Evidence for 5d-σ and 5d-π covalency in lanthanide sesquioxides from oxygen K-edge X-ray absorption spectroscopy

Abstract

The electronic structure in the complete series of stable lanthanide sesquioxides, Ln2O3 (Ln = La to Lu, except radioactive Pm), has been evaluated using oxygen K-edge X-ray absorption spectroscopy (XAS) with a scanning transmission X-ray microscope (STXM). The experimental results agree with recent synthetic, spectroscopic and theoretical investigations that provided evidence for 5d orbital involvement in lanthanide bonding, while confirming the traditional viewpoint that there is little Ln 4f and O 2p orbital mixing. However, the results also showed that changes in the energy and occupancy of the 4f orbitals can impact Ln 5d and O 2p mixing, leading to several different bonding modes for seemingly identical Ln2O3 structures. On moving from left to right in the periodic table, abrupt changes were observed for the energy and intensity of transitions associated with Ln 5d and O 2p antibonding states. These changes in peak intensity, which were directly related to the amounts of O 2p and Ln 5d mixing, were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by the O K-edge XAS is discussed in the context of several recent theoretical and physical studies on trivalent lanthanide compounds.

Crystal Field in Rare‐Earth Complexes: From Electrostatics to Bonding

“…[71,72] This tiny covalenti nteraction between the 4f orbitals and the ligandsd oes not affect the chemicalp roperties, but, regardless of how small it might be, plays aq uantitative, even qualitative, role in the splitting of the 4f orbitals, and consequently,i mpacts on the magnetic properties.T his work shows that, as is sometimes the case in transition-metal complexes [73] and recently shown for actinidec omplexes, [74] CF and covalency are closely entangled. Dependingo nb otht he nature of the central cation and the ligands,m agnetization of the ground state is either axial or equatorial.…”

Solution and Solid‐State Ligand K‐Edge XAS Studies of PdCl2 Diphosphine Complexes with Phenyl and Cyclohexyl Substituents

“…Ligand K‐edge X‐ray absorption spectroscopy (XAS) is a powerful method for experimentally quantifying differences in covalent metal–ligand bonding and electronic structure in transition metal, lanthanide, and actinide coordination compounds . Ligand K‐edge XAS – where ligand refers to the atom bound to the metal – uses synchrotron‐generated X‐rays to excite 1s electrons localized on the ligand into unoccupied molecular orbitals (MOs) containing ligand p character.…”

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029

“…11 These materials have been the subject of numerous synchrotron radiation based spectroscopy experiments, including X-ray photoelectron spectroscopy, 12 resonant inelastic X-ray scattering, 13 and X-ray absorption spectroscopies at the cerium L3, [7][8][14][15][16][17][18][19][20][21][22] M5,4-, 12,19,[23][24][25][26][27][28][29][30] and N5,4-edges, 12 as well as ligand K-edges. [31][32] Some of these studies have presented evidence for the importance of both 4f and 5d orbital contributions to covalency in Ce bonding. 15,18,[31][32][33][34][35][36] Extended solids including CeO2 have also served as important test cases for evaluating whether accurate theoretical methodology can treat the 4f-electrons simply as part of the core, or whether a more demanding approach should be followed that allows the 4f-electrons to delocalize and participate in bonding.…”

“…[31][32] Some of these studies have presented evidence for the importance of both 4f and 5d orbital contributions to covalency in Ce bonding. 15,18,[31][32][33][34][35][36] Extended solids including CeO2 have also served as important test cases for evaluating whether accurate theoretical methodology can treat the 4f-electrons simply as part of the core, or whether a more demanding approach should be followed that allows the 4f-electrons to delocalize and participate in bonding. [37][38][39][40][41][42][43][44][45][46] Cerium intermetallics have been at the forefront of condensed matter physics and development of the Anderson impurity model for heavy fermion systems and Kondo insulators.…”