4f States Research Articles

The key energy scales of Gd-based metallofullerene determined by resonant inelastic x-ray scattering spectroscopy

Endohedral metallofullerenes, formed by encaging Gd inside fullerenes like C80, can exhibit enhanced proton relaxitivities compared with other Gd-chelates, making them the promising contrast agents for magnetic resonance imaging (MRI). However, the underlying key energy scales of GdxSc3−xN@C80 (x = 1–3) remain unclear. Here, we carry out resonant inelastic x-ray scattering (RIXS) experiments on GdxSc3−xN@C80 at Gd N4,5-edges to directly study the electronic structure and spin flip excitations of Gd 4f electrons. Compared with reference Gd2O3 and contrast agent Gadodiamide, the features in the RIXS spectra of all metallofullerenes exhibit broader spectral lineshape and noticeable energy shift. Using atomic multiplet calculations, we have estimated the key energy scales such as the inter-site spin exchange field, intra-atomic 4f–4f Coulomb interactions, and spin-orbit coupling. The implications of these parameters to the 4f states of encapsulated Gd atoms are discussed.

First principles studies of hydrogen insertion effects on magnetic properties, bonding and structure reordering of UZr2

In the context of the need to recycle depleted uranium coming from nuclear waste, hydrogen absorption in uranium and mainly its alloys is an important solution. We here consider hydrogen insertion in UZr2 intermetallic leading to experimentally observed composition UZr2H6. Drastic changes are shown from density functional calculations to induce atom reordering within the metal substructures in the ground state. Particularly among different starting hypotheses for H sites, energy minimization is observed when the hydrogenated intermetallic undergoes metal substructures reordering with unmixed occupancy of Al (U) and B (Zr) sites in AlB2 type structure. Hydrogen is identified as covalently bonded within the plane of Zr metal, illustrated by anionic charged H−0.55 derived from Bader analysis of charge density. Hydrogen uptake leads to prevailing volume expansion effects versus chemical bonding ones, involving strong localization of uranium 5f states and increased magnetization versus the pristine intermetallic. Both the intermetallic and the ternary hydride are found ferromagnetic in the ground state.

Electronic structure, magnetic properties, and mixed valence character of Ce2 Ni3 Si5 from first principles calculations.

Cerium intermetallic compounds exhibit anomalous physical properties such as heavy fermion and Kondo behaviors. Here, an ab initio study of the electronic structure, magnetic properties, and mixed valence character of Ce2 Ni3 Si5 using density functional theory (DFT) is presented. Two theoretical methods, including pure Perdew-Burke-Ernzerhof (PBE) and PBE + U, are used. In this study, Ce3+ and Ce4+ are considered as two different constituents in the unit cell. The formation energy calculations on the DFT level propose that Ce is in a stable mixed valence of 3.379 at 0 K. The calculated electronic structure shows that Ce2 Ni3 Si5 is a metallic compound with a contribution at the Fermi level from Ce 4f and Ni 3d states. With the inclusion of the effective Hubbard parameter (Ueff ), the five valence electrons of 5 Ce3+ ions are distributed only on Ce3+ 4f orbitals. Therefore, the occupied Ce3+ 4f band is located in the valence band (VB) while Ce4+ 4f orbitals are empty and Located at the Fermi level. The calculated magnetic moment in Ce2 Ni3 Si5 is only due to cerium (Ce3+ ) in good agreement with the experimental results. The Ueff value of 5.4 eV provides a reasonable magnetic moment of 0.981 μB for the unpaired electron per Ce3+ ion. These results may serve as a guide for studying present mixed valence cerium-based compounds. © 2017 Wiley Periodicals, Inc.

Calculations of photoelectron momentum distributions and energy spectra at strong-field multiphoton ionization of sodium

Multiphoton ionization of sodium by a femtosecond laser pulse of 760 nm wavelength and different peak intensities is studied by inspecting the photoelectron angular and momentum distributions and the energy spectra. For this purpose a single-electron model of the atom interacting with the electromagnetic field is used, and the distributions are determined by calculating the evolution of the electron wave function. Beside the most prominent distribution maxima related to the four-photon ionization, the five-photon (above-threshold) ionization peaks are observed. Substructures in the main (nonresonant) maximum in the photoelectron spectra at the four-photon ionization are related to the resonantly enhanced multiphoton ionization via intermediate 4s, 4f, 5p, 5f and 6p states.

First-principles study on electronic structures and magnetic properties of Eu-doped phosphorene

The structural, electronic and magnetic properties of Eu-doped phosphorene with different doping concentrations were investigated by first-principles calculations for the first time. The calculations show that Eu-doped phosphorene systems are stable and have the large magnetic moments of more than 6 μB by 2.7, 6.25 and 12.5 at.% doping concentrations. The major contribution to the magnetic moment stems from the 4f states of Eu-doped atom. Meanwhile, Eu-doped atom introduces the impurity bands which can be changed by different doping concentrations. In order to determine the magnetic interaction, the different configurations for two Eu atoms doping in 3 × 3 × 1 phosphorene supercell were studied, which reveals that all of the configurations tend to form ferromagnetic. These results can provide references for inducing large magnetism of two-dimensional phosphorene, which are valuable for their applications in spintronic devices and novel semiconductor materials.

Giant Rashba effect at the topological surface of PrGe revealing antiferromagnetic spintronics

Rashba spin-orbit splitting in the magnetic materials opens up a new perspective in the field of spintronics. Here, we report a giant Rashba spin-orbit splitting on the PrGe [010] surface in the paramagnetic phase with Rashba coefficient αR = 5 eVÅ. We find that αR can be tuned in this system as a function of temperature at different magnetic phases. Rashba type spin polarized surface states originates due to the strong hybridization between Pr 4f states with the conduction electrons. Significant changes observed in the spin polarized surface states across the magnetic transitions are due to the competition between Dzyaloshinsky-Moriya interaction and exchange interaction present in this system. Presence of Dzyaloshinsky-Moriya interaction on the topological surface give rise to Saddle point singularity which leads to electron-like and hole-like Rashba spin split bands in the bar{{boldsymbol{Z}}}^{prime} -bar{{boldsymbol{Gamma }}}-bar{{boldsymbol{Z}}} and bar{{boldsymbol{X}}}^{prime} -bar{{boldsymbol{Gamma }}}-bar{{boldsymbol{X}}} directions, respectively. Supporting evidences of Dzyaloshinsky-Moriya interaction have been obtained as anisotropic magnetoresistance with respect to field direction and first-order type hysteresis in the X-ray diffraction measurements. A giant negative magnetoresistance of 43% in the antiferromagnetic phase and tunable Rashba parameter with temperature makes this material a suitable candidate for application in the antiferromagnetic spintronic devices.

Theoretical Prediction of Electronic Structures and Phonon Dispersion of Ce2XN2 (X = S, Se, and Te) Ternary

A systematic study of structural, electronic, vibrational properties of new ternary dicerium selenide dinitride, Ce2SeN2 and predicted compounds—Ce2SN2 and Ce2TeN2—is performed using first-principles calculations within Perdew–Burke–Ernzerhof functional with Hubbard correction. Our calculated results for structural parameters nicely agree to the experimental measurements. We predict that all ternary dicerium chalcogenide nitrides are thermodynamically stable. The predicted elastic constants and related mechanical properties demonstrate its profound mechanical stability as well. Moreover, our results show that Ce2XN2 are insulator materials. Trends of the structural parameters, electronic structures, and phonon dispersion are discussed in terms of the characteristics of the Ce (4f) states.

Investigation of Er doped zinc borate glasses by low-temperature photoluminescence

Er3+ doped (95-x)B2O3-xZnO-5TiO2 (x = 45, 50 and 55mol%) bulk glasses were prepared by conventional melt quenching method and investigated by transmission and photoluminescence (PL) spectroscopy. Transmission spectra exhibit sharp absorption bands at 364, 377, 406, 441, 448, 488, 521, 541, 651, 792, 974, and 1530nm, that correspond to absorption on Er3+ ions and they are attributed to the optical transitions from the ground state 4I15/2 to the excited states 4G9/2, 4G11/2,4F9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. The optical gap has been estimated around 3.5eV with a tendency to increase with increasing ZnO content. Low-temperature PL properties of the Er3+ doped glasses and of the host glass itself were investigated. To achieve this goal, two different wavelengths (325 and 514.5nm) were used for PL excitation. PL spectra excited by He-Cd laser at 325nm were dominated by the host glass broad-band luminescence centred at around 570nm. Simultaneously we observed narrow emission and absorption features due to 4f-4f electronic transitions in Er3+ ions superposed on the broad band host glass luminescence. We argue that these superposed narrow absorption dips represent a direct evidence of the energy transfer from the electronic structure of the host to 4f states of Er3+ ions. Fine structure of Er3+ emission bands at 980 and 1530nm, corresponding to radiative transitions from the two lowest excited states of Er3+ ions to the ground state manifold have been investigated at room temperature and at 4K, and a schematic energy diagram of Stark levels of 4I11/2, 4I13/2 and 4I15/2 manifolds has been deduced.

Intense orange emission in Pr3+ and NIR emission at 1.53 μm in Er3+ doped zinc phosphate glasses for potential broadband optical amplifier

Praseodymium (Pr3+) and Erbium (Er3+) doped zinc phosphate glasses with chemical compositions (60 − x) NH4H2PO4 + 20ZnO + 10BaF2 + 10NaF + xRE (where RE = Pr6O11 and Er2O3, x = 0.1, 0.5, 1.0, 1.5 and 2.0 mol%) have been prepared by melt quenching technique. Amorphous structure was ascertained by XRD analysis. The functional and vibrational bands have been assigned and clearly elucidated by FT-IR and Raman spectral profiles for all the glass samples. Judd–Ofelt (J–O) intensity parameters (Ω λ : λ = 2, 4, 6) have been obtained from the spectral intensities of different absorption bands of Pr3+ and Er3+ ions through optical absorption spectra. The radiative properties such as radiative transition probabilities (A R ), radiative lifetimes (τ R ) and branching ratios (β R ) for different excited states were calculated by using J–O intensity parameters. Visible photoluminescence spectra have exhibited six emission bands (3P1 → 3H5, 1D2 → 3H4, 3P0 → 3F2, 3P1 → 3F3, 3P0 → 3F3 and 3P0 → 3F4 states) for all the concentrations of Pr3+ ions. NIR photoluminescence spectra exhibited one emission band (4I15/2 → 4I13/2) of Er3+ ions. The energy transfer mechanism that leads to the quenching of lifetimes of 1D2 and 4I13/2 states have been discussed at higher concentrations of Pr3+ and Er3+ ions, respectively. These glasses were suggested as suitable hosts to produce intense orange emission at 608 nm for Pr3+ ion and efficient lasing action in NIR region at 1534 nm for Er3+ ion.

Ternary aurides La4In3Au10 and Yb4In3Au10 and platinide U4In3Pt10 with ordered Zr7Ni10 type structure

The ternary aurides La4In3Au10 and Yb4In3Au10 and the platinide U4In3Pt10 with ordered Zr7Ni10 type structure were synthesized from the elements by induction-melting in sealed tantalum tubes or via arc-melting. The polycrystalline samples were characterized by powder X-ray diffraction and the structures were refined from single crystal X-ray diffractometer data: Cmce, a = 1426.7(3), b = 1020.3(2), c = 1025.2(2) pm, wR2 = 0.0441, 1510 F2 values, 46 variables for La4In3Au10, a = 1361.5(3), b = 998.3(2), c = 1007.8(2), wR2 = 0.0804, 1404 F2 values, 46 variables for Yb4In3Au10 and a = 1344.4(3), b = 973.9(2), c = 978.9(2), wR2 = 0.0922, 741 F2 values, 48 variables for U4.15In3.03Pt9.82 (with small degrees of In/U, respectively Pt/In mixing on Wyckoff sites 4a and 8f). The La4In3Au10, Yb4In3Au10 and U4In3Pt10 structures contain pronounced two-dimensional gold, respectively platinum substructures which are filled and condensed by two crystallographically independent indium and rare earth atoms. The crystal chemical features clearly classify these intermetallics as aurides and platinides. The physical properties of U4In3Pt10 were characterized by means of magnetic and electrical transport measurements. The compound exhibits metallic conductivity and shows no magnetic ordering down to 1.72K. Its magnetic behavior is governed by hybridization between 5f and ligand electrons that results in significant delocalization of the 5f states.

Hard x-ray photoemission study of Yb1−xZrxB12: the effects of electron doping on the Kondo insulator YbB12

We have carried out hard x-ray photoemission spectroscopy (HAXPES) of Yb1−xZrxB12 () to study the effects of electron doping on the Kondo insulator YbB12. The Yb valences of Yb1−xZrxB12 at 300 K estimated from the Yb 3d HAXPES spectra decreased after substituting Yb with Zr from 2.93 for YbB12 to 2.83 for Yb0.125Zr0.875B12. A temperature dependent valence decrease was found upon cooling for all doping concentrations. We found peak shifts of the B 1s and Zr 3d5/2, and Yb3+ 4f spectra toward the deeper binding-energy with increasing Zr concentration, which indicates a shift of the Fermi level to the higher energy and that of the Yb 4f hole level close to the Fermi level, respectively, due to electron doping. These results qualitatively show the enhanced hybridization between the Yb 4f and conduction-band states with Zr substitution, consistent with magnetic susceptibility measurements.

Enhanced optical absorption and photocatalytic activity of anatase TiO2 through C[sbnd]Nd-codoped: A DFT+U calculations

The electronic and optical properties are investigated using density functional theory plus U approach in CNd monodoped and codoped anatase TiO2. The calculated results indicate that C dopant can enhance the intensity of optical absorption and Nd dopant can extend the optical absorption range into the whole visible spectrum in CNd-codoped anatase TiO2, and the Nd 4f states can effectively reduce the recombination of photo-excited electron-hole pairs thus achieving higher visible light photocatalytic activity for hydrogen production by water splitting than pure TiO2. Moreover, we also study the effects of doping concentration and impurity distance in CNd-codoped anatase TiO2 system, finding that the higher doping concentration, the stronger absorption activity, and absorption activity increases with the shortness of impurity distance due to the assemblage of impurity states.

Electronic, magnetic, and magnetocrystalline anisotropy properties of light lanthanides

Theoretical understanding of interactions between localized and mobile electrons and the crystal environment in light lanthanides is important because of their key role in much needed magnetic anisotropy in permanent magnet materials that have a great impact in automobile and wind turbine applications. We report electronic, magnetic, and magnetocrystalline properties of these basic light lanthanide elements studied from advanced density functional theory (DFT) calculations. We find that the inclusion of onsite 4f electron correlation and spin orbit coupling within the full-potential band structure is needed to understand the unique magnetocrystalline properties of these light lanthanides. The onsite electron correlation, spin orbit coupling, and full potential for the asphericity of charge densities must be taken into account for the proper treatment of 4f states. We find the variation of total energy as a function of lattice constants that indicate multiple structural phases in Ce contrasting to a single stable structure obtained in other light lanthanides. The 4f orbital magnetic moments are partially quenched as a result of crystalline electric field splitting that leads to magnetocrystalline anisotropy. The charge density plots have similar asphericity and environment in Pr and Nd indicating similar magnetic anisotropy. However, Ce and Sm show completely different asphericity and environment as both orbital moments are significantly quenched. In addition, the Fermi surface structures exemplified in Nd indicate structural stability and unravel a cause of anisotropy. The calculated magnetocrystalline anisotropy energy (MAE) reveals competing c-axis and in-plane anisotropies, and also predicts possibilities of unusual structural deformations in light lanthanides. The uniaxial magnetic anisotropy is obtained in the double hexagonal closed pack structures of the most of the light lanthanides, however, the anisotropy is reduced or turned to planar in the low symmetry structures. Through crystal field calculations we also illustrate the crystal field ground state 4f multiplets of light lanthanides.

Cerium doped TiO2 photoanode for an efficient quasi-solid state dye sensitized solar cells based on polyethylene oxide/multiwalled carbon nanotube/polyaniline gel electrolyte

We have synthesized rare earth element cerium (Ce3+) doped TiO2 nanoparticles by using hydrothermal method. This doped TiO2 is used as photoanode in dye sensitized solar cells (DSSCs). The nanoparticles were characterized by using X-ray diffraction (XRD), energy dispersive X-ray (EDX) and UV–visible spectroscopy. From XRD it was found that the anatase crystalline phase keeps unchanged after Ce3+ doping while the crystallite size decreases. There is a decrease in the band gap of doped TiO2 is observed, Ce3+ positively changes the conduction band minimum of TiO2 due to the introduction of unoccupied 4f states of Ce3+. The gel electrolyte was prepared by in situ polymerization of aniline in the mixture of PEO and c-MWCNT. The synthesized gel electrolyte was characterized by Fourier transform infrared spectroscopy (FTIR), SEM, and EDX analyses. The thermal stability of these gels also increased with the addition of c-MWCNT. The present study is concerned with effect of c-MWCNT and Ce3+ on the conversion efficiency of the quasi solid state DSSCs. DSSCs fabricated with 0.1% c-MWCNT c in PEO/PAni and TiO2 as photoanode achieved maximum conversion efficiency of 1.62%. The introduction of c-MWCNT improved ionic conductivity of composite electrolytes and enhanced interfacial contact between electrode and electrolyte. The Ce3+@TiO2 photoanode influences the performance of DSSCs due to the increased electron injection. 0.5wt% Ce3+@TiO2 photoanode gives a maximum PCE of 4.08%, Jsc of 7.36mAcm−2 and Voc of 0.76V.

Mechanism of Enhanced Sulfur Tolerance on Sm-Doped CeO2: A Density Functional Study

In this presentation, using first-principles calculation, the function of samarium (Sm) 4f states and Sm-perturbed O 2p states in enhancing the sulfur tolerance of Sm-doped CeO2 was elucidated. We find that the sulfur tolerance of Sm-doped CeO2 is closely related to the modification of O 2p states by the strong interaction between Sm 4f and O 2p states. In particular, the availability of unoccupied O 2p states near the Fermi level is responsible for enhancing the sulfur tolerance of Sm-doped CeO2 compared to the pure CeO2[1]. To better understand the importance of unoccupied O 2p states, the effect of various 4f shell dopants (Pr, Pm, and Eu) on the activity of surface lattice oxygen are also investigated. This work also hints on the importance of properly engineering the activity of oxygen ion in CeO2-based materials by adding dopant in order to develop sulfur-tolerant solid oxide fuel cell anode. [1] DH Lim, HS Kim, SP Yoon, J Han, CW Yoon, SH Choi, SW Nam, and HC Ham, Physical Chemistry Chemical Physics 16 (22), 10727-10733, (2014)

Photoluminescence excitation spectra of lanthanide doped YAlO3 in vacuum ultraviolet region

To understand luminescent mechanisms of lanthanide (Ln) doped phosphors, it is important to know the energy positions of unoccupied Ln2+ 4f and Ln3+ 5d states, as well as occupied Ln3+ 4f states, relative to the energy bands of host materials. Photoluminescence excitation (PLE) spectra of Ln doped YAlO3 were measured in a vacuum ultraviolet (VUV) region and the energy positions of Ln2+ 4f and Ln3+ 5d states in the wide-gap YAlO3 were elucidated. Peaks assignable to host lattice excitation were observed in all samples at approximately 8 eV in the PLE spectra. PLE peaks derived from charge transfer (CT) and 4f-5d transitions were observed at lower energy than the bandgap energy. Ln2+ 4f energy levels were obtained from the PLE peak energies for the CT transitions along with the valence band maximum. In contrast, Ln3+ 5d energy levels were evaluated from those for the 4f-5d transitions along with the Ln3+ 4f energy levels, which were obtained previously from X-ray photoelectron spectroscopy measurements. The elucidated Ln2+ 4f and Ln3+ 5d energy levels were exhibited in an energy diagram together with Ln3+ 4f energy levels and host energy bands. The experimental Ln2+ 4f and Ln3+ 5d energy levels were in good agreement with the reported theoretical data.

Influence of Gadolinium-doping on the microstructures and phase transition characteristics of VO2 thin films

Abstract Rare earth (RE)-doping is an effective approach for modulating the microstructures and properties of oxide semiconductors. We investigate the influence of Gadolinium-doping on the microstructures and semiconductor-to-metal transition (SMT) characteristics of VO2 thin films prepared by a reactively co-sputtering process. The chemical state of Gd, microstructures, surface morphologies, and electrical properties of Gd-doped VO2 thin films were analyzed by means of X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and four-point probe measurement, respectively. Gd-doping obviously reduces the grain size, and even induces amorphization in VO2 thin films for a relatively high doping level. The SMT temperature of Gd-doped VO2 thin films decreases with increasing the doping level (x, the concentration ratio of Gd to (V + Gd)). Furthermore, the SMT feature disappears as x increases up to 0.041. The resistivity of the Gd-doped VO2 thin film (x = 0.041) is lower than undoped VO2 thin films by nearly three orders. This can be attributed to weakened dimerization of V atoms and induced Gd 4f state above the lower d||V band due to Gd-doping.

Evolution of a magnetic order in the quasi-kagome lattice system CeRh1–xPdxSn (x ≤ 0.75)

The equiatomic compound CeRhSn with a quasi-kagome lattice of Ce atoms displays a non-Fermi liquid behavior at low temperatures. We have studied how the ground state changes with the substitution of Pd for Rh in CeRh1–xPdxSn (0 ≤ x ≤ 0.75) by measuring the specific heat C, magnetic susceptibility χ, and magnetization M. With increasing x, the lattice parameters a and c increase by 1.6%, the paramagnetic Curie temperature in χ(T) changes from –112 K to 16 K, and M(B = 5 T) at T = 1.8 K increases from 0.05μB/Ce to 1.4μB/Ce. A long-range magnetic order manifests in sharp peaks in C at 0.7, 1.0 and 1.5 K for x = 0.4, 0.5 and 0.65, respectively. An antiferromagnetic order for x = 0.75 is indicated by the observations of peaks in both χ(T) and C(T) at around 3.0 K and metamagnetic behavior in M(B) at B = 2 T. Our results point out that the doping of 4d electrons in the non-Fermi liquid system CeRhSn destroys the coherence of the quasi-kagome lattice and weakens the hybridization between the 4f state and conduction bands, leading to the evolution of antiferromagnetic order.

Dilution effects on the antiferromagnetic Kondo semiconductor CeOs2Al10

We have studied the effects of dilution of Ce sublattice on the unusual antiferromagnetic (AFM) order in the Kondo semiconductor CeOs2Al10 at 28.5 K by the magnetic, transport and specific-heat measurements of single crystals of Ce1-zLazOs2Al10. The effective magnetic moment and paramagnetic Curie temperature hardly change with z up to 0.5, indicating that the 4f state remains unchanged at high temperatures. The suppression of the Néel temperature TN is much weaker than that in 5d hole doped system, Ce(Os1-yRey)2Al10. Therefore, the AFM interaction is robust against the violation of the coherent Ce sublattice. The activation energy in the resistivity decreases in parallel with TN, confirming the argument that the presence of the c-f hybridization gap is a requisite for the unusual AFM order in this system.

Electronic properties of LaAlO3/SrTiO3 n-type interfaces: a GGA+U study

The role of electronic correlation effects for a realistic description of the electronic properties of / heterostructures as covered by the on-site Coulomb repulsion within the GGA+U approach is investigated. Performing a systematic variation of the values of the Coulomb parameters applied to the Ti 3d and La 4f orbitals we put previous suggestions to include a large value for the La 4f states into perspective. Furthermore, our calculations provide deeper insight into the band gap landscape in the space spanned by these Coulomb parameters and the resulting complex interference effects. In addition, we identify important correlations between the local Coulomb interaction within the La 4f shell, the band gap, and the atomic displacements at the interface. In particular, these on-site Coulomb interactions influence buckling within the LaO interface layer, which via its strong coupling to the electrostatic potential in the LAO overlayer causes considerable shifts of the electronic states at the surface and eventually controls the band gap.