The knowledge of the coordination environment around various atomic species in many functional materials provides a key for explaining their properties and working mechanisms. Many structural motifs and their transformations are difficult to detect and quantify in the process of work (operando conditions), due to their local nature, small changes, low dimensionality of the material, and/or extreme conditions. Here we use an artificial neural network approach to extract the information on the local structure and its in situ changes directly from the x-ray absorption fine structure spectra. We illustrate this capability by extracting the radial distribution function (RDF) of atoms in ferritic and austenitic phases of bulk iron across the temperature-induced transition. Integration of RDFs allows us to quantify the changes in the iron coordination and material density, and to observe the transition from a body-centered to a face-centered cubic arrangement of iron atoms. This method is attractive for a broad range of materials and experimental conditions.

Thermochromic phase transition was studied in CuMoO4 using the Cu and Mo K-edge X-ray absorption spectroscopy in the temperature range of 10-300 K. The hysteretic behavior has been evidenced from the temperature dependence of the pre-edge shoulder intensity at the Mo K-edge, indicating that the transition from brownish-red γ-CuMoO4 to green α-CuMoO4 occurs in the temperature range of 230-280 K upon heating, whereas the α-to-γ transition occurs between 200 and 120 K upon cooling. Such behavior of the pre-edge shoulder at the Mo K-edge correlates with the change of molybdenum coordination between distorted tetrahedral in α-CuMoO4 and distorted octahedral in γ-CuMoO4. This result has been supported by ab initio full-multiple-scattering X-ray absorption near edge structure (XANES) calculations.

Reversible thermochromic phase transition between αand γ-phases was studied in CuMoO 4 and CuMo 0.90 W 0.10 O 4 using X-ray absorption spectroscopy in the temperature range of 10-300 K. Reverse Monte Carlo modelling with evolutionary algorithm approach at several absorption edges simultaneously was applied to extract structural information encoded in the experimental EXAFS spectra. The obtained results show that an addition of 10 mol% of tungsten to CuMoO 4 induces local distortions in the structure and stabilizes the γ-phase, leading to an increase of the phase transition temperature by ∼50-100 K.

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Thin films of rhenium trioxide (ReO 3) were produced by reactive DC magnetron sputtering from metallic rhenium target followed by annealing in the air in the range of temperatures from 200C to 350C. Nanocrystalline singlephase ReO 3 films were obtained upon annealing at about 250C. The thin films appear bright red in reflected light and blue-green in transmitted light, thus showing an optical transparency window in the spectral range of 475-525 nm. The film exhibits high conductivity, evidenced by macro-and nano-scale conductivity measurements. The longrange and local atomic structures of the films were studied in detail by structural methods as X-ray diffraction and X-ray absorption spectroscopy. The oxidation state (6+) of rhenium was confirmed by X-ray photoemission and Xray absorption spectroscopies. The nanocrystalline morphology of the annealed films was evidenced by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The obtained results allowed us to propose the mechanism of rhenium oxide conversion from the initially amorphous ReO x phase to cubic ReO 3. 1. Introduction Rhenium oxides are known to exist in the three main phases ReO 2 , ReO 3 and Re 2 O 7 , corresponding to the oxidation states of Re 4+ , Re 6+ and Re 7+ , respectively. ReO 2 is dark blue or black solid and has a monoclinic phase α-ReO 2 below 300°C [1]. When heated above 300°C, it irreversibly turns into the orthorhombic phase β-ReO 2 , which is stable in vacuum to 850-1000°C but oxidizes in the air to Re 2 O 7 above 400°C. Both phases of ReO 2 have metallic conductivity [2]. Crystalline Re 2 O 7 is an inorganic polymer and it is electrically insulating material [2]. It consists of ReO 6 octahedra and ReO 4 tetrahedra. In each octahedral ReO 6 group, three of the Re-O bonds are longer than the others, and if weak bonds are broken (e.g., upon heating) volatile molecules of Re 2 O 7 are produced. Re 2 O 7 sublimes at temperatures above 360°C. Besides, Re 2 O 7 is highly hygroscopic, decomposing into perrhenic acid (HReO 4) when exposed to moisture [3]. ReO 3 is a red solid with a metallic luster. Its cubic crystalline structure is of perovskite-type and is formed by a network of regular ReO 6 octahedra, which have common vertices in three