Experimental evidence of an electronic transition in CeP under pressure using Ce L3 XAS

Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1-) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1--decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1-, OH1-, MeO1-, and O2- ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

Crystal-field mediated electronic transitions of EuS up to 35 GPa

An advanced experimental and theoretical model to explain the correlation between the electronic and local structure of Eu[Formula: see text] in two different environments within a same compound, EuS, is presented. EuX monochalcogenides (X: O, S, Se, Te) exhibit anomalies in all their properties around 14 GPa with a semiconductor to metal transition. Although it is known that these changes are related to the [Formula: see text] [Formula: see text] [Formula: see text] electronic transition, no consistent model of the pressure-induced modifications of the electronic structure currently exists. We show, by optical and x-ray absorption spectroscopy, and by ab initio calculations up to 35 GPa, that the pressure evolution of the crystal field plays a major role in triggering the observed electronic transitions from semiconductor to the half-metal and finally to the metallic state.

Probing Multiconfigurational States by Spectroscopy: The Cerium XAS L3 -edge Puzzle

The Ce L₃ edge XAS spectra of CeO₂ and cerocene [Ce(C₈ H₈ )₂ ] were calculated with relativistic ab-initio multireference wavefunction approaches capable of reproducing the observed spectra accurately. The study aims to resolve the decades-long puzzle regarding the relationship between the number and relative intensities of the XAS peaks and the 4f electron occupation in the ground state (GS) versus the core-excited states (ESs). CeO₂ and cerocene exemplify the different roles of covalent bonding and wavefunction configurational composition in the observed intensity patterns. Good agreement is found between the calculated GS 4f-shell occupations and the value derived from XAS measurements using peak areas (n_f ). The identity of the two-peaked Ce L₃ edge is fully rationalized from the perspective of the relaxed wavefunctions for the GS and core ESs. The states underlying the different peaks differ from each other in a surprisingly simple way that can be associated with 4f¹ vs. 4f⁰ sub-configurations. Furthermore, part of one of the cerocene spectral peaks is associated with 4f² sub-configurations. The pattern therefore reveals excited states that can be interpreted in terms of Ce IV and III oxidation numbers, as long assumed, with Ce II states additionally appearing in the cerocene spectrum. While this work demonstrates the rough accuracy of the conventional approach to determining n_f from Ce L₃ -edge XAS, limitations are highlighted in terms of the ultimate accuracy of this approach and the potential of observing new types of excited states. The need to determine the sources of n_f by calculations, is stressed.

First high-pressure XAFS results at the bending-magnet-based energy-dispersive XAFS beamline BL-8 at the Indus-2 synchrotron facility

The static focusing optics of the existing energy-dispersive XAFS beamline BL-8 have been advantageously exploited to initiate diamond anvil cell based high-pressure XANES experiments at the Indus-2 synchrotron facility, India. In the framework of the limited photon statistics with the 2.5 GeV bending-magnet source, limited focusing optics and 4 mm-thick diamond windows of the sample cell, a (non-trivial) beamline alignment method for maximizing photon statistics at the sample position has been designed. Key strategies include the selection of a high X-ray energy edge, the truncation of the smallest achievable focal spot size to target size with a slit and optimization of the horizontal slit position for transmission of the desired energy band. A motor-scanning program for precise sample centering has been developed. These details are presented with rationalization for every step. With these strategies, Nb K-edge XANES spectra for Nb₂O₅ under high pressure (0-16.9 GPa) have been generated, reproducing the reported spectra for Nb₂O₅ under ambient conditions and high pressure. These first HPXANES results are reported in this paper. The scope of extending good data quality to the EXAFS range in the future is addressed. This work should inspire and guide future high-pressure XAFS experiments with comparable infrastructure.