Ln 4f Research Articles

Are 4f-Orbitals Engaged in Covalent Bonding Between Lanthanides and Triphenylphosphine Oxide? An Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory Study.

The bonding covalency between trivalent lanthanides (Ln = La, Pr, Nd, Eu, Gd) and triphenylphosphine oxide (TPPO) is studied by X-ray absorption spectra (XAS) and density functional theory (DFT) calculations on the LnCl3(TPPO)3 complexes. The O, P, and Cl K-edge XAS for the single crystals of LnCl3(TPPO)3 were collected, and the spectra were interpreted based on DFT calculations. The O and P K-edge XAS spectra showed no significant change across the Ln series in the LnCl3(TPPO)3 complexes, unlike the Cl K-edge XAS spectra. The experimental O K-edge XAS spectra suggest no mixing between the Ln 4f- and the O 2p-orbitals in the LnCl3(TPPO)3 complexes. DFT calculations indicate that the amount of the O 2p character per Ln-O bond is less than 0.1% in the Ln 4f-based orbitals in all of the LnCl3(TPPO)3 complexes. The experimental spectra and theoretical calculations demonstrate that Ln 4f-orbitals are not engaged in the covalent bonding of lanthanides with TPPO, which contrasts the involvement of U 5f-orbitals in covalent bonding in the UO2Cl2(TPPO)2 complex. Results in this work reinforce our previous speculation that bonding covalency is potentially responsible for the extractability of monodentate organophosphorus ligands toward metal ions.

Strengths of covalent bonds in LnO2 determined from O K-edge XANES spectra using a Hubbard model.

In LnO2 (Ln = Ce, Pr, and Tb), the amount of Ln 4f mixing with O 2p orbitals was determined by O K-edge X-ray absorption near edge (XANES) spectroscopy and was similar to the amount of mixing between the Ln 5d and O 2p orbitals. This similarity was unexpected since the 4f orbitals are generally perceived to be "core-like" and can only weakly stabilize ligand orbitals through covalent interactions. While the degree of orbital mixing seems incompatible with this view, orbital mixing alone does not determine the degree of stabilization provided by a covalent interaction. We used a Hubbard model to determine this stabilization from the energies of the O 2p to 4f, 5d(eg), and 5d(t2g) excited charge-transfer states and the amount of excited state character mixed into the ground state, which was determined using Ln L3-edge and O K-edge XANES spectroscopy. The largest amount of stabilization due to mixing between the Ln 4f and O 2p orbitals was 1.6(1) eV in CeO2. While this energy is substantial, the stabilization provided by mixing between the Ln 5d and O 2p orbitals was an order of magnitude greater consistent with the perception that covalent bonding in the lanthanides is largely driven by the 5d orbitals rather than the 4f orbitals.

Electronic structure of rare-earth mononitrides: quasiatomic excitations and semiconducting bands

The electronic structure of the rare-earth mononitrides LnN (where Ln = rare-earth), which are promising materials for future spintronics applications, is difficult to resolve experimentally due to a strong influence of defects on their transport and optical properties. At the same time, LnN are challenging for theory, since wide semiconducting 2p and 5d bands need to be described simultaneously with strongly correlated 4f states. Here, we calculate the many-body spectral functions and optical gaps of a series of LnN (with Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) by a density-functional + dynamical mean-field theory (DFT + DMFT) approach treating the correlated Ln 4f shells within the quasi-atomic Hubbard-I approximation. The on-site Coulomb interaction in the 4f shell is evaluated by a constrained DFT + Hubbard-I approach. Furthermore, to improve the treatment of semiconducting bands in DFT + DMFT, we employ the modified Becke–Johnson semilocal exchange potential. Focusing on the paramagnetic high-temperature phase, we find that all investigated LnN are pd semiconductors with gap values ranging from 1.02 to 2.14 eV along the series. The pd band gap is direct for light Ln = La…Sm and becomes indirect for heavy rare-earths. Despite a pronounced evolution of the Ln 4f states along the series, empty 4f states are invariably found above the bottom of the 5d conduction band. The calculated spectra agree well with those available from x-ray photoemission, x-ray emission and x-ray absorption measurements.

Energy Diagrams of Lanthanide Energy Levels in Perovskite‐Type Calcium‐Based Double Oxides Examined by X‐Ray Photoelectron Spectroscopy

The energy levels of 4f ground states for trivalent lanthanide ions (Ln3+) in CaMO3 (M=Ti, Sn, Zr, Hf) are examined using X‐ray photoelectron spectroscopy (XPS). Some Ln3+ 4f energy levels referred to the valence band maximum (VBM), especially for Tb3+, are evaluated from the Ln3+ 4f states extracted by the difference XPS spectra between nondoped and Ln3+‐doped materials. The energy position of the zigzag curves for the Ln3+ 4f energy levels are referred to the Tb3+ 4f energy levels obtained from the XPS measurements. Energy gaps and Ln2+ 4f energy levels are also estimated from the absorption and photoluminescence excitation spectra of nondoped and Eu3+‐doped materials. On the basis of the experimental values, the energy diagrams aligned at the VBM are illustrated and compared to understand the Ln 4f energy levels in the electronic structures of the perovskite‐type calcium‐based double oxides.

Structural and electronic properties of exohedrally doped neutral silicon clusters LnSin (n = 5, 10; Ln = Sm, Eu, Yb)

Lanthanide-doped silicon clusters have been extensively studied in the fields of optoelectronics, magnetism and nanomaterials during the last decade. Herein, systematic structure searches for typical neutral clusters of lanthanide-doped silicon clusters LnSin (n = 5, 10; Ln = Sm, Eu, Yb) have been performed by means of density functional theory coupled with the "stochastic kicking" global search technique. It is found that the Ln atom in LnSin prefers to locate on the surface of Sin to form an exohedral structure, and this exohedral configuration may dominate the nascent structure of LnSin. The spin density and Mulliken population analyses indicate that LnSin clusters possess remarkable magnetic moments (except for YbSin), which are mainly supplied by the Ln 4f electrons (except for Yb). Density of states visually shows the significant spin polarization for open-shell structures of SmSin and EuSin. As for the YbSin (n = 5, 10) system, it has a closed-shell electronic structure with a large HOMO-LUMO gap of 2.72 eV. Bonding analysis, including localized orbital locator and electron density difference, shows that the Si-Si covalent interaction and Sm-Si electrostatic interaction are important for the structural stability of LnSin.

Chemical Bonding of Crystalline LnB6 (Ln = La-Lu) and Its Relationship with Ln2B8 Gas-Phase Complexes.

Solid-state lanthanide (Ln) borides of the simple LnB6 composition not only exhibit exciting physical behavior, in particular magnetic properties, but their electronic structure and chemical bonding are particularly intriguing as well. To shed more light on the latter, we have performed quantum-chemical (DFT+ U) electronic-structure calculations and bonding analyses of the entire LnB6 series with Ln from La to Lu. Trivially, the boron framework is held together by the B 2sp orbitals, and this framework bonds to the Ln atoms via covalent-ionic interactions. The Ln 4f electrons, however, are decisive for the magnetic properties. In more detail, the effective charges of the Ln atoms as calculated by (Mulliken or Löwdin) occupation numbers of the 6s/5d/4f orbitals are compatible with experimentally assigned oxidation numbers. The shorter inter-octahedral B-B bonds, dominated by 2s-2p interactions, turn out to be stronger than the intra-octahedral B-B bonding with a more 2p-2p-like character. Interestingly, there are strong structural similarities between the LnB6 motif studied here and gas-phase Ln2B8 species showing inverse sandwich structures, and these similarities are also reflected in the electronic structure. In particular, Ln2B8 is predicted to have a large electron affinity. Hence, this work aims at providing an intrinsic link between gas-phase complexes and solid-state crystal structures in order to better understand the former species.

Electronic structure of Ln2O2Te (Ln=La, Sm and Gd) by X-ray absorption spectroscopy

The oxytellurides are layered structure material which shows low lattice thermal conductivity and hence making them suitable for high temperature thermoelectric applications. We report the electronic structural properties of rare-earth doped oxytellurides [Ln2O2Te (Ln = La, Sm and Gd)] prepared by solid state reaction method. This research is focused on the suitability of rare-earth oxychalcogenide as a thermoelectric material using the electronic structure investigations. The X-ray absorption spectra at O K-edge show the O 2p bonding with Ln 5d and 4f and Te 5s states and the Ln M-edges show that the valences of Ln are to be trivalent or mixed states. The Ln M-edges are also simulated with Cowan code, and electronic structures of these oxytellurides are discussed. The substitution of different rare earth metals at the Ln-site, may enhance the thermoelectric property and stability of the material.

Exotic electronic structures of SmxCe3-xOy (x = 0-3; y = 2-4) clusters and the effect of high neutral density of low-lying states on photodetachment transition intensities.

Lanthanide (Ln) oxide clusters have complex electronic structures arising from the partially occupied Ln 4f subshell. New anion photoelectron (PE) spectra of SmxCe3-xOy- (x = 0-3; y = 2-4) along with supporting results of density functional theory (DFT) calculations suggest interesting x and y-dependent Sm 4f subshell occupancy with implications for Sm-doped ionic conductivity of ceria, as well as the overall electronic structure of the heterometallic oxides. Specifically, the Sm centers in the heterometallic species have higher 4f subshell occupancy than the homonuclear Sm3Oy-/Sm3Oy clusters. The higher 4f subshell occupancy both weakens Sm-O bonds and destabilizes the 4f subshell relative to the predominantly O 2p bonding orbitals in the clusters. Parallels between the electronic structures of these small cluster systems with bulk oxides are explored. In addition, unusual changes in the excited state transition intensities, similar to those observed previously in the PE spectra of Sm2O- and Sm2O2- [J. O. Kafader et al., J. Chem. Phys. 146, 194310 (2017)], are also observed in the relative intensities of electronic transitions to excited neutral state bands in the PE spectra of SmxCe3-xOy- (x = 1-3; y = 2, 4). The new spectra suggest that the effect is enhanced with lower oxidation states and with an increasing number of Sm atoms, implying that the prevalence of electrons in the diffuse Sm 6s-based molecular orbitals and a more populated 4f subshell both contribute to this phenomenon. Finally, this work identifies challenges associated with affordable DFT calculations in treating the complex electronic structures exhibited by these systems, including the need for a more explicit treatment of strong coupling between the neutral and PE.

Ln3+ 4f Energy Levels in CaTiO3 Analyzed by XPS Measurements

The energy levels of Ln3+ 4f ground states in CaTiO3 are analyzed using X‐ray photoelectron spectroscopy (XPS). The Ln 4f states in the XPS spectra are extracted utilizing large photoionization cross section differences between Ln 4f and O 2p states. The Ln3+ 4f energy levels referred to the valence band maximum are evaluated to be 0.8 eV for Pr3+, −5.0 eV for Gd3+, 0.9 eV for Tb3+, and −4.6 eV for Lu3+ from the extracted XPS spectra. Although the obtained values of Gd3+ and Lu3+ 4f levels are deeper in the valence band, the values of Pr3+ and Tb3+ 4f levels are in good agreement with those reported previously. The difference in the photoluminescence colors between Pr3+‐doped CaTiO3 and CaZrO3 is simply explained from the Pr3+ 4f energy levels located in the bandgaps.

Quantitative Evidence for Lanthanide-Oxygen Orbital Mixing in CeO2, PrO2, and TbO2.

Understanding the nature of covalent (band-like) vs ionic (atomic-like) electrons in metal oxides continues to be at the forefront of research in the physical sciences. In particular, the development of a coherent and quantitative model of bonding and electronic structure for the lanthanide dioxides, LnO2 (Ln = Ce, Pr, and Tb), has remained a considerable challenge for both experiment and theory. Herein, relative changes in mixing between the O 2p orbitals and the Ln 4f and 5d orbitals in LnO2 are evaluated quantitatively using O K-edge X-ray absorption spectroscopy (XAS) obtained with a scanning transmission X-ray microscope and density functional theory (DFT) calculations. For each LnO2, the results reveal significant amounts of Ln 5d and O 2p mixing in the orbitals of t2g (σ-bonding) and eg (π-bonding) symmetry. The remarkable agreement between experiment and theory also shows that significant mixing with the O 2p orbitals occurs in a band derived from the 4f orbitals of a2u symmetry (σ-bonding) for each compound. However, a large increase in orbital mixing is observed for PrO2 that is ascribed to a unique interaction derived from the 4f orbitals of t1u symmetry (σ- and π-bonding). O K-edge XAS and DFT results are compared with complementary L3-edge and M5,4-edge XAS measurements and configuration interaction calculations, which shows that each spectroscopic approach provides evidence for ground state O 2p and Ln 4f orbital mixing despite inducing very different core-hole potentials in the final state.

Theoretical Study on the Photoelectron Spectra of Ln(COT)2–: Lanthanide Dependence of the Metal–Ligand Interaction

We have performed a theoretical analysis of the recently reported photoelectron (PE) spectra of the series of sandwich complex anions Ln(COT)2- (Ln = La-Lu, COT = 1,3,5,7-cyclooctatetraene), focusing on the Ln dependence of the vertical detachment energies. For most Ln, the π molecular orbitals, largely localized on the COT ligands, have the energy order of e1g < e1u < e2g < e2u as in the actinide analogues, reflecting the substantial orbital interaction with the Ln 5d and 5p orbitals. Thus, it would be expected that the lanthanide contraction would increase the orbital interaction so that the overlaps between the COT π and Ln atomic orbitals tend to increase across the series. However, the PE spectra and theoretical calculations were not consistent with this expectation, and the details have been clarified in this study. Furthermore, the energy level splitting patterns of the anion and neutral complexes have been studied by multireference ab initio methods, and the X peak splittings observed in the PE spectra only for the middle-range Ln complexes were found to be due to the specific interaction between the Ln 4f and ligand π orbitals of the neutral complexes in e2u symmetry. Because the magnitude of this 4f-ligand interaction depends critically on the final state 4f electron configuration and the spin state, a significant Ln dependence in the PE spectra is explained.

Creation of 4fn-4fn-15d1 Transition Energy Diagram of Trivalent Lanthanide Ions in Fluorides Based on First-Principles Calculations

The theoretical prediction of the 4fn-4fn-15d1 transition energies of lanthanides in crystals is an important issue in the development of novel luminescent materials such as solid-state lasers or phosphors. In our previous work, the first-principles calculations of the entire multiplet structures of lanthanide (Ln) ions in LiYF4 (YLF) corresponding to 4fn and 4fn-15d1 configurations were performed using the relativistic discrete variational multi-electron (DVME) method [1]. However, the lowest 4fn-4fn-15d1transition energy was overestimated for light lanthanides while it was underestimated for heavy lanthanides. In this work, in order to improve the accuracy of the theoretical transition energy, a correction based on the molecular orbital (MO) calculation in the Slater’s transition state was introduced. The LnF8 clusters were constructed based on the crystal structures of YLF and CaF2. The fully-relativistic MO calculations were performed using the relativistic DV-Xα code. The multiplet calculations were performed using the relativistic DVME code. The Slater determinants corresponding to 4fn and 4fn-15d1 configurations were constructed from the four-component relativistic MOs and used as the basis functions to diagonalize the relativistic many-electron Hamiltonian. In order to correct the errors in the transition energy, MO calculations in the Slater’s transition state where a half electron was excited from the Ln 4f level to the Ln 5d level were also performed. A constant correction term was added to the diagonal matrix element of the many-electron Hamiltonian for 4fn-15d1 configuration so that the 4fn -4fn-15d1transition energy is consistent with the 4f-5d transition energy obtained by the MO calculation in the Slater’s transition state. The diagrams representing the lowest 4fn-4fn-15d1transition energies of trivalent lanthanide ions in YLF calculated with and without the above correction are shown in Fig. 1. The overestimation in the light lanthanides and the underestimation in the heavy lanthanides are significantly reduced by the correction and the theoretical transition energies approached to the experimental values in the literature [2-5].

Determination of 4f energy levels for trivalent lanthanide ions in YAlO3 by X-ray photoelectron spectroscopy

A simple method to analyze 4f energy levels of trivalent lanthanide (Ln) ions was demonstrated by conventional X-ray photoelectron spectroscopy (XPS) measurements using Ln ions doped YAlO3 sintered polycrystalline samples. Although XPS peaks derived from Ln 4f states overlapped with the host's valence band consisting of O 2p states, the difference XPS spectra between Ln doped and non-doped samples showed only the Ln 4f peaks due to the large difference of photoionization cross sections between Ln 4f and O 2p orbitals. The difference spectra showing Ln 4f states were aligned at the valence band maximum (VBM) making use of the peaks of Al 2p inner shells, and the Ln3+ 4f energy levels referred to the VBM were determined from the Ln3+ 4f peak energies. The Ln3+ 4f energy levels obtained by this simple method were in good agreement with those previously obtained by resonant ultraviolet photoelectron spectroscopy measurements using single crystal samples.

Highly Sensitive Dual-Phase Nanoglass-Ceramics Self-Calibrated Optical Thermometer.

A strategy to achieve high sensitivity of noncontact optical thermometer via the structure design of nanoglass-ceramic and the usage of Ln(3+) (Ln = Eu, Tb, Dy) luminescence as reference signal and Cr(3+) emission as temperature signal was provided. Specifically, the synthesized dual-phase glass-ceramics were evidenced to enable spatially confined doping of Ln(3+) in the hexagonal GdF3 nanocrystals and Cr(3+) in the cubic Ga2O3 nanoparticles, being beneficial to suppressing detrimental energy transfer between Ln(3+) and Cr(3+) and thus significantly enhancing their luminescence. As a consequence, completely different temperature-sensitive luminescence of Ln(3+)4f → 4f transition and Cr(3+) 3d → 3d transition in the present glass-ceramic resulted in obvious variation of Cr(3+)/Ln(3+) fluorescence intensity ratio with temperature and strikingly high detecting temperature sensitivity of 15-22% per K. We believe that this preliminary study will provide an important advance in exploring other innovative optical thermometry.

Evidence for 5d-σ and 5d-π covalency in lanthanide sesquioxides from oxygen K-edge X-ray absorption spectroscopy.

The electronic structure in the complete series of stable lanthanide sesquioxides, Ln2O3 (Ln = La to Lu, except radioactive Pm), has been evaluated using oxygen K-edge X-ray absorption spectroscopy (XAS) with a scanning transmission X-ray microscope (STXM). The experimental results agree with recent synthetic, spectroscopic and theoretical investigations that provided evidence for 5d orbital involvement in lanthanide bonding, while confirming the traditional viewpoint that there is little Ln 4f and O 2p orbital mixing. However, the results also showed that changes in the energy and occupancy of the 4f orbitals can impact Ln 5d and O 2p mixing, leading to several different bonding modes for seemingly identical Ln2O3 structures. On moving from left to right in the periodic table, abrupt changes were observed for the energy and intensity of transitions associated with Ln 5d and O 2p antibonding states. These changes in peak intensity, which were directly related to the amounts of O 2p and Ln 5d mixing, were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by the O K-edge XAS is discussed in the context of several recent theoretical and physical studies on trivalent lanthanide compounds.

Lanthanide-doped LaSi3N5 based phosphors: Ab initio study of electronic structures, band gaps, and energy level locations

The electronic structures and band gaps of lanthanide (Ln)-doped LaSi3N5 are calculated using the sophisticated screened Coulomb hybrid HSE06-PBE functional. Doping of LaSi3N5 with Ln3+ and Ln2+ dopants results in placing of the Ln 4f bands in the gap between the valence band and the conduction band of the host LaSi3N5. The location of Ln 4f states causes a change in the electronic structures and band gaps and a dramatic change in the luminescence properties of LaSi3N5-based phosphors. The energy level locations are constructed from the ab initio calculated electronic structures and compared with the empirical energy level diagram. The calculated scheme displays all the states which can be used to describe the luminescence process predicted in Ln-doped LaSi3N5. This study can be applied to all compounds doped with lanthanides dopants and provides a tool for materials development.

The electronic configurations of LnX (Ln = La–Eu, X = O, S, Se, Te): A FON–DFT investigation

Relativistic density functional theory calculation has been carried out for lanthanide monochalcogenide molecules LnX (Ln=La–Eu, X=O, S, Se, Te). Fractional occupation number approach was applied to investigate the electronic structure. LnX are mainly ionic molecules with 5d–np (n=2–5 for O–Te) covalent contributions. The ground state formal configuration is 4fkσ1 (k=0–6 for La–Eu) when k<3, and 4fk+1σ0 when k>3. NdX (k=3) molecules mostly behave as a mixture between them. Molecular orbital analysis and Mulliken population indicate orbital mixing of Ln (4f, 5d, 6s) in most molecules. Spin–orbit coupling has an increasing effect from La to Eu on bond lengths and dipole moments.

Ground and excited electronic state analysis of PrF²⁺ and PmF²⁺.

The ground state and excited state manifolds are computed for PrF(2+) and PmF(2+) at the CASSCF (n,8) level of theory where the active space spans the Ln 4f orbitals as well as the F 2pz orbital. Dynamical correlation is included using second-order multireference quasidegenerate perturbation theory (MCQDPT2). The spin-orbit multiplets for each of the excited states are resolved, and spin-orbit coupling constants are computed using the Breit-Pauli spin-orbit operator. Equilibrium geometries for each of the ground and excited states are computed, and the nature of the Ln-F bond is examined. Potential energy curves for the lowest four triplet states and lowest two quintet states are computed for PrF(2+), which split into 14 levels upon application of the spin-orbit Hamiltonian. Likewise, the lowest six quintet states are computed for PmF(2+) as well as the lowest triplet state and the lowest two septet states. These nine states split into 43 terms upon application of the spin-orbit Hamiltonian.

Novel lanthanide borohydrides: magnetism of all flavours

The rare-earth metals have high magnetic moments and a diverse range of magnetic structures. However, due to the inner-transition nature of lanthanide elements, the valence f orbitals of their trivalent cations usually do not mix substantially with the ligands' orbitals in the chemical compounds. The majority of lanthanide compounds is thus characterized by a rather ionic metal–ligand bonding and is hosting only weak crystal field effects. Several exceptions known encompass the valence fluctuation systems consisting of Sm, Ey, Tm or Yb combined with less electronegative nonmetal ligands (Si, S, Se, B etc.) or metals (Murani 2003 and references therein). This important class of lanthanide compounds for which crystal field effects are strong includes the classical systems: Yb3Si5 (Iandelli et al., 1979), YbB12 (Altshuler et al., 1998), and Yb3H8 (Drulis et al., 1999) . Even elemental Yb and Eu metals show valence transition at elevated pressure from di- to trivalent (Takemura &amp; Syassen, 1985). These valence fluctuations are typically accompanied by electric resistivity changes: Ln(2+) → Ln(3+) + e–. Lanthanide borohydrides, Ln(BH4)3, constitute a rather poorly explored and novel group of compounds (Olsen et al., 2014). They are conveniently prepared via mechanochemical synthesis approach (high-energy milling). Quasi-ternary alkali metal-lanthanide borohydrides, MLn(BH4)4, are also available using this synthetic procedure (Wegner et al., 2013 [1] &amp; Wegner et al., 2014 [2]). Here we explore for the first time the magnetic properties of Ln(BH4)3 and MLn(BH4)4 compounds, with particular emphasis on the thermally unstable systems (Ln= Sm, Yb and Eu) as contrasted with the reference case of much more thermally stable derivatives of ordinary lanthanides (Ln = Ho). We show that remarkably strong mixing of Ln(4f) and H(1s) states which causes thermal instability: Ln(3+) + BH4–→ Ln(2+) + BH4· leads in some cases to strong magnetic superexchange interactions between Ln(3+) centers [3].

Facile one-pot synthesis of porous Ln2Ti2O7 (Ln = Nd, Gd, Er) with photocatalytic degradation performance for methyl orange

Porous nanocrystalline Ln2Ti2O7 (Ln=Nd, Gd, Er) was prepared by a facile self-propagating combustion method using metal nitrates (Ln(NO3)3 (Ln=Nd, Gd, Er), TiO(NO3)2) and glycine. The photocatalytic activity of Ln2Ti2O7 (Ln=Nd, Gd, Er) was evaluated by the photocatalytic degradation of methyl orange (MO). The results showed that the photocatalytic activity of Ln2Ti2O7 (Ln=Gd, Er) with cubic pyrochlore structure was higher than that of Nd2Ti2O7 with monoclinic perovskite structure and Gd2Ti2O7 showed the best photocatalytic activity. The different photocatalytic activities observed for Ln2Ti2O7 (Ln=Nd, Gd, Er) could be related to its different crystal structures and Ln 4f shells.