Occupied 4f Research Articles

Electronic and geometric factors affecting oxygen vacancy formation on CeO2(111) surfaces: A first-principles study from trivalent metal doping cases

Oxygen vacancies (Ov) on CeO2 surfaces create polarons, involving both electron localization on Ce ions and local geometry distortions. The relative positions of reduced Ce(III) and Ov affect the energies. We use trivalent doping to study the factors affecting the Ov formation energy on the CeO2(111) surfaces. We find that it is easier to form an Ov adjacent to the dopant with a smaller radius for Al, Ga, In, Tl, Sc, and Y doped cases, and on the second nearest neighbor O to the dopant with a larger radius (La and Ac). It is ascribed to that the smaller dopant could give more space to relax when the Ce(III) is sharing neighboring O to it, while the larger ones not. The Ce(III) could also be considered as a case of trivalent doping, following the same trend varying with dopant radius. Compared with the other trivalent dopants, the reduced Ce(III) produces a new occupied 4f state in the band gap, increases the Ov formation energy, and would be more easily oxidized and favorable for the redox cycle. This study gives a theoretical view of the Ov formation process and takes us closer to the physical nature of the doped ceria system.

Mimicking π Backdonation in Ce-MOFs for Solar-Driven Ammonia Synthesis.

π Backdonation is the core process to break through the kinetically complex and energetic hurdle for catalyzing effectively the NH3 synthesis but only occurs on certain transition metals with empty and filled d orbitals. Herein, mimicking π backdonation enables MOF-76(Ce) materials to convert N2/NH3 effectively. Note that, by virtue of the intrinsic mechanism of ligand-to-metal charge transfer, metal cerium species in MOF-76(Ce) serve as an electron sink for accumulating the photogenerated electrons. Taken together, experimental and theoretical analyses reveal that such metal cerium species with coordination unsaturated state (Ce-CUS) on a MOF-76(Ce) nanorod surface can also provide unoccupied and occupied 4f orbitals to accept from and then donate electrons back to nitrogen molecules. Remarkably, it shows outstanding photocatalytic nitrogen reduction performance with high average NH3 yield (34 μmol g-1 h-1) under ambient conditions. This work provides fresh insights into rational designing and engineering highly active catalysts with rare earth elements.

Structural, electronic and optical properties of prominent M2Si5N8:Eu2+ phosphors (M = Mg, Ca, Sr, Ba) from the ground–state and excited–state first principles calculations

In this paper, the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional is employed to explore the structural, electronic, and optical properties of M2Si5N8:Eu2+ phosphors (M = Mg, Ca, Sr, Ba) on the ground–state and excited–state levels. From the detailed study of electronic structures of pure M2Si5N8, the top of the valence band is predominantly of N p character and the calculated band gaps are consistent with the reported experimental values. By Eu doping, the occupied 4f band is located directly in the band gap and results in the optical transition of the 4f–5d character. In order to confirm the location of the Eu 5d band below the conduction band, the excited–state calculation was executed. Using the position of the lowest excited–state 5d band below the conduction band, the calculated 4f–5d energy gaps agrees well with the experimental data. The excited–state calculation confirms that the Eu2+ d–f emission will be predominant in these phosphors. Furthermore, in order to understand and explain the excitation and emission observations, the energy level scheme was constructed from the calculated electronic structures. The findings in this article could well suggest a new Mg2Si5N8:Eu2+ phosphor with a stable Eu2+ d–f emission and can also be applied to select another rare-earth dopant.

First-principles search for efficient activators for LaI3

First-principles calculations were performed using density functional theory with Hubbard corrections or hybrid exchange-correlation functionals, as well as the GW approximation, to predict dopants that could serve as efficient activators for LaI3, a potentially very bright scintillator for which an appropriate activator has not been identified yet. The dopants considered in this work included a series of lanthanide ions (Ce, Pr, Nd, Eu, Gd, and Tb) and several ns2 ions (Tl, Pb, Bi, and Sb). Based on both ground-state calculations and constrained DFT calculations to simulate excited states, the trivalent lanthanide dopants were shown not to constitute an improvement over Ce3+, for which experimental data exist, as they showed occupied 4f states below the valence band maximum (VBM) and 5d states above the conduction band maximum (CBM). In contrast, the only divalent lanthanide considered, Eu2+, displayed occupied 4f states within the band gap of the host, but its 5d states were calculated to be above the CBM. Eu2+ could nonetheless be exploited as an activator by increasing the band gap energy slightly through substitution of iodide ions by bromide ions. Similar results were obtained for Tl+. Finally, Bi3+ and Sb3+ were predicted to be efficient activators in LaI3 by virtue of having unoccupied p states below the CBM, which can serve as electron traps and can combine with holes at the VBM to form localized luminescence centers. Therefore, Bi- and Sb-doped LaI3 should be grown and tested experimentally. Comparison with empirical relationships of the relative positions of the energy levels of rare-earth dopants and implications for efficient doping schemes and scintillation mechanisms in LaI3 are also discussed.

Luminescence Mechanistic Study of BaLaGa3O7:Nd Using Density Functional Theory Calculations

BaLaGa3O7:Nd (BLGO:Nd) has been investigated as a laser crystal material for about three decades. In the present work, the luminescence mechanism of BLGO:Nd is clarified by density functional theory (DFT) calculations. Structural optimization was first performed on the constructed supercell to obtain the equilibrium geometry. On the basis of the optimized crystal, the electronic structures of the BLGO host (without and with single defects) and the BLGO:Nd phosphor (without and with neighboring defects) were comprehensively investigated. Three important features are revealed by theoretical analyses. First, single defects in BLGO have little effect on the light emission, although the impurity levels appeared within the band gap. Second, luminescence can be realized by the introduction of Nd ions. Calculations of optical properties demonstrated that parity-forbidden transitions among the 4f levels are partially allowed because the mixing of 4f and 5d configurations occurs at higher empty 4f levels. It is thus clear that the electronic transitions between occupied 4f and empty 4f-5d states are electric-dipole-allowed. Therefore, light emission in BLGO:Nd can be achieved in the electronic transition process of Nd 4f electrons → empty 4f-5d levels → empty 5d levels → Nd 4f levels. The neighboring intrinsic defects play only an auxiliary role in prolonging the decay time. Third, co-doping of Tb in BLGO:Nd is considered to be beneficial to luminescence in theory because of its shallow to deep distribution of impurity orbitals in the band gap. Therefore, BLGO:Nd co-doped with other lanthanide ions will offer guidelines in the search for the best luminescent materials.

Native Point Defects in CaS: Focus on Intrinsic Defects and Rare Earth Ion Dopant Levels for Up-converted Persistent Luminescence.

We studied native point defects as well as Eu and Dy ion doping in CaS by the simple DFT + Hubbard U method. The electronic properties and formation energies of native point defects and dopants have been discussed. We found the neutral S vacancy has the lowest energy of 0.62 eV under the Ca-rich limit. The Schottky pair is another dominant defect with a cost of 1.51 eV per defect site from S-rich to Ca-rich chemical potential limits. Our calculations on the thermodynamic transition levels confirm the experimental observed intrinsic blue two-peak broad band emissions stimulated by near-infrared range irradiation for undoped CaS. Both Eu and Dy show an energetic favorable trend to be substitutionally doped in the CaS lattice. All of the positive charge states of the Eu ion contribute localized recombination trapping level in the gap while having very deep donor transition levels. The neutral state of Dy contributes to the occupied 4f level localized 1.3 eV below the conduction band edge with very shallow donor type transition level (0/+) of 0.56 eV below the conduction band. All of the positive charge states of Dy have two shallow 5d levels with 0.4 and 0.6 eV below the conduction band. In this work, we further analyzed that the Dy dopant contributed deeper trap levels in the CaS materials that can store the electron carriers with more evenly, wider, and deeper range of levels distributed so that they lengthen the decay-time of the persistent luminescence. The related 980 nm photo-stimulated luminescence is actualized with the help of native defects like VS(0), VS(+), and STK(-) as relay centers for a possible up-converted luminescence. We also summarized a narrow doping limit energy which has been determined as 1.33 eV constantly in CaS independent to different chemical potential limits. This gives a solid theoretical reference for lanthanide ion doping experiments in CaS.

Electronic structure and superconductivity of europium

We have calculated the electronic structure of Eu for the bcc, hcp, and fcc crystal structures for volumes near equilibrium up to a calculated 90GPa pressure using the augmented-plane-wave method in the local-density approximation. The frozen-core approximation was used with a semi-empirical shift of the f-states energies in the radial Schrödinger equation to move the occupied 4f valence states below the Γ1 energy and into the core. This shift of the highly localized f-states yields the correct europium phase ordering with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the bcc and hcp structures are also found to agree with and follow a Tc trend similar to recent measurement by Debessai et al. [1].

Electronic structure of the Sr 2MgSi 2O 7:Eu 2+ persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr 2MgSi 2O 7(:Eu 2+)) materials were studied by a combined experimental and theoretical approach. The UV–VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu 2+ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu 2+ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.

Magneto-optics as a tool to study occupied and empty 4f and 5f states

Magneto-optical spectroscopy is known to be particularly powerful for studying occupied 4f and 5f states through f→d excitations. The recent discovery of d→f transitions in LaSe has revived the idea to use magneto-optical spectroscopy also to study empty f states. We review the state of the art of optical and magneto-optical investigations of occupied and empty f states for lanthanum, cerium and uranium monochalcogenides and monopnictides.

Initial and final state contributions of the core level shifts for Gd(0001)

Abstract The initial state and final state contributions to the core level shift are separated by comparing the shifts of occupied and unoccupied Gd 4f levels, which exhibit the initial state shift with equal signs and the final state shift with opposite signs. For clean Gd(0001) we find a surface shift of −0.4 eV for both the unoccupied and occupied 4f level. On the other hand, with 3–4 L of oxygen the unoccupied surface 4f level is shifted by + 1.0 eV with respect to the bulk, and the occupied 4f by −1.0 eV. Therefore, the initial state effect dominates at the clean surface, while the final state effect plays a significant role at the oxidized surface. The difference is explained by metallic versus dielectric screening.

Anisotropic p-f mixing mechanism explaining anomalous magnetic properties in Ce monopnictides. II. Crystal-field splitting in rare-earth pnictides

For pt.I see ibid., vol.18, p.2697 (1985). The crystal-field splittings in CeP, PrP and NdP are calculated by considering the point-charge Coulomb interaction, the intra-atomic d-f Coulomb interaction, and the p-f and d-f mixings. The p-f mixing mechanisms, not only between the occupied 4f states and the conduction bands, but also between the unoccupied 4f states and the valence bands make an important contribution to the crystal-field splitting. The fact that the crystal-field potential in CeP is smaller than those in PrP and NdP is due to the occupied 4f level in CeP being shallower. The values of the Slater-Koster integrals (pf sigma ) and (pf pi ), are determined uniquely from the crystal-field fitting for PrP and NdP.