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Iron-bearing carbonates play an important role in Earth's carbon cycle. Owing to their stability at mantle conditions, recently discovered iron carbonates with tetrahedrally coordinated carbon atoms are candidates for carbon storage in the deep Earth. The carbonates' iron oxidation and spin state at extreme pressure and temperature conditions contribute to the redox conditions and element partitioning in the deep mantle. By laser heating FeCO3 at pressures of about 83 GPa, Fe43+C3O12 and Fe22+Fe23+C4O13 were synthesized and then investigated by x-ray emission spectroscopy to elucidate their spin state, both in situ and temperature quenched. Our experimental results show both phases in a high-spin state at all pressures and over the entire temperature range investigated, i.e., up to 3000 K. The spin state is conserved after temperature quenching. A formation path is favored where Fe43+C3O12 forms first and then reacts to Fe22+Fe23+C4O13, most likely accompanied by the formation of oxides. Density functional theory calculations of Fe22+Fe23+C4O13 at 80 GPa confirm the experimental findings with both ferric and ferrous iron in high-spin state with antiferromagnetic order at 80 GPa. As the intercrystalline cation partitioning between the Fe-bearing carbonates and the surrounding perovskite and ferropericlase depends on the spin state of the iron, an understanding of the redox conditions prevalent in subducted slab regions in the lower mantle has to take the latter into account. Especially, Fe22+Fe23+C4O13 may play a key role in subducted material in the lower mantle, potentially with a similar role as silicate perovskite.

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of T-K similar to 480 K. We report a negative in-plane thermal expansion alpha/T below 2 K, which passes through a broad minimum near 0.75 K. Volume and a-axis magnetostriction for B parallel to a are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6 T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1 K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at T << T-K result from geometrical frustration.

The archetypical heavy-fermion superconductor CeCu2Si2 is known to present two distinct superconducting phases under pressure. In the low-pressure region, the superconductivity is mediated by spin fluctuations while the superconducting phase observed in the high-pressure region could be associated with a first-order valence transition (FOVT). However, the critical end point (CEP) of the FOVT was shown to be located at negative temperature and only a continuous valence change (crossover regime) was so far observed at 14 K, i.e., far above the optimal superconducting temperature (Tc = 2.3 K). Here we present x-ray absorption measurements under pressure and applied magnetic field at the Ce L3 edge at 2.7 K, i.e., close to the optimal Tc. It was expected that the applied magnetic field could shift the CEP to positive temperature with the possibility to observe a FOVT. Our data indicate the valence of Ce increases continuously (crossover regime) from 3.11 at ambient pressure up to 3.20 at 8.5 GPa likewise for any applied magnetic field up to 6 T.

Resonant inelastic x-ray spectroscopy at the uranium N-4 absorption edge at 778 eV has been used to reveal the excitations in UO2 up to 1 eV. The earlier (1989) studies by neutron inelastic scattering of the crystal-field states within the H-3(4) multiplet are confirmed. In addition, the first excited state of the F-3(2) multiplet at similar to 520 meV has been established, and there is a weak signal corresponding to the next excited state at similar to 920 meV. This represents a successful application of soft x-ray spectroscopy to an actinide sample, and resolves an open question in UO2 that has been discussed for 50 years. The technique is described and important caveats are drawn about possible future applications.

Herein we show that nonresonant inelastic x-ray scattering involving an s core level is a powerful spectroscopic method to characterize the excited states of transition metal compounds. The spherical charge distribution of the s core hole allows the orientational dependence of the intensities of the various spectral features to produce a spatial charge image of the associated multiplet states in a straightforward manner, thereby facilitating the identification of their orbital character. In addition, the s core hole does not add an extra orbital angular momentum component to the multiplet structure so that the well-established Sugano-Tanabe-Kamimura diagrams can be used for the analysis of the spectra. For alpha-MnS we observe the spherical charge density corresponding to its high-spin 3d(5) ((6)A(1)) ground state configuration and we were able to selectively image its excited states and identify them as t(2g) (T-5(2)) and e(g) (E-5) with an energy splitting 10Dq of 0.78 eV.

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of T-K similar to 480 K. We report a negative in-plane thermal expansion alpha/T below 2 K, which passes through a broad minimum near 0.75 K. Volume and a-axis magnetostriction for B parallel to a are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6 T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1 K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at T << T-K result from geometrical frustration.

The archetypical heavy-fermion superconductor CeCu2Si2 is known to present two distinct superconducting phases under pressure. In the low-pressure region, the superconductivity is mediated by spin fluctuations while the superconducting phase observed in the high-pressure region could be associated with a first-order valence transition (FOVT). However, the critical end point (CEP) of the FOVT was shown to be located at negative temperature and only a continuous valence change (crossover regime) was so far observed at 14 K, i.e., far above the optimal superconducting temperature (Tc = 2.3 K). Here we present x-ray absorption measurements under pressure and applied magnetic field at the Ce L3 edge at 2.7 K, i.e., close to the optimal Tc. It was expected that the applied magnetic field could shift the CEP to positive temperature with the possibility to observe a FOVT. Our data indicate the valence of Ce increases continuously (crossover regime) from 3.11 at ambient pressure up to 3.20 at 8.5 GPa likewise for any applied magnetic field up to 6 T.

Herein we show that nonresonant inelastic x-ray scattering involving an s core level is a powerful spectroscopic method to characterize the excited states of transition metal compounds. The spherical charge distribution of the s core hole allows the orientational dependence of the intensities of the various spectral features to produce a spatial charge image of the associated multiplet states in a straightforward manner, thereby facilitating the identification of their orbital character. In addition, the s core hole does not add an extra orbital angular momentum component to the multiplet structure so that the well-established Sugano-Tanabe-Kamimura diagrams can be used for the analysis of the spectra. For alpha-MnS we observe the spherical charge density corresponding to its high-spin 3d(5) ((6)A(1)) ground state configuration and we were able to selectively image its excited states and identify them as t(2g) (T-5(2)) and e(g) (E-5) with an energy splitting 10Dq of 0.78 eV.

Resonant inelastic x-ray spectroscopy at the uranium N-4 absorption edge at 778 eV has been used to reveal the excitations in UO2 up to 1 eV. The earlier (1989) studies by neutron inelastic scattering of the crystal-field states within the H-3(4) multiplet are confirmed. In addition, the first excited state of the F-3(2) multiplet at similar to 520 meV has been established, and there is a weak signal corresponding to the next excited state at similar to 920 meV. This represents a successful application of soft x-ray spectroscopy to an actinide sample, and resolves an open question in UO2 that has been discussed for 50 years. The technique is described and important caveats are drawn about possible future applications.

While x-ray absorption spectroscopy is mainly governed by electric-dipole transitions, the technique of nonresonant inelastic x-ray scattering (NIXS) offers the possibility to explore higher-order multipole transitions. These transitions obey different selection rules that can reach final states of higher angular momenta, opening complementary spectroscopic perspectives. Here, we investigate the suitability of NIXS to study transuranium compounds. We show that the K edge of the Be encapsulation can be practically fully excluded by using the imaging capabilities of the technique arising from the position of the signals on the multidetector. Experimental results for the multipole transitions at the actinide O-4,O-5 edges (90-120 eV) in UO2, NpO2, PuO2, and Pu2O3 are compared with multielectronic calculations. The spectral features are shown to be very sensitive to the ratio of the triakontadipole and octupole transitions, which could potentially be used to assess the radial expansion of the 5f wave function, which is expected to occur in covalent mixing with the O 2p states.

While x-ray absorption spectroscopy is mainly governed by electric-dipole transitions, the technique of nonresonant inelastic x-ray scattering (NIXS) offers the possibility to explore higher-order multipole transitions. These transitions obey different selection rules that can reach final states of higher angular momenta, opening complementary spectroscopic perspectives. Here, we investigate the suitability of NIXS to study transuranium compounds. We show that the K edge of the Be encapsulation can be practically fully excluded by using the imaging capabilities of the technique arising from the position of the signals on the multidetector. Experimental results for the multipole transitions at the actinide O-4,O-5 edges (90-120 eV) in UO2, NpO2, PuO2, and Pu2O3 are compared with multielectronic calculations. The spectral features are shown to be very sensitive to the ratio of the triakontadipole and octupole transitions, which could potentially be used to assess the radial expansion of the 5f wave function, which is expected to occur in covalent mixing with the O 2p states.

We present core level nonresonant inelastic x-ray scattering (NIXS) data of the heavy-fermion compounds CeCoIn5 and CeRhIn5 measured at the Ce N-4,N-5 edges. The higher than dipole transitions in NIXS allow determining the orientation of the Gamma(7) crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the CeMIn5 compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wave function is the Gamma(+)(7) ((x2) - y(2)) or Gamma(-)(7) (xy orientation) remained undetermined. We show that the Gamma(-)(7) doublet with lobes along the (110) direction forms the ground state in CeCoIn5 and CeRhIn5. For CeCoIn5, however, we find also some contribution of the first excited state crystal-field state in the ground state due to the stronger hybridization of 4 f and conduction electrons, suggesting a smaller alpha(2) value than originally anticipated from x-ray absorption. A comparison is made to the results of existing density functional theory plus dynamical mean-field theory calculations.

We present core level nonresonant inelastic x-ray scattering (NIXS) data of the heavy-fermion compounds CeCoIn5 and CeRhIn5 measured at the Ce N-4,N-5 edges. The higher than dipole transitions in NIXS allow determining the orientation of the Gamma(7) crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the CeMIn5 compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wave function is the Gamma(+)(7) ((x2) - y(2)) or Gamma(-)(7) (xy orientation) remained undetermined. We show that the Gamma(-)(7) doublet with lobes along the (110) direction forms the ground state in CeCoIn5 and CeRhIn5. For CeCoIn5, however, we find also some contribution of the first excited state crystal-field state in the ground state due to the stronger hybridization of 4 f and conduction electrons, suggesting a smaller alpha(2) value than originally anticipated from x-ray absorption. A comparison is made to the results of existing density functional theory plus dynamical mean-field theory calculations.

We investigated the crystal-electric field ground state of the 4f manifold in the strongly correlated topological insulator SmB6 using core-level nonresonant inelastic x-ray scattering. The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the Gamma(8) quartet state of the Sm f(5) J = 5/2 configuration governs the ground-state symmetry and, hence, the topological properties of SmB6. Our findings contradict the results of density functional calculations repotted so far.

Yb2Si2Al may be a prototype for exploring different aspects of the Shastry-Sutherland lattice, formed by planes of orthogonally coupled Yb ions. Measurements of the magnetic susceptibility find incoherently fluctuating Yb3+ moments coexisting with a weakly correlated metallic state that is confirmed by measurements of the electrical resistivity. Increasing signs of Kondo coherence are found with decreasing temperature, including an enhanced Sommerfeld coefficient and Kadowaki-Woods ratio that signal that the metallic state found at the lowest temperatures is a Fermi liquid where correlations have become significantly stronger. A pronounced peak in the electronic and magnetic specific heat indicates that the coupling of the Yb moments to the conduction electrons leads to an effective Kondo temperature that is approximately 30 K. The valence of Yb2Si2Al has been investigated with electron spectroscopy methods. Yb2Si2Al is found to be strongly intermediate valent [v(F) = 2.68(2) at 80 K]. Taken together, these experimental data are consistent with a scenario where a coherent Kondo lattice forms in Yb2Si2Al from an incoherently fluctuating ensemble of Yb moments with incomplete Kondo compensation, and strong intermediate valence character.

Nonresonant inelastic x-ray scattering (NIXS) has been performed on single crystals of UO2 to study the directional dependence of higher-order-multipole scattering from the uranium O-4,O-5 edges (90-110 eV). By comparing the experimental results with theoretical calculations the symmetry of the ground state is confirmed directly as the crystal-field (CF) Gamma(5) triplet state within the J = 4 manifold. The results demonstrate that the directional dichroism of the NIXS spectra is sensitive to the CF strength and establish NIXS as a tool for probing CF interactions quantitatively.

We investigated the crystal-electric field ground state of the 4f manifold in the strongly correlated topological insulator SmB6 using core-level nonresonant inelastic x-ray scattering. The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the Gamma(8) quartet state of the Sm f(5) J = 5/2 configuration governs the ground-state symmetry and, hence, the topological properties of SmB6. Our findings contradict the results of density functional calculations repotted so far.

We present linear polarization-dependent soft-x-ray absorption spectroscopy data at the Ce M-4,M-5 edges of Cd- and Sn-dopedCeCoIn(5). The(4)f ground-state wave functions have been determined for their superconducting, anti-ferromagnetic, and paramagnetic ground states. The absence of changes in the wave functions in CeCo(In1-x Cd-x)(5) suggests that the 4f-conduction-electron (cf) hybridization is not affected by global Cd doping, thus supporting the interpretation of magnetic droplets nucleating long-range magnetic order. This is contrasted by changes in the wave function due to Sn substitution. Increasing Sn in CeCo(In1-y Sn-y)(5) compresses the 4f orbitals into the tetragonal plane of these materials, suggesting enhanced cf hybridization with the in-plane In(1) atoms and a homogeneous altering of the electronic structure. As these experiments show, the 4f wave functions are a very sensitive probe of small changes in the hybridization of 4f and conduction electrons, even conveying information about direction dependencies.

Yb2Si2Al may be a prototype for exploring different aspects of the Shastry-Sutherland lattice, formed by planes of orthogonally coupled Yb ions. Measurements of the magnetic susceptibility find incoherently fluctuating Yb3+ moments coexisting with a weakly correlated metallic state that is confirmed by measurements of the electrical resistivity. Increasing signs of Kondo coherence are found with decreasing temperature, including an enhanced Sommerfeld coefficient and Kadowaki-Woods ratio that signal that the metallic state found at the lowest temperatures is a Fermi liquid where correlations have become significantly stronger. A pronounced peak in the electronic and magnetic specific heat indicates that the coupling of the Yb moments to the conduction electrons leads to an effective Kondo temperature that is approximately 30 K. The valence of Yb2Si2Al has been investigated with electron spectroscopy methods. Yb2Si2Al is found to be strongly intermediate valent [v(F) = 2.68(2) at 80 K]. Taken together, these experimental data are consistent with a scenario where a coherent Kondo lattice forms in Yb2Si2Al from an incoherently fluctuating ensemble of Yb moments with incomplete Kondo compensation, and strong intermediate valence character.

Nonresonant inelastic x-ray scattering (NIXS) has been performed on single crystals of UO2 to study the directional dependence of higher-order-multipole scattering from the uranium O-4,O-5 edges (90-110 eV). By comparing the experimental results with theoretical calculations the symmetry of the ground state is confirmed directly as the crystal-field (CF) Gamma(5) triplet state within the J = 4 manifold. The results demonstrate that the directional dichroism of the NIXS spectra is sensitive to the CF strength and establish NIXS as a tool for probing CF interactions quantitatively.

We investigated the ground-state symmetry of the cubic hidden order compound CeB6 by means of core level non- resonant inelastic x-ray scattering (NIXS). The information is obtained from the directional dependence of the scattering function that arises from higher than dipole transitions. Our new method confirms that the ground state is well described using a localized crystal-field model assuming a G8 quartet ground state. Copyright (C) EPLA, 2017

We investigated the ground-state symmetry of the cubic hidden order compound CeB6 by means of core level non- resonant inelastic x-ray scattering (NIXS). The information is obtained from the directional dependence of the scattering function that arises from higher than dipole transitions. Our new method confirms that the ground state is well described using a localized crystal-field model assuming a G8 quartet ground state. Copyright (C) EPLA, 2017

We present bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) data of the Ce3d core levels and lifetime-reduced L-edge X-ray absorption spectroscopy (XAS) in the partial fluorescence yield (PFY) mode of the CeMIn5 family with M=Co, Rh, and Ir. The HAXPES data are analyzed quantitatively with a combination of full multiplet and configuration interaction model which allows correcting for the strong plasmons in the CeMIn5 HAXPES data, and reliable weights w(n) of the different f(n) contributions in the ground state are determined. The CeMIn5 results are compared to HAXPES data of other heavy fermion compounds and a systematic decrease of the hybridization strength V-eff from CePd3 to CeRh3B2 to CeRu2Si2 is observed, while it is smallest for the three CeMIn5 compounds. The f-occupation, however, increases in the same sequence and is close to one for the CeMIn5 family. The PFY-XAS data confirm an identical f-occupation in the three CeMIn5 compounds and a phenomenological fit to these PFY-XAS data combined with a configuration interaction model yields consistent results. (C) 2016 The Authors. Published by Elsevier B.V.

The crystal-field ground-state wave functions of the tetragonal, magnetically ordering Kondo lattice materials CeMAl4Si2 (M = Rh, Ir, and Pt) are determined with low-temperature linearly polarized soft-x-ray absorption spectroscopy, and estimates for the crystal-field splittings are given from the temperature evolution of the linear dichroism. Values for the dominant exchange field in the magnetically ordered phases can be obtained from fitting the influence of magnetic order on the linear dichroism. The direction of the required exchange field is vertical bar vertical bar c for the antiferromagnetic Rh and Ir compounds, with the corresponding strength of the order of lambda(ex) approximate to 6 meV (65 K). Furthermore, the presence of Kondo screening in the Rh and Ir compound is demonstrated on the basis of the absorption due to f(0) in the initial

We present bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) data of the Ce3d core levels and lifetime-reduced L-edge X-ray absorption spectroscopy (XAS) in the partial fluorescence yield (PFY) mode of the CeMIn5 family with M=Co, Rh, and Ir. The HAXPES data are analyzed quantitatively with a combination of full multiplet and configuration interaction model which allows correcting for the strong plasmons in the CeMIn5 HAXPES data, and reliable weights w(n) of the different f(n) contributions in the ground state are determined. The CeMIn5 results are compared to HAXPES data of other heavy fermion compounds and a systematic decrease of the hybridization strength V-eff from CePd3 to CeRh3B2 to CeRu2Si2 is observed, while it is smallest for the three CeMIn5 compounds. The f-occupation, however, increases in the same sequence and is close to one for the CeMIn5 family. The PFY-XAS data confirm an identical f-occupation in the three CeMIn5 compounds and a phenomenological fit to these PFY-XAS data combined with a configuration interaction model yields consistent results. (C) 2016 The Authors. Published by Elsevier B.V.

The crystal-field ground-state wave functions of the tetragonal, magnetically ordering Kondo lattice materials CeMAl4Si2 (M = Rh, Ir, and Pt) are determined with low-temperature linearly polarized soft-x-ray absorption spectroscopy, and estimates for the crystal-field splittings are given from the temperature evolution of the linear dichroism. Values for the dominant exchange field in the magnetically ordered phases can be obtained from fitting the influence of magnetic order on the linear dichroism. The direction of the required exchange field is vertical bar vertical bar c for the antiferromagnetic Rh and Ir compounds, with the corresponding strength of the order of lambda(ex) approximate to 6 meV (65 K). Furthermore, the presence of Kondo screening in the Rh and Ir compound is demonstrated on the basis of the absorption due to f(0) in the initial

The occupancy of the 4f(n) contributions in the Kondo semiconductors CeM2Al10 (M = Ru, Os and Fe) has been quantitatively determined by means of bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) on the Ce 3d core levels. Combining a configuration interaction scheme with full multiplet calculations allowed to accurately describe the HAXPES data despite the presence of strong plasmon excitations in the spectra. The configuration interaction parameters obtained from this analysis - in particular the hybridization strength V-eff and the effective f binding energy Delta(f) - indicate a slightly stronger exchange interaction in CeOs2Al10 compared to CeRu2Al10, and a significant increase in CeFe2Al10. This shows the existence of a substantial amount of Kondo screening in these magnetically ordered systems and places the entire CeM2Al10 family in the region of strong exchange interactions. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.

The occupancy of the 4f(n) contributions in the Kondo semiconductors CeM2Al10 (M = Ru, Os and Fe) has been quantitatively determined by means of bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) on the Ce 3d core levels. Combining a configuration interaction scheme with full multiplet calculations allowed to accurately describe the HAXPES data despite the presence of strong plasmon excitations in the spectra. The configuration interaction parameters obtained from this analysis - in particular the hybridization strength V-eff and the effective f binding energy Delta(f) - indicate a slightly stronger exchange interaction in CeOs2Al10 compared to CeRu2Al10, and a significant increase in CeFe2Al10. This shows the existence of a substantial amount of Kondo screening in these magnetically ordered systems and places the entire CeM2Al10 family in the region of strong exchange interactions. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.