2O3 stack exhibits a larger extent of silicon oxidation at the La2O3/Si interface. For the CeO2/La2O3 gate stack, the out-diffused lanthanum atoms can promote the reduction of CeO2 which produce more atomic oxygen. This result confirms the significant improvement of electrical properties of CeO2/La2O3 gated devices as the excess oxygen would help to reduce the oxygen vacancies in the film and would suppress the formation of interfacial La-silicide also.

Strain dependence of the electronic properties of LaTiO3 thin films

We report on the transport and the core-level X-ray photoemission spectroscopy data of fully-strained LaTiO3 thin films grown on GdScO3 and SrTiO3 substrates. We observed that LaTiO3 thin film grown on GdScO3 showed insulating behavior but that grown on SrTiO3 exhibited a metallic character. We found that while the La 4d photoemission spectra of the two films were nearly the same, their Ti 2p and O 1s data revealed a difference. Our results suggest that strain-induced changes in the Ti-O bonding play an important role in the electronic properties of LaTiO3 thin films.

Structural, thermodynamic, and mechanical properties of bulk La and A-La2O3

First principles calculations are combined with the quasi-harmonic approximation and thermal electronic excitation, to have a systematic study of structural, thermodynamic (CV, CP, and αL), and mechanical (Cij, B, G, and E) properties of bulk La and A-La2O3. It is found that the generalized gradient approximation gives a much better reflection of the ground state properties than local density approximation, and that a very strong covalent bonding is formed in A-La2O3, mainly due to the pronounced hybridization between O 2p and La 5d as well as 5p states. Calculations also reveal that the electronic contribution to heat capacity of fcc La should be bigger than that of dhcp La, and that elastic anisotropy of A-La2O3 would cause the big difference of thermal expansion along the a and c axes. Moreover, the elastic properties of La and A-La2O3 decease almost linearly with the increase of temperature, and the descending sequence of bulk moduli is E→B→G within the entire temperature range. The strong covalent bonding of A-La2O3 would be enhanced (reduced) with the decrease (increase) of temperature, and the calculated results are in good agreement with experimental observations in the literature.

Study of Band Structure Properties of Pnictide LaO1−xF x FeAs (x = 0, 0.2) Superconducting Compound

This paper reports on the band structure properties and changes in band structure of fluorine-doped LaO1−x Fx FeAs (x = 0, 0.2) compound, measured using X-ray photoemission spectroscopy (XPS). The band structure of the superconducting compound is compared with nonsuperconducting parent compound LaOFeAs. With fluorine doping, a shift of the shallow core level is observed in XPS spectra, which may be a response of the band structure due to fluorine doping in the system. The balance of the chemical potential shift with the screening effect of conduction electrons near the Fe and As ions is discussed using nearly unchanged Fe 2p and As 3d core-level spectra. The La 3d core-level spectra shift towards the high energy, ∼0.36 eV, may be due to the chemical potential shift caused by fluorine doping. In our valence band spectra, a small peak at around 0.2 eV is observed, which disappeared with the fluorine doping in the system, indicating a change of Fe 3d state from low spin to high spin states and also confirming the nature of Fe 3d electrons as itinerant, which is responsible for superconductivity in these compounds.

Luminescence of CNT coated LaPO4:Tm phosphors and their field emission lamp application

When the phosphor LaPO4:Tm was coated with carbon nanotubes (CNTs), carbon atoms were diffused into the phosphor lattice. Binding energy of La 3d and C 1s were not shifted by CNT coating, while that of O 1s was lowered slightly as a result of the formation of CO and CO bonds on the phosphor surface. CNT coating increased the atomic concentrations of O and C by 27.4% and 9%, respectively. Photoluminescence intensity decreased significantly, by 52%, when CNT concentration of 1.5wt% was coated. This is attributed to covering the phosphor with black CNTs/carbons and the formation of CO and CO bonds on the phosphor surface. Conversely, cathodoluminescence (CL) intensity was improved by a factor of 3.86 when 1.25wt% of CNTs was coated, attributing to improved conductivity, reduced charging, and increased CL excitation probability of the phosphor. The influence of CNT coating on the luminescence of LaPO4:Tm was compared with that of LaPO4:Eu phosphor. Blue CL was observed from LaPO4:Tm, it is potential for field emission lamp application.

Sm-Doped LaSi3 N5 : Synthesis, Computed Electronic Structure, and Band Gaps

LaSi3N5-based phosphor doped with Sm was prepared by the nitridation of LaSi-Si-Si3N4-Sm2O3 powder mixture. The emission spectrum shows two main bands with maxima at 595 nm in the orange region and at ~650 nm in the red region. The excitation spectrum of Sm-doped LaSi3N5 shows a maxima at 585, 570, and 405 nm. First-principles density-functional theory calculations were performed using Vienna ab initio simulation package to enhance the understanding of the electronic structure of the stoichiometric LaSi3N5 and Sm-doped LaSi3N5. The electronic structure and band gaps were calculated in 2 × 1 × 2 supercell with 144 atoms using the more precise screened Coulomb hybrid functional HSE06. Both La3+/Sm3+ and La3+/Sm2+ substitutions were calculated. The calculated band gap of Sm(III)-doped LaSi3N5 is 2.01 eV, in reasonable agreement with the experimental value of 2.12 eV, but corresponds to the unrealistic transition between the N, Si p states, and unoccupied Sm 4f states. The band gap of 1.43 eV calculated for Sm(II)-doped LaSi3N5 is smaller than the available experimental value, but corresponds to the correct transition between nonbonding Sm 4f states and empty La 5d states. Optical properties are found to be governed by f electrons of the Sm(II) dopant.

High-Energy X-ray Photoemission and Structural Study of Ultrapure LaF3 Superionic Conductor Thin Films on Si

LaF3 films in the 5–40 nm thickness range were grown on Si(111) by molecular beam epitaxy. The substrates were kept at 450 °C during deposition. The films were investigated by high-energy X-ray photoemission flanked by conventional X-ray photoemission, reflection high-energy electron diffraction, and atomic force microscopy. The film growth was layer-by-layer. The surface of the films presented flat terraces, ∼100 nm wide, separated by monatomic steps, reproducing the morphology of the substrate. La 3d, F 1s, O 1s, and Si 2p core levels and valence band were measured by high-energy photoemission to investigate the reactivity of the system and the surface and bulk composition of the films, following varying sample treatments (X-ray irradiation, sputtering, heating). The fresh prepared films resulted of high purity, with no traces of reaction or intermixing at the buried interface between the substrate and the trifluoride. The X-ray beam was seen to induce F depletion at the surface and promote oxide formation. F depletion enhancement was obtained through Ar ion sputtering. An irreversible variation of the film composition was finally observed for samples heated above 300 °C, with the development of La oxides and oxofluorides. These effects were related to the high mobility of F ions in the LaF3 lattice and to the high tendency of defects formation involving F sites.

Influence of lanthanum on the optomagnetic properties of zinc ferrite prepared by combustion method

Pure and lanthanum doped zinc ferrite nanoparticles were synthesized by a combustion method using glycine as fuel. The mechanism of formation of these nanoferrites is discussed briefly. The prepared nanoparticles characterized using powder X-ray diffraction analysis (XRD) revealed the formation of cubic spinel phase with high crystallinity. Average crystallite size, X-ray density and bulk density were found to decrease with an increase in La3+ concentration. The chemical elements and states on the surface of these ferrites were determined using X-ray photoelectron spectroscopy (XPS). The detailed core level spectra of the photoelectron peaks of Zn 2p, Fe 2p, La 3d and O 1s were analyzed. The magnetic behavior of these nanoparticles was studied using a vibrating sample magnetometer (VSM) and corresponding changes in the saturation magnetization (Ms), coercivity (Hc) and remanent magnetization (Mr) were analyzed. The optical behavior of these ferrite nanoparticles was characterized by UV–Diffuse reflectance studies (UV–DRS). From the UV–DRS studies, the optical band gap was found to be in the range of 1.87–1.97eV. The combustion method significantly produces large amount of products within a short time. Therefore, this method is potentially suitable for manufacturing industries for preparing the magnetic nanoparticles.

Growth, chemical composition, and structure of thin La x Hf1 − x O y films on Si

The chemical structure, phase composition, and crystal structure of LaxHf1 − xOy films grown on Si using volatile metalorganic compounds as Hf and La precursors have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray microanalysis, and atomic force microscopy. By varying the lanthanum and hafnium source temperatures, we were able to grow films with 2 at % < CLa < 30 at %. The Hf 4f and La 3d peak positions in the XPS spectra of the films correspond to hafnium and lanthanum in the Hf4+ and La3+ states. With increasing La concentration, the reflections in the X-ray diffraction patterns of the films shift to smaller 2θ angles, indicating the formation of solid solutions. At 18 at % La, we observed a transition from a fluorite-like structure to the pyrochlore structure (La2Hf2O7). The film containing 30 at % La consisted of a mixture of c-La2O3 and La2Hf2O7. The surface roughness of the films was shown to increase with increasing La concentration. Capacitance-voltage (C-V) measurements were used to assess the relative permittivity (k) of the films as a function of La concentration. The minimum k value was obtained at the La concentration corresponding to the transition from the fluorite structure to an ordered pyrochlore structure (second-order phase transition).

Cerium-doped LaSi3N5: Computed electronic structure and band gaps

Abstract First-principle calculations are performed using VASP to enhance the understanding of the electronic structure of stoichiometric LaSi3N5 and Ce-doped LaSi3N5. Electronic structure and band gaps are calculated using DFT and the screened Coulomb hybrid functional HSE06. Five doping schemes are investigated: La3+/*Ce3+, La3+/□ (□ = cation vacancy), La3+/Ce3+, La3+/Ce2+ compensated by one framework N3−/O2− substitution, and combined La3+/Ce3+and La3+/□ substitutions. The vacancy is electronically balanced by N3−/O2− substitutions. Two pseudopotential files for CeIII are used to investigate the effect of f orbitals on the calculated band gap. There was no change in the calculated value of the band gap using DFT method for the *CeIII doped LaSi3N5 compared to the stoichiometric LaSi3N5 (3.15 eV). HSE06 provided more precise band gaps. The calculated band gap of Ce-doped LaSi3N5 is 4.65 eV with the possible electronic transitions from Ce spin-up 4f to Ce spin-up 4f states, or La 5d states..

One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF₃ and BaLaF₅ Core-Shell Upconverting Nanocrystals.

We report a one-pot solvothermal synthesis of sub-10 nm, dominant ultraviolet (UV) emissive upconverting nanocrystals (UCNCs), based on sodium-codoped LaF3 and BaLaF5 (0.5%Tm; 20%Yb) and their corresponding core@shell derivatives. Elemental analysis shows a Na-codopant in these crystal systems of ~20% the total cation content; X-ray diffraction (XRD) data indicate a shift in unit cell dimensions consistent with these small codopant ions. Similarly, X-ray photoelectron spectroscopic (XPS) analysis reveals primarily substitution of Na+ for La3+ ions (97% of total Na+ codopant) in the crystal system, and interstitial Na+ (3% of detected Na+) and La3+ (3% of detected La3+) present in (Na)LaF3 and only direct substitution of Na+ for Ba2+ in Ba(Na)LaF5. In each case, XPS analysis of La 3d lines show a decrease in binding energy (0.08–0.25 eV) indicating a reduction in local crystal field symmetry surrounding rare earth (R.E.3+) ions, permitting otherwise disallowed R.E. UC transitions to be enhanced. Studies that examine the impact of laser excitation power upon luminescence intensity were conducted over 2.5–100 W/cm2 range to elucidate UC mechanisms that populate dominant UV emitting states. Low power saturation of Tm3+ 3F3 and 3H4 states was observed and noted as a key initial condition for effective population of the 1D2 and 1I6 UV emitting states, via Tm-Tm cross-relaxation.

XPS study of sol–gel produced lanthanum oxide thin films

La2O3 thin films were prepared by a sol–gel method and annealed in air and vacuum at various temperatures. The X-ray photoelectron spectroscopy (XPS) was used to investigate the properties and composition of films. The La 3d and O 1s spectra of films were analysed. It was shown that oxygen ions in La2O3 are in two states – O2– anions connected with lanthanum, and oxygen in the (OH)– group. The thermal transformation of lanthanum hydroxide to oxide process was triggered by thermal treatment at temperatures above 600 K. It was shown that after annealing thin films for longer time intervals in vacuum the intensity of O2– component increases and the intensity of oxygen in (OH)– decreases. The drop of oxygen in the (OH)– group was attributed to dehydration process.

The first examples of thiogermanate anion [GeS3(SH)]3−as a bridging ligand to a lanthanide complex

Two new lanthanoid thiogermanates [Ln(dien)(2)(μ-η(1),η(2)-GeS(3)(SH))](n) {Ln = La(), Nd(); dien = diethylenetriamine} were solvothermally synthesized and structurally characterized. Both and represent the first examples of the soft Lewis basic [GeS(3)(SH)](3-) anions as tridentate bridging ligand to hard Lewis acidic Ln(3+) ions. Both and exhibit semiconducting properties with absorption band edges of 2.11 eV for and 2.28 eV for . The DFT study of indicates a direct band gap with an electronic transfer excitation of S 3p to both Ge 4s and La 5d orbital electrons.