Enhanced Thermoelectric Power and Electronic Correlations in RuSe2_2

We report the electronic structure, electric and thermal transport properties of Ru$_{1-x}$Ir$_{x}$Se$_2$ ($x \leq 0.2$). RuSe$_2$ is a semiconductor that crystallizes in a cubic pyrite unit cell. The Seebeck coefficient of RuSe$_2$ exceeds -200 $\mu$V/K around 730 K. Ir substitution results in the suppression of the resistivity and the Seebeck coefficient, suggesting the removal of the peaks in density of states near the Fermi level. Ru$_{0.8}$Ir$_{0.2}$Se$_{2}$ shows a semiconductor-metal crossover at about 30 K. The magnetic field restores the semiconducting behavior. Our results indicate the importance of the electronic correlations in enhanced thermoelectricity of RuSb$_{2}$.Comment: 6 pages, 4 figure

X-ray diffraction studies of the effects of N incorporation in amorphous CNx, materials

The effects of nitrogen incorporation on the atomic-scale structure of amorphous CNx samples have been studied for 0, 5, 20, and 30 at. % N concentration, by x-ray diffraction. Significant differences in the structure are observed on the incorporation of only 5 at. % N, and the changes in structure continue as further N is added. From the experimental data, we are able to obtain directly the average bond distances and then calculate the average bond angles for each of the samples. The average first neighbor distance shows a gradual decrease from 1.55 Angstrom for 0 at. % N, to 1.44 Angstrom for 30 at. % N, and a similar trend is observed in the position of the second neighbor peak. This gives a corresponding increase in the average bond angle from 108 degrees to 114 degrees. The results show an increase in the fraction of sp(2) bonded carbon atoms with increasing N concentration, and there is evidence for the presence of significant numbers of C=N and C=N bonds. These results are also consistent with stress, hardness, and optical gap measurements for these samples. (C) 1998 American Institute of Physics. [S0021-8979(98)03907-3]

Compressibility and thermal expansion of cubic silicon nitride

The compressibility and thermal expansion of the cubic silicon nitride (c-Si3N4) phase have been investigated by performing in situ x-ray powder-diffraction measurements using synchrotron radiation, complemented with computer simulations by means of first-principles calculations. The bulk compressibility of the c-Si3N4 phase originates from the average of both Si-N tetrahedral and octahedral compressibilities where the octahedral polyhedra are less compressible than the tetrahedral ones. The origin of the unit cell expansion is revealed to be due to the increase of the octahedral Si-N and N-N bond lengths with temperature, while the lengths for the tetrahedral Si-N and N-N bonds remain almost unchanged in the temperature range 295-1075 K

Étude par diffraction des rayons X des vernis rouges des sigillées du sud de la Gaule. Les ateliers de la Graufesenque

La Graufesenque (Millau, Aveyron) 1$^{\rm rst}$ and 2$^{\rm nd}$, centuries AC Sigillata red slips have been studied by x-ray diffraction at ESRF and on a conventional diffractometer. Although x-ray diffraction is a non-destructive characterisation technique adapted to surface study, it has not been used so mutch for Sigillata slip characterisation. The poor quality of the slip surface can be an explanation. In this work we demonstrate that it is possible to get good quality diffraction diagrams on 20-120 AC Sigillata slips. The results confirm the productions homogeneity and the hypothesis about a non-local clay used for the slips. On a counterpart, the late (second half of 2$^{\rm nd}$ century AC) Sigillata study needs because of their crackled surface a high quality beam, which can only be accessible on a synchrotron installation. These productions seems to be more heterogeneous: some Sigillata present similar diffraction diagrams for the slip and ceramic body (same clay?), some others present results similar to the precedent production period ones.Les vernis rouges de poteries sigillées (I$^{\rm er}$ et II$^{\rm e}$ siècles) de la Graufesenque (Millau, Aveyron) ont été étudiés par diffraction des rayons X à la fois à l'ESRF et sur une installation classique de laboratoire. Bien que la diffraction des rayons X soit une technique adaptée à l'étude des surfaces, cette technique a jusqu'à présent assez peu servi dans l'examen de ces objets. Ceci peut en partie s'expliquer par le mauvais état de surface de certains vernis. Dans ce travail, nous montrons que moyennant quelques précautions, il est possible d'obtenir sur une installation classique de bons diffractogranunes des vernis des sigillées datés entre 20 et 120 ap. J.C. Les résultats confirment l'homogénéité de ces productions et l'hypothèse selon laquelle ces vernis n'ont pas été fabriqués à partir des argiles locales du jurassique environnant le site. Par contre, l'étude des sigillées tardives de la deuxième moitié du II$^{\rm e}$ siècle, dont les vernis sont très écaillés, nécessite une qualité de faisceau inaccessible sur une installation classique, et ne peut être réalisée que sur une source de lumière de type synchrotron. Ces productions semblent plus hétérogènes : certaines sigillées tardives ont un vernis qui pourrait avoir été élaboré à partir de l'argile locale, tandis que d'autres ont encore le vernis des sigillées de la période précédente

X-Ray Structural Determination of a Multilayered Magnetic Dielectric Ceramic Ba42Ti51Fe20O174 in the BaO–TiO2–Fe2O3 System

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