Influence of crystallographic environment on scandium K-edge X-ray absorption near-edge structure spectra.

The absence of an extensive series of Sc K-edge X-ray absorption near-edge structure spectroscopy (XANES) reference spectra and the scarcity of direct structural data on Sc are major hurdles to develop our understanding of Sc chemistry. However, this first step is essential to develop new Sc-based applications and to better understand the formation of Sc deposits. Here, we present a detailed comparative study of Sc K-edge XANES spectra of three Sc-bearing compounds: a garnet (Ca3Sc2Si3O12), an oxide (Sc2O3) and a phosphate (ScPO4·2H2O). First-principles calculations have been performed to interpret the origin of the K pre-edge spectral features. We demonstrate the validity of our approach by reproducing satisfyingly the experimental spectra. The densities of states projected on the absorbing Sc atom and its first neighbours give the possibility to interpret the position and intensity of the pre-edge XANES features in terms of Sc local environment. The pre-edge features provide information on p-d mixing of the absorber orbitals giving clues on the centrosymmetry of the site and on the mixing of the empty 4p orbitals of the absorber with empty 3d orbitals of the neighbours via the empty p orbitals of the ligands. We also show that these features give a first estimate of the crystal-field splitting energy (ca. 1.5 eV), inaccessible using other spectroscopic methods. Comparisons with K-edge spectra of other 3d0 ions from the literature reveal the specificities of the Sc pre-edge, indicating that core-hole screening is weaker than for Ti4+-bearing compounds. This study provides a dataset of spectral signatures and a theoretical basis for their interpretation, a requirement for future studies on Sc chemical form in synthetic and natural systems.

Temperature dependence of X-ray absorption and nuclear magnetic resonance spectra: probing quantum vibrations of light elements in oxides.

A combined experimental-theoretical study on the temperature dependence of the X-ray absorption near-edge structure (XANES) and nuclear magnetic resonance (NMR) spectra of periclase (MgO), spinel (MgAl2O4), corundum (α-Al2O3), berlinite (α-AlPO4), stishovite and α-quartz (SiO2) is reported. Predictive calculations are presented when experimental data are not available. For these light-element oxides, both experimental techniques detect systematic effects related to quantum thermal vibrations which are well reproduced by density-functional theory simulations. In calculations, thermal fluctuations of the nuclei are included by considering nonequilibrium configurations according to finite-temperature quantum statistics at the quasiharmonic level. The influence of nuclear quantum fluctuations on XANES and NMR spectroscopies is particularly sensitive to the coordination number of the probed cation. Furthermore, the relative importance of nuclear dynamics and thermal expansion is quantified over a large range of temperatures.

Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)2]2.

Theoretical predictions show that depending on the populations of the Fe 3d xy , 3d xz , and 3d yz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe-ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)-high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.

Detecting non-bridging oxygens: non-resonant inelastic X-ray scattering in crystalline lithium borates.

Probing the local environment of low-Z elements, such as oxygen, is of great interest for understanding the atomic-scale behavior in materials, but it requires experimental techniques allowing it to work with versatile sample environments. In this paper, the local environment of lithium borate crystals is investigated using non-resonant inelastic X-ray scattering (NRIXS) at energy losses corresponding to the oxygen K-edge. Large variations of the spectral features are observed close to the edge onset in the 535-540 eV energy range when varying the Li2O content. Calculations allow identification of contributions associated with bridging oxygen (BO) and non-bridging oxygen (NBO) atoms. The main result resides in the observed core-level shift of about 1.7 eV in the spectral signatures of the BO and NBO. The clear signature at 535 eV in the O K-edge NRXIS spectrum is thus an original way to probe the presence of NBOs in borates, with the great advantage of making possible the use of complex environments such as a high-pressure cell or high-temperature device for in situ measurements.

Contribution of molecular dynamics simulations and ab initio calculations to the interpretation of Mg K-edge experimental XANES in K(2)O-MgO-3SiO(2) glass.

The structural environment of Mg in a K-bearing silicate glass of composition K(2)MgSi(3)O(8) is investigated by x-ray absorption near-edge structure (XANES) spectroscopy at the Mg K-edge. XANES calculations are performed using a plane-wave electronic structure code, and a structural model obtained by classical molecular dynamics coupled to ab initio relaxation. Bond valence theory is used to validate plausible environments within the structural models. Comparison between the experimental and calculated spectra enables us to conclude that the Mg atoms are located in distorted tetrahedral sites. Site distortions are found to be correlated to the theoretical shift of the XANES edge position.

First-principles calculations of X-ray absorption spectra at the K-edge of 3d transition metals: an electronic structure analysis of the pre-edge.

We first present an extended introduction of the various methods used to extract electronic and structural information from the K pre-edge X-ray absorption spectra of 3d transition metal ions. The K pre-edge structure is then modelled for a selection of 3d transition metal compounds and analyzed using first-principles calculations based on the density functional theory (DFT) in the local density approximation (LDA). The selected compounds under study are presented in an ascending order of electronic structure complexity, starting with the Ti K-edge of rutile and anatase, and finishing with the Fe K-edge of the cyanomet-myoglobin. In most cases, the calculations are compared to polarized experimental spectra. It is shown that DFT-LDA methods enable us to reproduce satisfactorily the experimental features and to understand the nature of the electronic transitions involved in the pre-edge region. The limiting aspects of such methods in modelling the core-hole electron interaction and the 3d electron-electron repulsion are also pointed out.