4f Hybridization Research Articles

Boosting CO2 Electroreduction to C2H4 via Unconventional Hybridization: High-Order Ce4+ 4f and O 2p Interaction in Ce-Cu2O for Stabilizing Cu.

Efficient conversion of carbon dioxide (CO2) into value-added materials and feedstocks, powered by renewable electricity, presents a promising strategy to reduce greenhouse gas emissions and close the anthropogenic carbon loop. Recently, there has been intense interest in Cu2O-based catalysts for the CO2 reduction reaction (CO2RR), owing to their capabilities in enhancing C-C coupling. However, the electrochemical instability of Cu+ in Cu2O leads to its inevitable reduction to Cu0, resulting in poor selectivity for C2+ products. Herein, we propose an unconventional and feasible strategy for stabilizing Cu+ through the construction of a Ce4+ 4f-O 2p-Cu+ 3d network structure in Ce-Cu2O. Experimental results and theoretical calculations confirm that the unconventional orbital hybridization near Ef based on the high-order Ce4+ 4f and 2p can more effectively inhibit the leaching of lattice oxygen, thereby stabilizing Cu+ in Ce-Cu2O, compared with traditional d-p hybridization. Compared to pure Cu2O, the Ce-Cu2O catalyst increased the ratio of C2H4/CO by 1.69-fold during the CO2RR at -1.3 V. Furthermore, in situ and ex situ spectroscopic techniques were utilized to track the oxidation valency of copper under CO2RR conditions with time resolution, identifying the well-maintained Cu+ species in the Ce-Cu2O catalyst. This work not only presents an avenue to CO2RR catalyst design involving the high-order 4f and 2p orbital hybridization but also provides deep insights into the metal-oxidation-state-dependent selectivity of catalysts.

Electron doping induced stable ferromagnetism in two-dimensional GdI3 monolayer

As a two-dimensional material with a hollow hexatomic ring structure, Néel-type anti-ferromagnetic (AFM) GdI3 can be used as a theoretical model to study the effect of electron doping. Based on first-principles calculations, we find that the Fermi surface nesting occurs when more than 1/3 electron per Gd is doped, resulting in the failure to obtain a stable ferromagnetic (FM) state. More interestingly, GdI3 with appropriate Mg/Ca doping (1/6 Mg/Ca per Gd) turns to be half-metallic FM state. This AFM–FM transition results from the transfer of doped electrons to the spatially expanded Gd-5d orbital, which leads to the FM coupling of local half-full Gd-4f electrons through 5d–4f hybridization. Moreover, the shortened Gd–Gd length is the key to the formation of the stable ferromagnetic coupling. Our method provides new insights into obtaining stable FM materials from AFM materials.

Temperature‐dependent electronic properties for 4f states in cerium mononitride

AbstractIn order to elucidate the temperature‐dependent occupancy number of Ce 4f electrons and valence state of Ce ions in cerium mononitride (CeN), we perform an ab initio calculation by using a many‐body scheme combing density functional theory with dynamical mean field theory, taking into account the spin‐orbit coupling (SOC) interaction and on‐site Coulomb repulsion between Ce 4f electrons. Results demonstrate that Ce 4f j = 5/2 and j = 7/2 manifolds undergo insulating‐metallic transition with increasing of temperature. Ce 4f‐conduction electrons hybridization, f‐f correlation, SOC interaction and final state effects yield a complicated spectrum function in CeN. Ce 4f atomic configuration transition and hybridization might be responsible for the temperature‐dependent occupancy number of Ce 4f electrons and the mixed‐valence state in CeN. A fact that localization of Ce 4f electrons, that is, 4f1 configuration or Ce3+ valence, increases with increasing of temperature could account for the experimentally observed lattice constant versus temperature data. Finally, the so‐called quasiparticle band structure is also discussed for comparison with experimental angle‐resolved photoemission spectrum.

Temperature-independent localization of Ce 4f electrons in cerium monoarsenide

In order to reveal the valence state of Ce ions and the occupancy number of Ce 4f electrons versus temperature in cerium monoarsenide (CeAs), we perform an ab initio calculation by using a many-body scheme combing density functional theory (DFT) with dynamical mean field theory (DMFT), taking into account the spin-orbit coupling (SOC) interaction and on-site Coulomb repulsion between Ce 4f electrons. Results demonstrate that Ce 4f j=5/2 and j=7/2 manifolds exhibit the metallic and insulating characters, respectively, irrespective of temperature. Ce 4f-4f correlation, Ce 4f-conduction electrons hybridization, SOC interaction and final state effects yield a complicated spectrum function for CeAs. Coupling of 4f atomic configuration transition, hybridization and temperature does not change the localized Ce 4f regime in CeAs. Finally, momentum-resolved electronic spectrum function is also discussed for comparison with experimental angle-resolved photoemission spectrum (ARPES).

Half-metallic and thermoelectric properties of Sr2EuReO6

Structural parameters, electronic band structure, half-metallic and thermoelectric properties of Sr2EuReO6 compound materials are investigated. The computations are performed using density-functional calculations within the generalized gradient approximation. Electronic results show half-metallic nature with majority spin channels as metallic and minority spin channels as semi-conducting. Magnetic moment calculated within GGA + U was found equal to 8.00 μB. The half metallicity is attributed by the double-exchange interaction mechanism via the Re(5d)–O(2p)–Eu(4f) hybridization. Boltzmann theory is employed to check the thermoelectric response, where Seebeck coefficient of 297.97 μVK−1, electrical conductivity of 44.96 × 1019 Ω−1m−1s−1 and lattice thermal conductivity of 5.94 × 1015 W/mK−1s−1 is seen. The thermoelectric efficiency measured through ZT calculation with a value of 1 at 300 K, indicating that Sr2EuReO6 can work at low as well as high temperature thermoelectric devices operations. Based on our calculations, Sr2EuReO6 combine both good spintronic and thermoelectric behaviors that may be used in spin injection applications.

Electronic structure studies on single crystalline Nd2PdSi3, an exotic Nd-based intermetallic: evidence for Nd 4f hybridization

In the series R2PdSi3, Nd2PdSi3 is an anomalous compound in the sense that it exhibits ferromagnetic order unlike other members in this family. The magnetic ordering temperature is also unusually high compared to the expected value for a Nd-based system, assuming 4f localization. Here, we have studied the electronic structure of single crystalline Nd2PdSi3 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Theoretical results obtained for the effective on-site Coulomb energy of 6 eV corroborate well with the experimental valence band spectra. While there is significant Pd 4d–Nd 4f hybridization, the states near the Fermi level are found to be dominated by hybridized Nd 4f–Si 3p states, which is possibly responsible for the ferromagnetism in Nd compound. Nd 3d core level spectrum exhibits multiple features manifesting strong final state effects due to electron correlation, charge transfer and collective excitations. These results serve as one of the rare demonstrations of hybridization of Nd 4f states with the conduction electrons possibly responsible for the exoticity of this compound.

Ground state anomalies in SmB6

SmB6 has drawn much attention in recent times due to the discovery of anomalies in its ground state properties as well as prediction of topologically protected gapless surface states. Varied theories have been proposed to capture the ground state anomalies. Here, we studied the electronic structure of SmB6 employing density functional theory using different exchange correlation potentials, spin-orbit coupling and electron correlation strength. We discover that a suitable choice of interaction parameters such as spin-orbit coupling, electron correlation strength and exchange interaction within the generalized gradient approximation provides a good description of the spectral functions observed in the angle-resolved photoemission spectroscopy (ARPES) studies. The Fermi surface plots exhibit electron pockets around X-point and hole pockets around ΓX line having dominant Sm 4f character. These observations corroborate well with the recent experimental results involving quantum oscillation measurements, ARPES, etc. In addition to primarily Sm 4f contributions observed at the Fermi level, the results exhibit significantly large contribution from B 2p states compared to weak Sm 5d contributions. This suggests important role of B 2p - Sm 4f hybridization in the exotic physics of this system.

Valence instability in the bulk and at the surface of the antiferromagnet SmRh2Si2

Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh2Si2. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4f emission strongly dominates, a small divalent 4f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RERh2Si2 (RE = rare-earth elements) compounds are discussed.

Implications of magnetic and magnetodielectric behavior of GdCrTiO5

We have carried out dc magnetization, heat-capacity, and dielectric studies down to 2 K for the compound GdCrTiO5, crystallizing in orthorhombic Pbam structure, in which well-known multiferroics RMn2O5 (R = Rare-earths) form. The points of emphasis are: (i) The magnetic ordering temperature of Cr appears to be suppressed compared with that in isostructural Nd counterpart, NdCrTiO5. This finding on the Gd compound suggests that Nd 4f hybridization plays an uncommon role in the magnetism of Cr in contrast to a proposal long ago. (ii) Dielectric constant does not exhibit any notable feature below about 30 K in the absence of external magnetic field, but a peak appears and gets stronger with the application of external magnetic fields, supporting the existence of magnetodielectric coupling. (iii) The dielectric anomalies appear even near 100 K, which can be attributed to short-range magnetic-order. We also observe a gain in spectral weight below about 150 K in Raman spectra in the frequency range 150–400 cm−1, which could be magnetic in origin, supporting short-range magnetic order. It is of interest to explore whether geometrically frustration plays any role in the dielectric properties of this family, as in the case of RMn2O5.

Relationship between Torsion and Anisotropic Exchange Coupling in a Tb(III)-Radical-Based Single-Molecule Magnet.

The incorporation of paramagnetic ligands within rare-earth ion clusters exhibiting large magnetic anisotropy has provided significant advancement in the design of single-molecule magnets (SMMs) with large blocking temperatures. However, the exchange interaction in such systems is complex and difficult to probe by conventional magnetometry techniques, and little is known about the structural relationships. Inelastic neutron scattering and terahertz electron paramagnetic resonance measurements are used complimentarily to investigate the large exchange interaction between a rare earth-radical pair in a Tb(III)-based SMM complex. The origin of the exchange interaction is investigated for two molecular species in the crystallographic unit cell that exhibit different bonding structures between Tb(III) and a 2pyNO radical. A correlation between the Tb-O-N-C torsion angles and the magnitudes of exchange couplings is found. Interestingly, a large nondegeneracy within the ground-state doublet is present for the larger torsion angle species. It is essential to consider the balance of two channels of exchange coupling, 2p-4f hybridization and 2p-5d charge transfer, to explain this characteristic behavior. The former channel gives the antiferromagnetic interaction, and the latter gives the ferromagnetic one. When an effective Ĵ = (1)/2 Ising-type Hamiltonian is applied, the exchange couplings are evaluated to be antiferromagnetic J(z) = 9.89 meV (79.8 cm(-1)) for the low torsion angle (3.8°) species and J(z) = 7.39 meV (59.6 cm(-1)) for the larger torsion angle (15.8°) species. It is also found that a small percentage of the transverse exchange component must be included for the larger torsion angle to account for the observed nondegenerate ground state. The symmetry of the exchange couplings is discussed by considering the characters of d and f orbitals.

The nature of the first order isostructural transition in GdRhSn

We present structural, magnetic, thermal, magnetocaloric, and electrical transport properties of polycrystalline GdRhSn. Magnetization data show that it orders antiferromagnetically at TN=16.2K. The compound has the ZrNiAl type hexagonal crystal structure at room temperature and undergoes a first order iso-structural transition in the paramagnetic state at 245K. The unit cell volume change at the transition is small (−0.07%) but discontinuous, in agreement with the first-order nature of the transition observed by magnetic, transport, and heat capacity measurements. The anisotropic changes of the lattice parameters are Δa/a=0.28% and Δc/c=−0.64% on cooling. A substantial change in the 4f and conduction electron hybridization, giving rise to an increased integrated DOS, occurs when the high temperature phase transforms to the low temperature phase. A moderate magnetocaloric effect at TN (ΔSM=−6.5J/kgK and ΔTad=4.5K for ΔH=50kOe) has been measured using both magnetization and heat capacity data.

Rare earth 4f hybridization with the GaN valence band

The placement of the Gd, Er and Yb 4f states within the GaN valence band has been explored by both experiment and theory. The 4d–4f photoemission resonances for various rare-earth(RE)-doped GaN thin films (RE = Gd, Er, Yb) provide an accurate depiction of the occupied 4f state placement within the GaN. The resonant photoemission show that the major Er and Gd RE 4f weight is at about 5–6 eV below the valence band maximum, similar to the 4f weights in the valence band of many other RE-doped semiconductors. For Yb, there is a very little resonant enhancement of the valence band of Yb-doped GaN, consistent with a large 4f14-δ occupancy. The placement of the RE 4f levels is in qualitative agreement with theoretical expectations.

4fhybridization effect on the magnetism of Nd2PdSi3

Among the members of the series R2PdSi3 (R= Rare-earths), the magnetic behavior of the Nd compound is interesting in some respects. This compound is considered to order ferromagnetically (below 16 K), unlike other members of this series which order antiferromagnetically. In addition, magnetic ordering temperature (To) is significantly enhanced with respect to de Gennes scaled value. In order to understand the magnetism of this compound better, we have investigated the magnetic behavior in detail (under external pressure as well) and also of its solid solutions based on substitutions at Nd and at Si sites, viz., on the series, Nd(2-x)(Y,La)(x)PdSi(3-y)Ge(y) (x, y= 0.2, 0.4, 0.8, and 1.2) by bulk measurements. The results overall establish that Nd2PdSi3 orders ferromagnetically below 16 K, but antiferromagnetic component seems to set in at very low temperatures. Notably, there is a significant suppression of To for Y and Ge substitutions, compared to La substitution, for a given magnitude of unit-cell volume change, however qualitatively correlating with the separation between the layers of Nd and Pd-Si(Ge). On the basis of this observation, we conclude that 4f(Nd) hybridization plays a major role on the magnetism of the former solid solutions. To our knowledge, this work serves as a rare demonstration of 4f-hybridization effects on the magnetism of a Nd-based intermetallic compound.

Study of 4f hybridization in CeNiX with X=Sn δGe 1−δ, 0≤ δ≤1

We report inelastic neutron scattering and core-level X-ray photoemission spectroscopy experiments for studying the Kondo problem in the CeNiX, X=Sn δ Ge 1− δ 0≤ δ≤1 series. The neutron results confirm that they behave like a Kondo lattice for δ≥0.85, showing broad maxima at around 30 meV, typical of a crystal field magnetic scattering. So, the Ge doping could produce the suppression of the cerium magnetism observed for δ≤0.25. To open a more deep sight on this point, we have analyzed the 3d core-level XPS spectra by using the well-known Gunnarsson–Schönhammer model. From this analysis, we have obtained the “on-site” Coulomb bare repulsion for f states, U, and hybridization parameter, Δ, related with the hopping from the f states to the conduction ones. These U values are very similar for all compounds, about 7 eV, but the hybridization parameter slightly changes from 0.2 to 0.16 eV on increasing the Sn concentration. In Sn-rich compounds, the 4f occupation is close to spin limit fluctuation, which allows us to obtain an estimation of the Kondo temperatures, ≈1200 K, and the static 0 K susceptibility, ≈1.1×10 −3 emu/mol. Finally, we have done “ab-initio” calculations based on the LDA+U+SO which confirm the existence of a small electronic gap opening in the DOS of Ge-rich compounds for U values lower than 7 eV.

4f hybridization and band dispersion in gadolinium thin films and compounds

AbstractThere is interplay between intra‐atomic orbital hybridization and extra‐atomic hybridization in various gadolinium systems, which affects magnetic coupling and electron itinerancy (localization). The results do not always follow expectation. The experimental band structure of thin Gd(0001) films, grown on the Mo(112) surface, along the $ \bar \Gamma $ – $ \bar M $ does not agree with expectations even qualitatively. In particular, the dispersion of the gadolinium band, with strong 5d weight near 2 eV binding energy provides considerable evidence to support the case for 4f‐5d hybridization, with increasing 5d localization. On the other hand, there is also evidence of extra‐atomic Gd 4f hybridization leading band dispersion in the occupied 4f levels in Gd2O3 (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

K- and spin-dependent hybridization effects in Ce monolayer

Here we present applications of the periodic Anderson model (PAM) to consideration of wave vector (k)- and spin-dependent hybridization effects in Ce metal. It was shown that k-dependent splitting of the 4f ionization peak of Ce/W(1 10) are correctly described in the framework of the PAM (Coulomb repulsion between two f electrons localized on the same lattice site Uff→∞). Our results show that the wave vector is conserved upon hybridization. In case of the magnetically ordered Ce monolayer, spin- and angle-resolved resonant photoemission spectra reveal spin-dependent changes of the Fermi-level peak intensities (which reflect the hybridization strength). That indicate a spin-dependence of 4f hybridization and, thus, of 4f occupancy and local moment. The phenomenon was also described in the framework of PAM by 4f electron hopping into the exchange split Fe 3d derived bands that form a spin-gap at the Fermi energy around the T point of the surface Brillouin zone.

Myriad of correlated electron effects found in the [formula omitted] family

The dilute, rare earth bearing family of RT 2 Zn 20 (R = rare earth, T = transition metal) intermetallic compounds can be tuned to a nearly ferromagnetic Fermi liquid that is closer to the Stoner limit than Pd (for R = Y, Lu and T = Fe). The submersion of moment bearing rare earths (Gd–Tm) into this highly polarizable matrix gives rise to exceptionally high temperature ferromagnetism (e.g. T C = 86 K for GdFe 2 Zn 20 ) for a compound with only one moment bearing ion out of 23 in the formula unit. In addition to this d-shell correlated electron behavior, 4f hybridization gives rise to six, new, Yb-based heavy fermion compounds: YbT 2 Zn 20 (T = Fe, Ru, Os, Co, Rh, Ir). These half dozen compounds manifest a range of Yb ion degeneracy values ( N ) for T ∼ T K , ranging from N = 4 – 8 . Not only do these compounds offer a clear testing ground for the effects of low temperature degeneracy on the correlated electron state, they also present an exceptionally clear experimental confirmation of the generalized Kadowaki–Woods formalism.

A resonant photoemission study of the Ce and Ce-oxide/Pd(1 1 1) interfaces

We have investigated the Ce 4f electronic states in the Ce/Pd(1 1 1) and Ce-oxide/Pd(1 1 1) systems, using resonant photoemission (Ce 4d → 4f transitions), and XPS to understand Pd–Ce interactions in ultra thin layers of cerium and ceria deposited on Pd(1 1 1). Cerium deposited on Pd(1 1 1) at room temperature forms surface Ce–Pd alloys with Ce rich character, while a Pd rich Ce–Pd alloy is formed by heating to 700 °C. A modification of the chemical state of Ce can also be seen after oxygen exposure. RPES provides evidence that Ce-oxide layers deposited on Pd(1 1 1) have a CeO 2 (Ce 4+) character, however a net contribution of the Ce 3+ states is also revealed. The Ce 3+ states have surface character and are accompanied by oxygen vacancies. Heating to 600 °C causes Ce-oxide reduction. A significant shift of Pd 4d-derived states, induced by Pd 4d and Ce 4f hybridization, was observed. The resonant features in the valence band corresponding to Ce 4+, Ce 3+ and Ce 0 states have been investigated for various Pd−Ce(CeO x ) coverages.

Bulk 3-D Fermi surfaces of [formula omitted] proved by soft X-ray ARPES

Bulk sensitive angle-resolved photoemission spectroscopy (ARPES) study was performed to probe the three-dimensional bulk Fermi surfaces of CeRu 2 Ge 2 in the paramagnetic phase, which is not accessible by the de Haas–van Alphen (dHvA) studies. Since Ce 4f electrons of CeRu 2 Ge 2 are thought to be localized, we compared the observed Fermi surfaces with the band-structure calculation of 4f electron localized model LaRu 2 Ge 2 . The observed electronic states are slightly different from both the band calculation for LaRu 2 Ge 2 and dHvA measurement for CeRu 2 Ge 2 in the ferromagnetic phase. Magnetic interaction and Ce 4f hybridization should be considered to fully understand the differences in electronic structures.

Calculated lanthanide contractions for molecular trihalides and fully hydrated ions: The contributions from relativity and 4f-shell hybridization

Abstract The calculated lanthanide contraction in the title systems is typically 18–21 pm of which about 9–23% comes from relativistic effects. A pronounced 4f hybridization is found for LuF 3 using three different relativistic methods of calculation. Large lanthanide nuclear quadrupole coupling constants are predicted for the molecular trihalides.