4f States Research Articles

Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed.

Low-temperature scanning tunneling microscopy study on the electronic properties of a double-decker DyPc2 molecule at the surface.

To fully achieve potential applications of the double-decker molecules containing rare earth elements as single-molecule magnets in molecular spintronics, it is crucial to understand the 4f states of the rare earth atoms sandwiched in the double-decker molecules by metal electrodes. In this study, low-temperature scanning tunneling microscopy and spectroscopy were employed to investigate the isolated double-decker DyPc2 molecule adsorbed on Au(111) via its differential conductance measurements. The experimental results revealed that the differential conductance maps acquired at a constant height mode simply depicted the authentic molecular orbitals; moreover, the differential conductance maps achieved at a constant current mode could not directly probe the 4f states of the sandwiched Dy atom. This was consistent with the spectra obtained over the molecule center around the Fermi level, indicative of no Kondo feature. Upon decreasing the tip-molecule distance, the CH-mode images presented high-resolution structure but no information of the 4f states. All results indicated that the Dy atom barely contributed to the tunneling current because of the absence of coupling with the microscope tip, echoing the inaccessibility of the Dy 4f states in the double-decker DyPc2 molecule.

Proton conduction of BaCe0.90Y0.10O3 -δ thin film with lattice distortion

The proton conduction of BaCe0.9Y0.1O3-δ (BCY) film on Al2O3 (0001) substrate grown by RF magnetron sputtering has been studied as a function of film thickness (20–600nm) in the temperature region of 300–700°C. The lattice constants and grain size of the as-deposited film increase with increasing film thickness. The film thickness above ~200nm, which close to the lattice constants of the bulk, shows high proton conductivity with small activation energy (EA) of ~0.3eV below 500°C. The BCY thin film has the mixed valent states of Ce4+ and Ce3+ created by the charge transfer from O 2p to Ce 4f states. The EA corresponds to the energy difference between the top of valence band and the Fermi level. These results originate to the changes of O–O bond length and oxygen vacancies in the a–c plane of the BCY thin film.

Orientationally textured thin films of WOx deposited by pulsed laser deposition

Pulsed laser deposition from a compound target in an oxygen atmosphere has been used to produce sub-stoichiometric WOx films of 30 nm thickness on Si(100) and SrTiO3(100) substrates. The growth temperature was 500 °C and the pressure of the O2 background was 2.5 × 10−2 mbar. The films have been assessed using x-ray photoelectron spectroscopy, x-ray reflectivity (XRR), x-ray diffraction (XRD) and scanning electron microscopy. The chemical shift of the tungsten 4f states showed that the tungsten was close to fully oxidized. XRR measurements and scanning electron micrographs showed the films on SrTiO3(100) to be much smoother than those on Si(100) which were granular. XRD in the Bragg–Brentano geometry combined with texture analysis showed that the films were textured with the [001], [010], [100] directions normal to the surface. The films on SrTiO3(100) were found to be biaxially textured with the film directions aligning with those in the substrate. The nature of the texture was sensitive to the laser fluence used. Higher fluence promoted [001] texture whereas lower fluence promoted [010] and [100]. Intermediate fluences produced smooth, highly ordered films with biaxial texture. Investigations using the laser repetition rate indicate that the mechanism for the difference is the overall deposition rate, which is affected by fluence. On Si(100) the films were rougher and exhibited only uniaxial texture.

Magnetic hysteresis of cerium doped bismuth ferrite thin films

Abstract The influence of Cerium doping on the structural and magnetic properties of BiFeO 3 thin films have been investigated. Rietveld refinement of X-ray diffraction data and successive de-convolution of Raman scattering spectra of Bi 1− x Ce x FeO 3 (BCFO) thin films with x =0–0.20 reflect the single phase rhombohedral (R3c) formation for x x ≥0.08. All low wavenumber Raman modes ( −1 ) showed a noticeable shift towards higher wavenumber with increase in doping concentration, except Raman E-1 mode (71 cm −1 ), shows a minor shift. Sudden evolution of Raman mode at 668 cm −1 , manifested as A 1 -tetragonal mode, accompanied by the shift to higher wavenumber with increase in doping concentration ( x ) affirm partial structural phase transition. Anomalous wasp waist shaped ( M–H ) hysteresis curves with improved saturation magnetization ( M s ) for BCFO thin films is attributed to antiferromagnetic interaction/hybridization between Ce 4f and Fe 3d electronic states. The contribution of both hard and soft phase to the total coercivity is calculated. Polycrystalline Bi 0.88 Ce 0.12 FeO 3 thin film found to exhibit better magnetic properties with M s =15.9 emu/g without any impure phase.

Stability and diffusion properties of Ti atom on α-uranium surfaces: A first-principles study

Titanium (Ti) film is experimentally used to provide good protection for uranium (U) surface from corrosion, but the performance of the protector is limited by the strength of the Ti/U interface. Therefore, it is important to further understand the interaction between Ti atoms and U surfaces. Here the first-principles density functional theory approaches with spin polarization and non-spin polarization have been employed to study the stability and diffusion properties of Ti atoms on α-uranium (001) and (100) surfaces. The energetic, geometric parameters and electronic structures have been analyzed. For Ti/U(001) surface, it is found that the most energetically favoured configuration is Ti atom occupying the hollow2 site. Ti atom tends to diffuse from hollow1 site to hollow2 site via the longbridge site. The U–Ti bond is dominated by the hybridization between Ti 3d and U 5f states, and the negative charge is transferred to the first U layer from the adsorbed Ti atom. For Ti/U(100) surface, the hollow site is the most preferred site for the adsorption of Ti atom. Ti atom will overcome a low barrier of ∼0.2eV when it diffuses from one hollow site to another via the longbridge site. The topmost two U layers get electrons from Ti atom and the deeper U layers.

Ab initio calculations of B2 type RHg (R = Ce, Pr, Eu and Gd) intermetallic compounds

Spin polarized ab initio calculations have been carried out to study the structural, electronic, elastic and thermal properties of RHg (R = Ce, Pr, Eu and Gd) intermetallic compounds in B2 structure. The calculations have been performed by using both generalized gradient approximation (GGA) and local spin density approximation (LSDA). The calculated value of lattice constant (a0) for these compounds with GGA is in better agreement with the experimental data than those with LSDA. Bulk modulus (B), first-order pressure derivative of bulk modulus and magnetic moment (μB) are also presented. The energy band structure and electron density of states show the occupancy of 4f states for light as well as heavy rare earth atom. The elastic constants are predicted from which all the related mechanical properties like Poisson’s ratio (σ), Young’s modulus (E), shear modulus (GH) and anisotropy factor (A) are calculated. The ductility or brittleness of these compounds is predicted from Pugh’s rule (B/GH) and Cauchy pressure (C12 − C44). The Debye temperature (θD) is estimated from the average sound velocity, which have not been calculated and measured yet.

The effect of Sm-doping on optical properties of LaB6 nanoparticles

Nanocrystalline particles of LaB6, SmB6 and Sm-doped LaB6 have been prepared by a solid-state reaction in order to investigate the optical properties of ternary rare-earth hexaborides. The sizes of prepared nanoparticles range from dozens to more than 200nm, as confirmed by XRD, SEM and TEM examinations. The optical property concerning the absorption spectra was tested with ultraviolet–visible–near infrared (UV–vis–NIR) absorption spectrum. All samples exhibit high absorbance in NIR range and UV range. The increase of Sm-doping amount shifts the position of minimum absorptance value of LaB6 to the long-wave direction. Density functional theory (DFT) is employed to interpret the optical properties of Sm-doped LaB6, and results indicate that Sm 4f states change the DOS at near Fermi surface of LaB6 after Sm doping and the reduced number of conduction electrons results into the change of absorption spectra.

Deduction of the chemical state and the electronic structure of Nd2Fe14B compound from X-ray photoelectron spectroscopy core-level and valence-band spectra

Characterization of chemical state and electronic structure of the technologically important Nd2Fe14B compound is attractive for understanding the physical nature of its excellent magnetic properties. X-ray photoelectron spectroscopy (XPS) study of such rare-earth compound is important and also challenging due to the easy oxidation of surface and small photoelectron cross-sections of rare-earth 4f electrons and B 2p electrons, etc. Here, we reported an investigation based on XPS spectra of Nd2Fe14B compound as a function of Ar ion sputtering time. The chemical state of Fe and that of B in Nd2Fe14B compound can be clearly determined to be 0 and −3, respectively. The Nd in Nd2Fe14B compound is found to have the chemical state of close to +3 instead of +3 as compared with the Nd in Nd2O3. In addition, by comparing the valence-band spectrum of Nd2Fe14B compound to that of the pure Fe, the contributions from Nd, Fe, and B to the valence-band structure of Nd2Fe14B compound is made more clear. The B 2p states and B 2s states are identified to be at ∼11.2 eV and ∼24.6 eV, respectively, which is reported for the first time. The contribution from Nd 4f states can be identified both in XPS core-level spectrum and XPS valence-band spectrum. Although Nd 4f states partially hybridize with Fe 3d states, Nd 4f states are mainly localized in Nd2Fe14B compound.

Valence States in CeVO4 and Ce0.5Bi0.5VO4 Probed by Density Functional Theory Calculations and X-ray Photoemission Spectroscopy

Ce is one of the few lanthanide elements to exhibit well-defined (III) and (IV) oxidation states in solid-state environments, and there is therefore ambiguity as to whether CeVO4 should be formulated as Ce(III)V(V)O4 or Ce(IV)V(IV)O4. To address this question, CeVO4 and Ce0.5Bi0.5VO4 have been studied by density functional theory calculations and X-ray photoemission spectroscopy. A peak above the main O 2p valence band in photoemission is attributed to localized Ce 4f states, in agreement with the calculations which show occupation of Ce 4f states. The Ce 3d core level spectrum is diagnostic of Ce(III) with no sign of a peak associated with 4f0 final states that are characteristic of Ce(IV) compounds. The experimental and theoretical results thus confirm that both compounds contain Ce(III) and V(V), rather than Ce(IV) and V(IV). In agreement with experiment, the calculations also show that the tetragonal zircon phase adopted by CeVO4 is more stable for Ce0.5Bi0.5VO4 than the monoclinic clinobisvanite phase adopted by BiVO4, so that the formation of the stereochemically active Bi(III) lone pairs is suppressed by Ce doping.

Structural and magnetic properties of Gd-doped ZnO

We use density functional theory to investigate structural and magnetic properties of Gd doped ZnO, accounting for the impurity 4f states using the GGA + U method. (i) We calculate the binding energy of forming [Gd–Gd] dimers, [VO − GdZn] and [VZn − GdZn] complexes and find that while the formation of [VZn − GdZn] is favourable, [GdZn − GdZn] and [VO − GdZn] complexes are less likely to form. Next, (ii) we investigate the spacial arrangement of two (and three) GdZn impurities in a 3 × 3 × 2 supercell and find that the magnetic impurities are energetically favourable when occupying distant lattice sites. Finally, we study the nature of interactions between the magnetic impurities (iii) for Gd in nearest-neighbour and non-nearest-neighbour Zn sites, (iv) in the presence of Zn or O vacancies, and (v) with and without additional charge carriers. Our results show mainly paramagnetic behaviour. In a few cases, e.g. magnetic impurities occupying in-plane nearest-neighbour zinc sites with n-type carrier doping, weak ferromagnetic coupling is observed. This magnetic ordering is of the order of a few meV and can be easily destroyed by thermal fluctuations. We thus expect Gd:ZnO to show paramagnetic behaviour at temperatures approaching room temperature.

Enhanced visible light photocatalytic activity of Gadolinium doped nanocrystalline titania: An experimental and theoretical study

HypothesisUndoped TiO2 nanoparticles are considered as a poor photocatalytic candidate in visible light due to the wide band gap. Incorporation of Gd ions is expected to modulate the electronic structure of the material and thereby enhance the photocatalytic properties of the material. ExperimentsGadolinium doped TiO2 nanoparticles were fabricated via a simple sol–gel method. FindingsThe surface area of Gd doped TiO2 (225m2/g) nanoparticles is much higher than that of undoped TiO2 (95m2/g). Doping of Gadolinium enhances the visible light absorption property of TiO2 nanoparticles. Photoluminescence intensity increases at 0.03 and 0.05mol and thereafter reduces at 0.07mol. The photocatalytic efficiency of these nanoparticles is evaluated by observing degradation of phenol in aqueous solution under visible light. The doped nanoparticles are found to exhibit better photocatalytic activity. This enhancement has been attributed to the introduction of the Gd 4f energy levels in the band gap of TiO2. The presence of these states has been further confirmed by theoretical study based on density functional theory (DFT). It is speculated that the 4f states of Gd act as efficient electron trap centers. These 4f states facilitate electron migration to the surface making available free carriers to take part in photocatalysis.

Cerium induced ferromagnetic exchange interactions in GaN

Experimental and first principle investigations on Ce:GaN diluted magnetic semiconductor were carried out to search the ferromagnetism . Magnetic properties of Ce implanted p-GaN films were studied using a SQUID magnetometer . Magnetization measured as function of temperature and applied field indicated the presence of room temperature ferromagnetism. First principle calculations results indicated Ce 4f states hybridized with bottom of conduction band of Ce:GaN. The delocalization of 4f electrons seems to be responsible for the observed ferromagnetism in terms of itinerant electrons. • M – T curves indicate room temperature ferromagnetism in Ce implanted GaN. • M – H / T curves indicate the absence of supermagnetism. • First principle calculations indicate Ce 4f states hybridized with bottom of CB. • Possible exchange interaction is discussed. • Larger valance effective mass indicate electron carriers be more effective for stabilization of ferromagnetism.

Theoretical studies of strongly correlated rare-earth intermetallics RIn3 and RSn3 (R = Sm, Eu, and Gd)

In this paper, the structural, elastic, and electronic properties of RIn3 and RSn3 (R = Sm, Eu, Gd) compounds have been investigated using the full potential linearized augmented plane wave plus local orbital method within the density functional theory. The structural properties are investigated using the LDA, GGA, and the band correlated LDA+U and GGA+U schemes. The lattice parameters are in good agreement with the available experimental results and the divalent state of Eu is also verified. The spin-orbit coupling is included in order to predict the correct electronic properties and splitting of 4f states of the rare earth elements is also incorporated. We calculated Bulk modulus, shear modulus, Young's modulus, anisotropic ratio, Kleinman parameters, Poisson's ratio, Lame's co-efficient, sound velocities for shear and longitudinal waves, and Debye temperature. We also predict the Cauchy pressure and B/G ratio in order to explore the ductile and brittle behaviors of these compounds.

Surface chemical and photocatalytic consequences of Ca-doping of BiFeO3 as probed by XPS and H2O2 decomposition studies

Pure and Ca-doped Bi1−xCaxFeO3 samples were prepared with x=0.0–0.2, adopting a sol–gel method. Previously reported studies performed on similarly composed and prepared samples revealed that Ca-doping, above solubility limit (namely at ≥10%-Ca), results in phase separation and formation of BiFeO3/α(γ)-Fe2O3 nanocomposite particles. Hetero p/n nanojunctions thus established were considered to help separating photo-generated electron–hole pairs and, therefore, explain consequent promotion of photo-Fenton catalytic activity of BiFeO3 towards methylene blue degradation in presence of H2O2 additive. However, the encompassed decomposition of H2O2 was not addressed. To bridge this gap of knowledge, the present investigation was designed to assess Ca-doping-effected surface chemical modifications and gauge its impact on the heterogeneous photo-/thermo-catalytic activity of BiFeO3 towards H2O2 decomposition, by means of X-ray photoelectron spectroscopy (XPS) and H2O2 decomposition gravimetry. XPS results revealed generation of high binding energy Bi 4f and Fe 2p states, as well as enhancement of the surface basicity, upon doping to 10%-Ca. These surface chemical consequences are rendered hardly detectable upon further increase of the dopant magnitude to 20%-Ca. In parallel, the H2O2 decomposition activity of the ferrite, under natural visible light, is enhanced to optimize upon Ca-doping at 10%, and, then, decreased on further doping to 20%. H2O2 decomposition experiments carried out in absence of light indicate that the doping promoting impact is reflected essentially in the photocatalytic activity. Accordingly, the observed surface chemical consequences of Ca-doping are considered to consolidate the p/n nanojunctions consequently established in the material bulk, by retarding recombination of visible light generated electron–hole pairs, thus enhancing the heterogeneous photocatalytic activity of BiFeO3.

An efficient 1.8 μm emission in Tm3 + and Yb3 +/Tm3 + doped fluoride modified germanate glasses for a diode-pump mid-infrared laser

The competition between infrared down-conversion (DC) and visible up-conversion (UC) is systemically studied in Tm3+ and/or Yb3+/Tm3+ co-doped fluoride modified germanate glasses. Based on the absorption spectra, the Judd–Ofelt intensity parameters, absorption and emission cross-sections, figure of merit (FOM), and saturated gain coefficient are calculated and analyzed. An intense 1.8μm emission is obtained upon 808nm laser diode (LD) excitation in Tm3+ doped samples, while an efficient 1.8μm emission is observed by 980nm LD in Yb3+/Tm3+ co-doped samples, along with visible UC emissions. It is found that 3F4 state of Tm3+ can be easily populated and then further excited to higher excited states through Yb3+ upon 980nm LD. Our results suggest that these Tm3+ doped fluoride modified germanate glasses are of great significance for searching efficient mid-infrared laser materials.

First-Principles Investigation of Electronic Structure Features of TbOFeAs Compound

We report a theoretical investigation on the electronic and magnetic properties of rare-earth pnictide parent compound, such as TbOFeAs. Employing first-principles method supplemented by the local spin density approximation (LSDA), we discuss the electronic structure with the incorporation of the role of Coulomb on-site repulsion (U) of Tb 4f states as well as the spin-orbit (SO) coupling on the magnetic and nonmagnetic phases. For ferromagnetic (FM) and antiferromagnetic (AFM) phases, we have determined the spin and orbital magnetic moments of Tb ions and confer the significance of the spin-orbit interaction of Tb 4f states in this parent compound. In the FM state, the reduction of Fe moment is about a factor of 3.5 with respect to AFM configuration. The most energetically favorable state is AFM configuration. Our theoretical findings surmise that the magnetic moments on Fe sites carry an AFM order. Based on LSDA + U + SO approximation, we infer that the Tb magnetic moments also carry an AFM order, albeit the spin Tb sites in TbO layer possess the same orientation as the Fe spins in FeAs layer. With the incorporation of on-site Coulomb repulsion and spin-orbit interaction in AFM state, the Fe 3d states are large near the Fermi level and this phase is illustrating a metallic behavior. Moreover, the Fermi surface topology and nesting features are presented.

First-principles theory of multipolar order in actinide dioxides

Abstract Magnetic phase transitions that involve multipolar degrees of freedom have been widely studied during the last couple of decades, challenging the common approximation which assumes that the physical properties of a magnetic material could be effectively described by purely dipolar degrees of freedom. Due to the complexity of the problem and to the large number of competing interactions involved, the simple (fcc) crystal structure of the actinide dioxides made them the ideal playground system for such theoretical and experimental studies. In the present paper, we summarize our recent attempts to provide an ab initio description of the ordered phases of UO2, NpO2, and AmO2 by means of state-of-the-art LDA + U first-principles calculations. This systematic analysis of the electronic structures is here naturally connected to the local crystalline fields of the 5f states in the actinide dioxide series. Related to these we find that the mechanisms which lead to the experimentally observed insulating ground states work in distinctly different ways for each compound.

Electronic structure and magnetic susceptibility of monoclinic α-plutonium

The electronic structure of the α-phase of plutonium has been calculated by the band methods with allowance for the spin-orbit interaction and Coulomb correlations in the complete matrix form (the LDA + U + SO method). The strong spin-orbit interaction of the 5 f electrons is manifested in the splitting of the calculated density of the 5f states, which makes a small contribution at the Fermi level on the order of the contribution from the 6d states. Using the results of the ab initio calculations, the spin and orbit contributions to the magnetic susceptibility of α-plutonium have been determined. Along with the impurity contribution, they describe well the experimental data on the susceptibility of this plutonium phase to a temperature of 300 K.

First-principles prediction of the magnetism of 4f rare-earth-metal-doped wurtzite zinc oxide

Electronic structure and magnetic properties of wurtzite ZnO semiconductor doped with rare earth (RE=La, Ce, Pr, Pm, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) atoms were studied using spin-polarized density functional theory based on the full-potential linear augmented plane wave (FP-LAPW) method as implemented in the Wien2k code. In this approach the generalized gradient approximation (GGA) was used for the exchange-correlation (XC) potential. Our results showed that the substitution of RE ions in ZnO induced spins polarized localized states in the band gap. Moreover, the studied DMSs compounds retained half metallicity at dopant concentration x=0.625% for most of the studied elements, with 100% spin polarization at the Fermi level (EF). The total magnetic moments of these compounds existed due to RE 4f states present at EF, while small induced magnetic moments existed on other non-magnetic atoms as well. Finally, the energy difference between far and near configurations was investigated. It was found that the room temperature ferromagnetism was possible for RE-doped ZnO at near configuration. Since the RE-RE separation was long enough (far configuration) for magnetic coupling, the system became paramagnetic or antiferromagnetic ground state.