Yb Valence Research Articles

Selective electro-oxidative separation of Sm, Yb, and Eu in low-temperature molten salt AlCl3-NaCl

The variable valence rare earth elements are difficult to separate due to their similar properties and intermediate valence states in molten salts. In order to realize their efficient and selectivity separation, the properties and separation methods of Sm, Yb, and Eu have been investigated in a low-temperature AlCl3-NaCl molten salt. It has been found that the oxidation reactions of different Ln(II) to Ln(III) have different potentials. The Ln(II)/Ln(III) reaction is also irreversible when the AlCl3 content is less than 50 mol%, Ln(III) exists in the form of LnCl3 precipitation. Therefore, selectively modulating the valence of Sm, Yb, and Eu by electrochemical methods can achieve their separation. The LnCl3 produced from the Ln(II) electro-oxidation precipitates and separates from the other unoxidized soluble Ln(II) in AlCl3-NaCl melt. The efficient separation of Ln(II) in AlCl3-NaCl can be achieved by pulse potentiostatic electrolysis. Under these conditions, the separation factors of Sm-Eu, Sm-Yb, and Yb-Eu can reach 103. The maximum removal of Ln(II) exceeded 99 % with a maximum current efficiency of more than 90 %. Most importantly, the separation of variable valence rare earth elements has been realized in a low-temperature AlCl3-NaCl melt.

Phase equilibria and crystal structures in the ytterbium–copper–zinc system

Phase equilibria in the isothermal section (600 °C) of the ytterbium-copper-zinc ternary system were determined over the entire concentration range by powder X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Extended substitutional Cu/Zn homogeneity regions are mainly formed from the Yb-Zn and Yb-Cu binary compounds. There are four ternary compounds, namely τ1-YbCu5−xZnx (cubic AuBe5 type, 0.7 < x ≤ 1.5), τ2-YbCu5−xZnx (hexagonal CaCu5 type, 1.8 ≤ x ≤ 4.4), τ3-Yb2CuZn7 and τ4-Yb5Cu2−xZn1+x. Magnetic measurements were performed for compositions along the phases τ1-YbCu5−xZnx and τ2-YbCu5−xZnx, as well as for the compound Yb33Cu50Zn17 belonging to the pseudobinary system YbCu2-YbZn2. While the Yb ion has a valence of 2 + for compositions ≥33% at% Yb, the Yb valence along the YbCu5−xZnx composition line is influenced by the crystal structure: Yb3+ for the cubic AuBe5 type and nonmagnetic Yb2+ for the hexagonal CaCu5 type.

Electronic and Crystal Structures of YbInCu4-Based Compounds under Pressure

The electronic structures of YbInCu4, Y0.1Yb0.9InCu4, YbIn0.95Ag0.05Cu4, and YbIn0.9Ag0.1Cu4 were measured under pressure at 300 K. The Yb valence of Y0.1Yb0.9InCu4 increases gradually with pressure up to 27 GPa. In YbInCu4 and YbIn0.9Ag0.1Cu4, the anomalous Yb-valence decrease under pressure without a structural phase transition was observed for the first time. In YbInCu4 the Yb valence started to decrease above 15 GPa and in YbIn0.9Ag0.1Cu4 it decreased in the pressure range of 5–21 GPa. The anomalous decrease in Yb valence was enhanced at 13 K in a similar high-pressure range. We analyzed and compared the XRD patterns of YbInCu4 and Y0.1Yb0.9InCu4 under high pressure. No structural phase transition was observed in the pressure below 53 GPa for YbInCu4 and 30 GPa for Y0.1Yb0.9InCu4. For YbInCu4 the results showed an anomaly in the pressure dependence of the volume. This could increase the c–f hybridization, resulting in a decrease in the Yb valence and thus correlating it with the anomalous valence crossover.

Pressure-induced Yb valence transition and magnetism in YbMn6Ge6−xSnx

We investigate the external pressure dependence of the crystal structure, Yb valence, and local magnetization in ${\mathrm{YbMn}}_{6}{\mathrm{Ge}}_{6\ensuremath{-}x}{\mathrm{Sn}}_{x}$ using x-ray diffraction, x-ray absorption near edge spectroscopy (XANES), and x-ray magnetic circular dichroism (XMCD). The application of physical pressure results in a weakly anisotropic unit cell volume contraction without modifying the crystal structure. The external pressure allows crossing the Yb magnetic instability, which occurs for strongly hybridized Yb (valence $\ensuremath{\approx}2.76$), with a concomitant slope change in the valence-pressure curve. In the Ge-rich compounds, Yb local magnetism vanishes at very high pressure ($P \ensuremath{\ge}39.8$ GPa) likely because of the simultaneous collapse of the Mn magnetism. The results underline similarities between external and chemical pressure increase, which favors trivalent and magnetic Yb. Some differences are observed in the Sn-rich compounds highlighting potential local pressure and/or electronic effects.

Valence Transitions in Yb1+xIn1−xCu4 Studied by High-resolution X-ray Absorption Spectroscopy, X-ray Diffraction, and Photoelectron Spectroscopy

Temperature- and pressure-induced valence transitions in the Yb-rich compounds Yb1+xIn1−xCu4 were studied directly by the high-resolution x-ray absorption spectroscopy. We observed a correlation between the temperature dependence of χT and the Yb valence which could be described by the Anderson model, where χ and T are magnetic susceptibility and temperature, respectively. The valence transition temperature of Yb1+xIn1−xCu4 was found to be insensitive to the chemical pressure compared to to the case of YbIn1−xAgxCu4. This difference could be attributed to the difference in the electronic structure above the Fermi level. The shift of Tv to higher temperatures with increasing x in Yb1+xIn1−xCu4 is interpreted to be caused by the stronger hybridization contributed by the additional Yb site at the In site. Hydrostatic pressure dependence of the Yb valence in Yb1.06In0.94Cu4 showed an anomaly. The Yb valence decreased at the pressure range of 5–20 GPa where an anomaly was also observed in the pressure dependence of the lattice constant.

Temperature and pressure dependence of the Yb valence state in YbMn∼0.17Si∼1.89

Single crystals of the non-stoichiometric YbMn∼0.17Si∼1.89 intermetallic compound were investigated by room-temperature x-ray diffraction as well as by temperature and pressure-dependent x-ray absorption experiments at the Yb L3 edge. The crystal structure can be described in the orthorhombic space group Cmcm or in the non-centrosymmetric P21 monoclinic space group. In both cases, the cell comprises two Yb sites of identical multiplicity with close coordination numbers and interatomic distances. The average Yb valence is found independent of temperature νav ∼ 2.42. Such a low Yb valence is hardly compatible with the occurrence of local magnetism on Yb inferred from a previous study. The pressure dependence of the Yb valence at 10 K does not reveal any first-order transition. The average Yb valence increases continuously under pressure but remains far from trivalency (νav ∼2.63) even under the highest external pressure used of 37 GPa.

Temperature Dependence of Yb Valence in an Au–Al–Yb Intermediate-Valence Quasicrystal Investigated by Yb &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt;-Edge X-ray Absorption Near-Edge Structure Spectroscopy

In this paper, we investigate the temperature dependence of Yb valence in an Au–Al–Yb intermediate-valence quasicrystal by Yb L3-edge synchrotron X-ray absorption near-edge structure spectroscopy. The Yb valence of 2.61+ at room temperature decreases monotonically as the temperature decreases, indicating that the Yb ionic radius increases as the temperature decreases and is consistent with the anomalous suppression of the thermal shrinkage of this quasicrystal by cooling, as previously reported. The Yb valence changes by −0.053 on cooling from 298 to 2 K, and its change is more gradual on the high-temperature side than on the low-temperature side with the boundary around 200 K. Additionally, the effect of the changes in the Yb valence on the volume is quantitatively examined. The ratio of the volume changes due to the change in Yb valence per Yb atom, ΔVYb, to the amount of change in Yb valence, Δν, which is expressed as ΔVYb/Δν, was evaluated. The value of ΔVYb/Δν was estimated to be −2.8 Å3/unit Yb valence/Yb atom, whose value was close to that evaluated for YbInCu4.

Electronic structure of YbFe4Al8 antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

Depending on their chemical composition, Yb compounds often exhibit different valence states. Here we investigate the valence state of YbFe4Al8 using X-ray photoelectron spectroscopy (XPS) and first-principles calculations. The XPS valence band of YbFe4Al8 consists of two contributions coming from divalent (Yb2+) and trivalent (Yb3+) configurations. The determined value of the valence at room temperature is 2.81. Divalent and trivalent contributions are also observed for core-level Yb 4d XPS spectra. We study several collinear antiferromagnetic models of YbFe4Al8 from the first-principles and for comparison we also consider LuFe4Al8 with a fully filled 4f shell. We predict that only Fe sublattices of YbFe4Al8 carry significant magnetic moments and that the most stable magnetic configuration is AFM-C with antiparallel columns of magnetic moments. We also present a Mulliken electronic population analysis describing charge transfer both within and between atoms. In addition, we also study the effect of intra-atomic Coulomb U repulsion term applied for 4f orbitals on Yb valence and Fe magnetic moments.

Intermediate valence state in YbB4 revealed by resonant x-ray emission spectroscopy

We report the temperature dependence of the Yb valence in the geometrically frustrated compound from 12 to 300 K using resonant x-ray emission spectroscopy at the Yb transition. We find that the Yb valence, v, is hybridized between the v = 2 and v = 3 valence states, increasing from at 12 K to at 300 K, confirming that is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction in is substantial, and is likely to be the reason why does not order magnetically at low temperature, rather than this being an effect of geometric frustration. Furthermore, the zero-point valence of the system is extracted from our data and compared with other Kondo lattice systems. The zero-point valence seems to be weakly dependent on the Kondo temperature scale, but not on the valence change temperature scale T v .

Pressure-induced changes of valence fluctuation in β−YbAlB4 probed by x-ray absorption spectroscopy

We report detailed valence fluctuation phenomena in the unique quantum-critical compound $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$ under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature ($\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$. Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).

Temperature and Pressure Dependences of the Electronic and Crystal Structures of Yb4TGe8 (T = Cr, Mn, Fe, Co, and Ni) Studied by High-resolution X-ray Absorption Spectroscopy, X-ray Diffraction, and Photoelectron Spectroscopy

Temperature and pressure dependences of the electronic structure of the Kondo lattice systems of Yb4CrGe8 and Yb4MnGe8 have been investigated in detail using high-resolution x-ray absorption spectroscopy. Temperature-induced valence changes of Yb are on the order of 0.08 for Yb4MnGe8 and 0.1 for Yb4CrGe8 at the temperature range between 12 and 300 K. The c–f hybridization remains at the low temperatures where the crossover from Fermi liquid to non-Fermi liquid like behavior occurs. We also measured the x-ray absorption spectra of Yb4CrGe8 at the Cr-K and Ge-K absorption edges and found the hybridization of 3d orbitals of Cr with the Yb band at low temperatures. Photoelectron spectroscopy of Yb4TGe8 (T = Cr, Mn, Fe, Co, and Ni) showed an anomaly of the energy shift of the Yb3+ component in the valence band spectra. The Yb valence increased rapidly with pressure up to 6–7 GPa, and further increase in the pressure did not change the Yb valence significantly. The compressibility along the b axis is larger than those along the a and c axes.

Yb Valence Fluctuations and Kondo-Like Behavior in the Yb1−Xscxco2zn20 System

Yb Valence Fluctuations and Kondo-Like Behavior in the Yb1−Xscxco2zn20 System

Direct Observation of Pressure-induced Yb Valence Transition in YbInCu4

The Yb valence state of YbInCu4 was measured at 300 and 13 K under pressure by high-resolution x-ray absorption spectroscopy (XAS). The electronic structure of Cu was also measured by XAS at the Cu-K absorption edge. Pressure-induced disappearance of the valence transition was observed directly at 13 K. The present results suggest a charge transfer from Yb to Cu sites at 13 K after the disappearance of the valence transition under pressure.

Thermodynamic properties of the Yb-Sb system predicted from first-principles calculations

In the present work, thermodynamic properties of the Yb-Sb system have been studied by first-principles based quasiharmonic phonon approach, reporting the temperature-dependent isobaric heat capacity, entropy, formation enthalpy, isothermal bulk modulus, and thermal expansion. The effects by using different exchange-correlation functionals and the pseudopotentials on thermodynamic properties are examined. The results reveal that the combined Perdew-Burker-Ernzerhof functional revised for solids (PBEsol) together with the pseudopotential with all f-electrons frozen in the core is the best choice. This combination leads to an overall good description of the relative phase stabilities and thermodynamic properties, except for the rocksalt YbSb compound which is calculated to have an energy of ~25 kJ/mol-atom above the convex hull. The discrepancy is due probably to the change of the Yb valence states between the rocksalt YbSb structure and the other Yb-Sb compounds. Furthermore, the present calculations show that the Yb16Sb11 compound is stable. The analysis of stretching force constants demonstrates that the extremely strong Yb-Sb bonding is responsible for the stabilities of Yb11Sb10, Yb16Sb11, h-Yb5Sb3, and Yb4Sb3 at high temperatures.

Pressure influence on the valence and magnetic state of Yb ions in noncentrosymmetric heavy-fermionYbNiC2

We present studies of neutron diffraction, electrical resistivity, magnetic susceptibility, magnetization, and specific heat of noncentrosymmetric $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$. At normal pressure, $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$ is a moderate heavy-fermion compound with Kondo lattice. At 16 K we observe an anomaly in the temperature dependence of specific heat which is ascribed to an abrupt valence change of Yb ions. At pressures above 7 GPa, the valence change and the Kondo lattice state are suppressed, and near a temperature of 10 K we detect the appearance of magnetic order. Above 5 GPa, the temperature dependence of the resistivity behaves similarly to other compounds of the $\mathrm{Ce}\mathrm{Ni}{\mathrm{C}}_{2}$-type family, indicating the formation of charge density waves. This is attributed to the nesting properties of the Fermi surface (FS) found within the density functional $\mathrm{theory}+\mathrm{dynamical}$ mean field theory treatment. Our ab initio calculations also show that the valence of Yb in $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$ at normal conditions is 2.85, which increases with temperature and pressure. The FS of $\mathrm{Yb}\mathrm{Ni}{\mathrm{C}}_{2}$ is anisotropic in shape in comparison with the 3D surface of $\mathrm{Yb}\mathrm{Co}{\mathrm{C}}_{2}$.

Influence of Fe doping on the Yb valence state and the magnetic structures in HoFe6Sn6-type YbMn6−yFeySn6 alloys (y ≤ 1.00)

We determine the magnetic structures of HoFe6Sn6-type (Immm) YbMn6−yFeySn6 alloys (y=0.25, 0.50, 0.75 and 1.00) from powder neutron diffraction experiments. The valence state and local magnetism of Yb are investigated using X-ray Near Edge Spectroscopy (XANES) and X-ray Magnetic Circular Dichroism (XMCD) at the Yb L3 edge. The mean Yb valence decreases upon Fe doping (from υ~2.66 down to υ~2.46) and there is no 4f Yb magnetism throughout the series. The Fe for Mn substitution leads to change in the spins arrangement of the 3d sublattice by reinforcing antiferromagnetic exchange interactions without significant change in T=Mn/Fe moment magnitude (m~2.35 µB). The ground state of the alloy with y=0.25 is ferromagnetic whilst Fe richer alloys are helimagnets with wave-vector k=0, qy, 0. The turn-angle increase upon Fe doping manifests by a mitigation of the Yb L3 dichroic effect, solely due to hybridization between its 5d states and the self-polarized 3d states of T.

Crystal, local atomic, and local electronic structures of YbFe2Zn20−xCdx(0≤x≤1.4) : A multiband system with possible coexistence of light and heavy fermions

The partial (up to 7 %) substitution of Cd for Zn in the Yb-based heavy-fermion material YbFe$_2$Zn$_{20}$ is known to induce a slight ($\sim 20$ %) reduction of the Sommerfeld specific heat coefficient $\gamma$ and a huge (up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient $A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods ratio $A/\gamma ^2$. Here, Yb $L_{3}$-edge X-ray absorption spectroscopy shows that the Yb valence state is close to $3+$ for all $x$, whereas X-ray diffraction reveals that Cd replace the Zn ions only at the $16c$ site of the $Fd\bar{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Zn occupation. Ab-initio electronic structure calculations in pure and Cd-doped materials, carried out without considering correlations, show multiple conduction bands with only minor modifications of the band dispersions near the Fermi level and therefore do not explain the resistivity drop introduced by Cd substitution. We propose that the site-selective Cd substitution introduces light conduction bands with substantial contribution of Cd($16c$) $5p$ levels that have weak coupling to the Yb$^{3+}$ $4f$ moments. These light fermions coexist with heavy fermions originated from other conduction bands with larger participation of Zn($48f$ and $96g$) $4p$ levels that remain strongly coupled with the Yb$^{3+}$ local moments.

Lattice instability coupled with valence degrees of freedom in valence fluctuation compound YbPd

Inelastic x-ray scattering (IXS) was used to examine the valence fluctuation compound YbPd at 300 K in the $\ensuremath{\Gamma}\ensuremath{-}X$, $\ensuremath{\Gamma}\ensuremath{-}M, X\ensuremath{-}M$, and X-R directions. The obtained spectra and determined phonon dispersion relations demonstrate lattice instabilities related to the successive transitions and correlations between phonon spectra and valence degrees of freedom in YbPd. The low-lying branches in the phonon dispersion relations as well as the broadened linewidths of the IXS spectra observed around the longitudinal $X$ point provide supporting evidence of correlation between a valence-lattice interaction and the successive transitions in YbPd. X-ray absorption spectroscopy was also performed in order to discuss correlations between lattice instabilities and Yb valence degrees of freedom in YbPd.

Intermediate Yb valence in the Zintl phases Yb14MSb11(M=Zn,Mn,Mg) : XANES, magnetism, and heat capacity

${\mathrm{Yb}}_{14}\mathrm{Mn}{\mathrm{Sb}}_{11}$ is a magnetic Zintl compound as well as being one of the best high temperature $p$-type thermoelectric materials. According to the Zintl formalism, which defines intermetallic phases where cations and anions are valence satisfied, this structure type is nominally made up of 14 ${\mathrm{Yb}}^{2+}$, 1 ${\mathrm{MnSb}}_{4}^{9\ensuremath{-}}$, 1 ${\mathrm{Sb}}_{3}^{7\ensuremath{-}}$, and 4 ${\mathrm{Sb}}^{3\ensuremath{-}}$ atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 ${\mathrm{Yb}}^{2+}$, 1 ${\mathrm{Yb}}^{3+}$, 1 (Mg, Zn)${\mathrm{Sb}}_{4}^{10\ensuremath{-}}$, 1 ${\mathrm{Sb}}_{3}^{7\ensuremath{-}}$, and 4 ${\mathrm{Sb}}^{3\ensuremath{-}}$. The predicted existence of ${\mathrm{Yb}}^{3+}$ based on simple electron counting rules of the Zintl formalism calls the Yb valence of these compounds into question. X-ray absorption near-edge structure, magnetic susceptibility, and specific heat measurements on single crystals of the three analogs show signatures of intermediate valence Yb behavior and in particular, reveal the heavy fermion nature of ${\mathrm{Yb}}_{14}{\mathrm{MgSb}}_{11}$. In these isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate ($M=\mathrm{Zn}$), mostly 2+ ($M=\mathrm{Mn}$), to 3+ ($M=\mathrm{Mg}$). In all cases, there is a small amount of intermediate valency at the lowest temperatures. The amount of intermediate valency is constant for $M=\mathrm{Mn}$, Mg and temperature dependent for $M=\mathrm{Zn}$. The evolution of the Yb valence correlated to the transport properties of these phases is highlighted. The presence of Yb in this structure type allows for fine tuning of the carrier concentration and thereby the possibility of optimized thermoelectric properties along with unique magnetic phenomena.

Strong in-plane anisotropy in the electronic structure of fixed-valence β−LuAlB4

The origin of intrinsic quantum criticality in the heavy-fermion superconductor $\beta$-YbAlB$_4$ has been attributed to strong Yb valence fluctuations and its peculiar crystal structure. Here, we assess these contributions individually by studying the isostructural but fixed-valence compound $\beta$-LuAlB$_4$. Quantum oscillation measurements and DFT calculations reveal a Fermi surface markedly different from that of $\beta$-YbAlB$_4$, consistent with a `large' Fermi surface there. We also find an unexpected in-plane anisotropy of the electronic structure, in contrast to the isotropic Kondo hybridization in $\beta$-YbAlB$_4$.