Xavier Rocquefelte

Thermochromism in yttrium iron garnet compounds.

Polycrystalline yttrium iron garnet (Y3Fe5O12, hereafter labeled YIG) has been synthesized by solid-state reaction, characterized by X-ray diffraction, Mössbauer spectroscopy, and UV-vis-NIR diffuse reflectance spectroscopy, and its optical properties from room temperature (RT) to 300 °C are discussed. Namely, its greenish color at RT is assigned to an O(2-) → Fe(3+) ligand-to-metal charge transfer at 2.57 eV coupled with d-d transitions peaking at 1.35 and 2.04 eV. When the temperature is raised, YIG displays a marked thermochromic effect; i.e., the color changes continuously from greenish to brownish, which offers opportunities for potential application as a temperature indicator for everyday uses. The origin of the observed thermochromism is assigned to a gradual red shift of the ligand-to-metal charge transfer with temperature while the positioning in energy of the d-d transitions is almost unaltered. Attempts to achieve more saturated colors via doping (e.g., Al(3+), Ga(3+), Mn(3+), ...) remained unsuccessful except for chromium. Indeed, Y3Fe5O12:Cr samples exhibit at RT the same color than the undoped garnet at 200 °C. The introduction of Cr(3+) ions strongly impacts the color of the Y3Fe5O12 parent either by an inductive effect or, more probably, by a direct effect on the electronic structure of the undoped material with formation of a midgap state.

The η6,η1-Coordination of Beryllium Atoms in the Graphite Analogue BeB2C2

The structure of BeB2C2 (see picture; Be white, B dark gray, C light gray) has been solved using near-edge fine-structure analysis of electron energy loss spectra and high-resolution powder diffractometry. It exhibits the unusual features of graphite-analogous B/C layers and a nonsymmetrical coordination of the beryllium atoms. The latter reveals a striking parallelism of the molecular and solid-state chemistries of Be compounds.

Site preference of Eu2+ dopants in the (Ba,Sr)13−xAl22−2xSi10+2xO66 phosphor and its effect on the luminescence properties: a density functional investigation

First principles density functional calculations were carried out for model compounds Ba12EuAl22Si10O66 and Sr12EuAl22Si10O66 to account for the luminescence properties of Eu2+ doped Ba13−xAl22−2xSi10+2xO66 and Eu2+ doped (Ba,Sr)13−xAl22−2xSi10+2xO66. Both host lattices have three different crystallographic alkaline earth sites, namely, AE(1), AE(2) and AE(3), at which the dopants Eu2+ are located. The fluorescence properties of Eu2+ doped Ba13−xAl22−2xSi10+2xO66 are not explained if the dopant ions Eu2+ are considered to randomly occupy the three different Ba sites, but are explained if the dopants are considered to preferentially occupy the Ba(3) sites. In contrast, the fluorescence properties of Eu2+ doped (Ba,Sr)13−xAl22−2xSi10+2xO66 are explained only when both Sr2+ and Eu2+ cations are considered to occupy the AE(3) sites, which suggests that the activators are more randomly distributed over the three AE sites in Eu2+ doped (Ba,Sr)13−xAl22−2xSi10+2xO66 than in Eu2+ doped Ba13−xAl22−2xSi10+2xO66. Calculations evidence the strong tendency for the Ba(3) sites to accommodate defects in Ba13−xAl22−2xSi10+2xO66, namely, Eu dopants, Sr substituents, and Ba vacancies. This arises from the higher malleability of the Ba(3) site compared to the Ba(1) and Ba(2) sites.

Synergetic theoretical and experimental structure determination of nanocrystalline materials: study of LiMoS2

A combined approach is proposed to solve the structure of badly crystallized materials. It couples poor quality powder X-ray diagram (XRD) and XRD simulation deduced from first-principle geometry optimization. It is used to completely solve the LiMoS2 structure.

Spectromicroscopy of C60 and azafullerene C59N: Identifying surface adsorbed water

C60 fullerene crystals may serve as important catalysts for interstellar organic chemistry. To explore this possibility, the electronic structures of free-standing powders of C60 and (C59N)2 azafullerenes are characterized using X-ray microscopy with near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy, closely coupled with density functional theory (DFT) calculations. This is supported with X-ray photoelectron spectroscopy (XPS) measurements and associated core-level shift DFT calculations. We compare the oxygen 1s spectra from oxygen impurities in C60 and C59N, and calculate a range of possible oxidized and hydroxylated structures and associated formation barriers. These results allow us to propose a model for the oxygen present in these samples, notably the importance of water surface adsorption and possible ice formation. Water adsorption on C60 crystal surfaces may prove important for astrobiological studies of interstellar amino acid formation.

Corrigendum: Unravelling the origin of the giant Zn deficiency in wurtzite type ZnO nanoparticles

Owing to its high technological importance for optoelectronics, zinc oxide received much attention. In particular, the role of defects on its physical properties has been extensively studied as well as their thermodynamical stability. In particular, a large concentration of Zn vacancies in ZnO bulk materials is so far considered highly unstable. Here we report that the thermal decomposition of zinc peroxide produces wurtzite-type ZnO nanoparticles with an extraordinary large amount of zinc vacancies (>15%). These Zn vacancies segregate at the surface of the nanoparticles, as confirmed by ab initio calculations, to form a pseudo core-shell structure made of a dense ZnO sphere coated by a Zn free oxo-hydroxide mono layer. In others terms, oxygen terminated surfaces are privileged over zinc-terminated surfaces for passivation reasons what accounts for the Zn off-stoichiometry observed in ultra-fine powdered samples. Such Zn-deficient Zn1-xO nanoparticles exhibit an unprecedented photoluminescence signature suggesting that the core-shell-like edifice drastically influences the electronic structure of ZnO. This nanostructuration could be at the origin of the recent stabilisation of p-type charge carriers in nitrogen-doped ZnO nanoparticles.

Crystal Structures Of Several Inorganic-Organic Hybrids Solved From Powder XRD

When dealing with the crystal structure solving of low-crystalline materials, it is of paramount importance to combine DFT geometry optimization with direct-space methods in order to find the correct structural model. Particularly, in the case of inorganic-organic hybrids prepared by solvothermal route, finite dimensional single crystal is very difficult to obtain. Hence, we present several successfully elucidated crystal structures from the powder XRD of the low-crystalline inorganic-organic hybrids: Cr(HL)Cl3, HL=Hbdmpza (1), [1] VO(C10H7COO)2 (2), VO(C14H9COO)2 (3). All of these structures have been solved from powder XRD data, either from laboratory or synchrotron source, by combining direct-space methods (simulated annealing), DFT geometry optimization, and constraint Rietveld refinement. The solid compound 1 was isolated in acetonitrile by starting CrCl3·9H2O and HL only in a concentrated solution, from which microcrystalline aggregates precipitated. Additionally, the presence of a carboxylate proton in HL stabilizes the structure by H-bonds, but also enables adsorption of moisture. Other two hybrids were synthesized in o-xylene by reacting vanadium (V) triisopropoxide with 1-naphthalenecarboxylic [2] or 9-anthracenecarboxylic acid. [2] They contain onedimensional chain of corner-sharing tetrahedra in the case of VO(C10H7COO)2 (Figure 1(a), (c)), and corner-sharing octahedra for VO(C14H9COO)2 (Figure 1(b), (d)) oriented along orthorhombic/monoclinic c-axis, respectively. While VO(C14H9COO)2 exhibits bidentate bridging binding of organic moiety to the metal center, VO(C10H7COO)2 shows monodentate mode as evidenced from DFT and infrared spectroscopy. The obtained crystal structures were further verified by direct methods (ab initio approach). I. D. acknowledges financial support from the Unity through Knowledge Fund (www.ukf.hr) of the Croatian Ministry of Science, Education and Sports (Grant Agreement No. 7/13).