Soft Report - Dipartimento di Fisica - Sapienza

FacilitiesX-ray Diffraction Laboratory - UdR CamerinoX-ray diffraction set-upThree different equipments, relevant to the activity of the center anddeveloped in recent times are: an angular dispersive x-ray diffractionset-up for measurements at high temperature conditions (up to2300K) and room pressure; a high pressure (up to 10 Gpa) and hightemperature (up to 1500 K) energy dispersive x-ray diffraction set-upand a high pressure (up to 100 Gpa) angular x-ray diffraction set-upallowing measurements at moderate temperature (up to 700 K).The x-ray source for the first two equipments is the same and consistsof a powerful Mo rotating anode (18 KW) delivering two independentbeams (one for each set-up). In the angular dispersive the beam ismonochromatic using the (002) Bragg peak of pyrolytic graphite at anenergy of 17.47 keV. This set-up is particularly useful to study andidentify phase transition and for accurate measurements of thermalexpansion in a very wide class of materials. In particular we haveespecially metals and semiconductors like Pd, Fe, Zn, Si and Ge. As anexample we report in the picture the behaviour of the (002) peak ofAg as function of temperature in the 293-1215 K temperature range.It is possible to see clearly the shift of the (002) peak for increasingtemperature as a consequence of the variation of the thermalexpansion, while the two graphite peaks (100) and (101) remain quiteat the same angular value. The red line has been recorded at 1250 Kand show the melting of Ag.The high pressure and high temperature energy dispersive x-raydiffraction set-up is realized using the second white x-ray beamcoming out from the 18 KW anode. The diffractometer includes anenergy-sensitive Ge detector with precise collimation system (sollerslits) and motorization for confining the scattering region in thevolume sample. The detection system is an LEGe detector withenergy resolution of 195 eV at 5.9 keV. The scattering angle can betuned continuously. The sample can be confined in a large volumeParis-Edimburgh high pressure cell, the only presently available inItaly and suitable for measurements up to pressures andtemperatures of 10 Gpa and 1500 K respectively. The press installedon motorized stages is shown in the picture. Successful experimentshave been performed on Ag, Ge and Polyethylene samples. We reportin the figure a spectrum of a Ge high-pressure sample under ambientconditions. This set-up allows accurate investigation of structure andphase transition in materials under extreme condition (high pressure /high temperature).The high-pressure angular x-ray diffraction set-up consist in amodified 4-circle geometry Diffractometer (KUMA-Oxford) combinedwith a miniaturized Diamond Anvil Cell (D'Anvils Ltd.) allowing thestudy of the structure and phase transitions of condensed matter in aa wide high pressure range (up to 100 Gpa) and moderatetemperature (up to 700K). The Kuma-Oxford diffractometer isequipped with a 3KW Mo anode (17.49 keV wavelength). In the nextfigure we show the special holder developed for the DAC that allowsmicrometric positioning of the centre of the cell. Besides the KUMA-Oxford detection systems that permits to collect standard scans, wehave developed a very efficient low-cost solution using an in-housemade detector holder with an high resolution x-ray photographic film(Structurix D7 Agfa). In this way we collect the entire Debye-Scherrerrings increasing statistic. The DAC used has an aperture angle of 40degrees, from -20 to 20 degs but tilting the cell respect to the x-raybeam we can obtain -10 to 30 degs. In the following picture we showthe typical Debye-Scherrer rings of Si powders at a pressure of 1.1Gpa and room temperature. Bragg spots of diamonds and spuriousring of inconel gasket are also visible. The integration of the 2-D data(fit2D software) produces one-dimensial plot and this is shown in thenext figure where it is possible to identify clearly the (111), (200) and(311) Si peaks.[1] A. Di Cicco, R. Gunnella, R. Marassi, M. Minicucci, R. Natali, G.Pratesi, E. Principi, S. Stizza, J. Non-Cryst. Sol. 352 4155 (2006).[2] http://gnxas.unicam.it, website of the Camerino research group.SOFT Scientific Report 2004-0618