We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s-and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems. CONTENTSNO CO Ions of Two Atom Oxides 4.4. Molecules with Three Atoms Renner Teller Effect O 3 NO 2 CO 2 4.5. Atomic Adsorption 4.6. Molecular Adsorption Linear Dichroism CO on Metal Surfaces 4.7. Analysis of Catalytic Reactions 5. Liquids and Solutions 5.1. Water 5.2. Bio-Organic Molecules Amino Acids, Polypeptides, and Proteins DNA Components 6. Solid Oxides of s-and p-Elements 6.1. Alkali Metal Oxides 6.2. Alkaline Earth Oxides 6.3. General Considerations of p-Element Oxides 6.4. Overview of p-Element Oxides 6.4.1. Boron Oxides

We report a full characterization of PuO₂ nanoparticles at the atomic level and probe their local and electronic structure by a variety of methods available at the synchrotron and theoretical approaches.

Within the development of future nuclear reactors, wet chemistry routes have been investigated for the fabrication of advanced oxide fuels. In this frame, a multi-parametric study focused on the hydrothermal conversion of uranium(IV) oxalate U(C2O4)2.nH2O into uranium oxides was undertaken in order to unravel the effects of temperature, pH and kinetics. For pH  1, the lowest temperatures explored (typically from 180 to 200°C) led to stabilize UO2+x/U4O9 mixtures exhibiting a global O/U ratio evaluated to 2.38 ± 0.10 from U M4-edge HERFD-XANES experiments. Higher temperatures (220-250°C) led the oxide stoichiometry to decrease down to 2.13 ± 0.04 which corresponds to a lower fraction of U4O9 in the mixture. Additionally, increasing the temperature of the hydrothermal treatment efficiently improved the elimination of residual carbon species and water. Hydrothermal conversion of U(C2O4)2.nH2O also led to a drastic modification of the powders morphology. With this aim, pH tuning could be used to shift from bipyramidal aggregates (up to pH = 1), microspheres (2 ≤ pH ≤ 5) then to nanometric powders (pH > 5). Finally, a kinetics study showed that uranium oxides can be obtained from the hydrothermal decomposition of oxalate within only few hours. If the samples early collected during the treatment always presented the characteristic XRD lines of UO2+x/U4O9 fluorite-type structure, they were found to be strongly oxidized (O/U = 2.65 ± 0.14) which suggested the existence of a U(VI)-bearing amorphous secondary phase. This latter further tended to reduce through time. Hydrothermal conversion then probably proceeds as a two-step mechanism composed by the oxidative decomposition of uranium(IV) oxalate followed by the reduction of uranium by organic moieties and its hydrolysis. It appears as an easy and efficient way to yield highly pure uranium oxide samples in solution.

Potash‐ and soda‐lime‐stained glasses from the 12th–13th centuries, blue‐colored by cobalt, have been investigated by Mn, Fe, and Cu K‐edge X‐ray and optical absorption spectroscopies in order to determine the oxidation state of these elements and their impact on the blue color. Remelting these historical glasses in air at 1200°C, the estimated temperature of medieval furnaces, revealed that these four glasses are more reduced before remelting. This favors Mn as weakly absorbing Mn2+, Fe as Fe2+ and Cu as colorless Cu+. Therefore Fe2+ is the second blue chromophore and copper was not intentionally used by glassmakers to obtain a blue color. A colorimetric analysis indicates that these specific melting conditions have a limited effect on the blue color of these glasses. Based on the spectroscopic determination of the redox state of Fe, Mn, and Cu, we estimate the oxygen partial pressure in medieval furnaces to be 10−7–10−9 and 10−5 bar for the potash‐ and soda‐lime samples, respectively. The comparison with previous results enables to prove the evolution of furnace technology over centuries.