Ultrasonic velocity,isothermal compressibility,free volume andP i/T f values in molten stearic,palmitic and lauric acids. Reflections on fusion

Ultrasonic measurements on molten stearic, palmitic and lauric acids enable us to calculate the internal pressure (P _i), free volume, isoentropic compressibility, isothermal compressibility and heat capacity at constant volume as functions of the temperature. TheP _i/T _f values are compared withS _f /b — v_intr) values previously published and a general discussion on the fusion phenomenon is given. An equation connecting internal pressure, molar volume and temperature (P _i V=RT + (at/V)) is proposed and thea values are given.

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Zusammenfassung Die an geschmolzener Stearin-, Palmitin und Laurinsäure durchge-führten Ultraschallmessungen ermöglichen die Berechnung des inneren Druckes, des freien Volumens, der isoenthropischen Kompressibilität, der isothermen Kompressibilität und der Wärmekapazität bei konstantem Volumen als Funktion der Temperatur. DieP _i T _f Werte werden mit den früher veröffentlichten Werten von S_f (b —V _intl) verglichen und eine allgemeine Beschreibung des Schmelzvorgangs wird gegeben. Eine Gleichung zur Verbindung des inneren Druckes, des Molarvolumens und der Temperatur (P_i,V=RT + (a/V)) wird vorgeschlagen und diea Werte angegeben.

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Résumé Le calcul de la variation de la pression interne, du volume libre, de la compressibilité isoentropique, de la compressibilité isotherme et de la chaleur spécifique à volume constant des acides stéarique, palmitique et laurique, en fonction de la température, a été effectué à partir de mesures aux ultrasons. Les valeurs P_i/T_i sont comparées à celles de S_f(b- V_intl déjà publiées. Une discussion générale porte sur le phénomène de fusion. Une équation reliant la pression interne, le volume molaire et la température (P V=RT + (a/V)) est proposée et les valeurs de a' sont données.

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, , , , , , . PiTs. gD_fS/(b — V_tr) . , , : P_iV=RT + (a/V) a.

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We thank the CNR (Rome) for financial support.     

Size-dependent pressure-induced amorphization in nanoscale TiO2

We investigated the size-dependent high-pressure phase transition behavior of nanocrystalline anatase TiO2 with synchrotron x-ray diffraction and Raman spectroscopy to 45 GPa at ambient temperature. Pressure-induced amorphization results in a high-density amorphous (HDA) form when the starting crystallite size is < 10 mm. The HDA-TiO2 transforms to a low-density amorphous form at lower pressures. Harnessing the nanometer length scale thus provides a new window for experimental investigation of amorphization in poor glass formers and a synthesis route for new amorphous materials.     

Local pressure-induced metallization of a semiconducting carbon nanotube in a crossed junction

The electronic and vibrational density of states of a semiconducting carbon nanotube in a crossed junction was investigated by elastic and inelastic scanning tunneling spectroscopy. The strong radial compression of the nanotube at the junction induces local metallization spatially confined to a few nanometers. The local electronic modifications are correlated with the observed changes in the radial breathing and G band phonon modes, which react very sensitively to local mechanical deformation. In addition, the experiments reveal the crucial contribution of the image charges to the contact potential at nanotube-metal interfaces.     

DIOPTAS: a program for reduction of two-dimensional X-ray diffraction data and data exploration

The amount of data collected during synchrotron X-ray diffraction (XRD) experiments is constantly increasing. Most of the time, the data are collected with image detectors, which necessitates the use of image reduction/integration routines to extract structural information from measured XRD patterns. This step turns out to be a bottleneck in the data processing procedure due to a lack of suitable software packages. In particular, fast-running synchrotron experiments require online data reduction and analysis in real time so that experimental parameters can be adjusted interactively. Dioptas is a Python-based program for on-the-fly data processing and exploration of two-dimensional X-ray diffraction area detector data, specifically designed for the large amount of data collected at XRD beamlines at synchrotrons. Its fast data reduction algorithm and graphical data exploration capabilities make it ideal for online data processing during XRD experiments and batch post-processing of large numbers of images.     

Structure of 2,2′-anhydro-3′-O-acetyl-2′thio-1-β-D-arabinofuranosylcytosine hydrochloride1

2,2-Anhydro-3-O-acetyl-2-thio-1--D-arabinofuranosylcytosine hydrochloride crystallizes in space group P2₁2₁2₁ witha=10.373(1),b=14.517(2),c=9.496(1) Å. Reflections were measured on a diffractometer and the structure was solved by direct methods. Least-squares refinement converged atR=0.056 andR _w =0.063. The study of this crystal structure showed that the alterations in the geometry of 2,2-anhydro-1--D-arabinofuranosylcytosine when the bridge oxygen is replaced by sulfur are localized in the region of the fused ring. The angle at the bridge atom decreases to nearly 90°, with concomitant enlargements of about 5°–7° in the angles opposite to the bridge atom. Angle C(1)-N(1)-C(6) decreases by 4°. Also, the amount of double bond character in the bond formed by C(2) and the bridge atom decreases. A survey of the conformational features of S,2-cyclonucleosides and comparison with O,2-cyclonucleosidcs showed that their preferred conformations are the same. However, S,2-cyclonucleosides exhibit a wider range of P and values. This correlates with a greater ease of the sulfur containing five-membered ring to pucker as compared to the oxygen-cyclo ring.This paper is part of the authors' dissertation (Vitali, 1986).