Trimerization of NaC2N3 to Na3C6N9 in the Solid

Sodium dicyanamide NaC2N3 was found to undergo two phase transitions. According to thermal analysis and temperature-dependent X-ray powder diffractometry, the transition of α-NaC2N3 (1a) to β-NaC2N3 (1b) occurs at 33°C and is displacive. 1a crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 647.7(1), b = 1494.8(3), c = 357.25(7) pm, β = 93.496(1)°, and Z = 4. The structure was solved from powder diffraction data (Cu Kα1, T = 22°C) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.072, Rp = 0.053, and RF = 0.074. 1b crystallizes in the orthorhombic system, space group Pbnm (no. 62), with a = 650.15(5), b = 1495.1(2), c = 360.50(3) pm, and Z = 4. The structure was refined by the Rietveld method using the atomic coordinates of 1a as starting values (Mo Kα1, T = 150°C). The final agreement factors were wRp = 0.044, Rp = 0.034, RF = 0.140. The crystal structures of both polymorphs contain sheets of Na+ and N(CN)2- ions which are in 1a nearly and in 1b exactly coplanar. Above 340°C, 1b trimerizes in the solid to Na3C6N9 (2). 2 crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 1104.82(1), b = 2338.06(3), c = 351.616(3) pm, β = 97.9132(9)°, and Z = 4. The structure was solved from synchrotron powder diffraction data (λ = 59.733 pm) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.080, Rp = 0.059, and RF = 0.080. The compound contains Na+ and the planar tricyanomelaminate C6N93-. The phase transition from 1b to 2 is reconstructive. It occurs in the solid-state without involvement of other phases or intermediates. The crystal structures of 1b and 2 indicate that there is no preorientation of the N(CN)2- in the solid before their trimerization to C6N93-

Conserving quasiparticle calculations for small metal clusters

A novel approach for GW-based calculations of quasiparticle properties for finite systems is presented, in which the screened interaction is obtained directly from a linear response calculation of the density-density correlation function. The conserving nature of our results is shown by explicit evaluation of the $f$-sum rule. As an application, energy renormalizations and level broadenings are calculated for the closed-shell Na$_9^+$ and Na$_{21}^+$ clusters, as well as for Na$_4$. Pronounced improvements of conserving approximations to RPA-level results are obtained

Retrieval and molecule sensitivity studies for the global ozone monitoring experiment and the scanning imaging absorption spectrometer for atmospheric chartography

The Global Ozone Monitoring Experiment (GOME) and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) are diode based spectrometers that will make atmospheric constituent and aerosol measurements from European satellite platforms beginning in the mid 1990's. GOME measures the atmosphere in the UV and visible in nadir scanning, while SCIAMACHY performs a combination of nadir, limb, and occultation measurements in the UV, visible, and infrared. A summary is presented of the sensitivity studies that were performed for SCIAMACHY measurements. As the GOME measurement capability is a subset of the SCIAMACHY measurement capability, the nadir, UV, and visible portion of the studies is shown to apply to GOME as well

Problemeset cossibilites d'evaluation de procedes des analyses Cluster, III. Appendix: Courte description des Algorithmes analyse Cluster les plus rependues

Es wird eine relativ einfach gehaltene Kurzcharakteristik derjenigen Clusteranalyse-Algorithmen gegeben, die aufgrund eines Literaturüberblicks (SCHNEIDER & SCHEIBLER 1983a) als die in der Fonchung hauptsächlich benutzten Verfahren einzustufen sind. Die Kurzbeschreibung verzichtet im wesentlichen auf statistische Details und verfolgt speziell das Ziel, dem Leser eine Vorstellung von Gemeinsamkeiten und Untenchieden in der Funktionsweise von hierarchischen Clusteranalysen, Optimierungs- bzw. Partitionierungstechniken, Dichteverfahren, "Clumping Techniques" und anderen Prozeduren zu geben.This paper presents a summary of 18 clustering algorithms most frequently applied in reseuch (cf. SCHNEIDER & SCHEIBLEK 1983a). Only a short description of each procedure is provided which aims at highlighting the basic differences and comrnonalities of hierarchical clustering algorithms, iterative partitioning methods, mode seeking techniques, clumping techniques, and other procedures.Les Algorithmes analyse Cluster qui sont decritent par (Schneider & Scheibler 1983) comme etant les procedes les plus rependus dans Ia recherche sont relates ici de facon courte. Le description chematique exclue l'ennumeration des details statistiques et a pour but essentiel de transmettre au lecteur, entre autre, une representation des rapports et des differences dans Je mode de fonction des analyses Cluster hierarchiques, des techniques d'optimation, et des procedes de population «Clumping techniques» etc