RICERCA ITALIANA     

Research program

Vibrational dynamics and relaxation in densified glasses and confined disordered systems

University Co-ordinator

Università degli Studi di CAMERINO - FISICA - ()

Research Unit Leader

Andrea Di Cicco

Description

The research program of the Camerino group will be focused on:
1) ex-situ and in-situ angular and energy-dispersive x-ray diffraction experiments (ADXD, EDXD) of densified glasses in the 0-10 GPa pressure region, eventually extended to higher pressures;
2) the development of original preparation procedures for producing samples suitable for measurements under non-equilibrium conditions (densified glasses and deeply undercooled systems) at high pressure;
3) the performance of ADXD, EDXD, and density measurements of a-Ge, a-GeO2 and related systems in a wide pressure and temperature range in our University and of x-ray absorption spectroscopy (XAS), single-energy x-ray absorption detection (SEXAD) temperature scans, and energy-scanning x-ray diffraction measurements using synchrotron radiation (in collaboration also with L'Aquila unit).
4) the study of the structure of covalent systems and glasses, also produced and characterized by other units, under extreme conditions using state-of-the-art x-ray techniques and paying particular attention to densification, amorphization and undercooling which are functions of the external pressure. Polyamorphism and crystal nucleation a function of pressure and temperature will be studied.

The Camerino group is currently able to perform ADXD experiments as a function of temperature using a rotating anode source, and has accumulated a large expertise in the production of suitable samples for measuring liquids under extreme conditions, with the construction of a highly developed sample preparation laboratory and suitable high-temperature devices. [1] The group participated to the first pioneering experiments with the large-volume high-pressure Paris-Edinburgh cell, originally conceived for neutron diffraction measurements and applied to XAS only in 1996.[2] The technology for the complex sample environment needed to operate the large-volume press has been completely replicated and improved in our laboratory, including the epoxy-boron gaskets, the small parts for sample container and crucible. [1] A large-volume Paris-Edinburgh cell is currently available within our research unit. A 4-circles diffractometer, adaptable for very high-pressure measurements with diamond anvil cells (DACs), is also available within our group.

Recent experimental efforts allowed us to study the structure of deeply undercooled liquids and the characteristic of the crystal nucleation processes, as well as the formation of solid metastable states.[3,4] These progresses have been made using a combination of x-ray techniques (XAS, SEXAD, EDXD, ESXD) [5] exploiting the intensity and collimation of synchrotron radiation for accurate measurements under extreme conditions. High rates of undercooling at low and high temperatures were obtained by emulsifying or dispersing fine submicrometric droplets of the specimen in suitable inert substances forming a solid pellet-shaped sample (see also [6,7]). First measurements obtained using a large-volume high-pressure Paris-Edinburgh cell have also shown that accurate XAS, EDXD, and SEXAD measurements are possible under high-pressure and high-temperature conditions, giving important information on phase transitions, structure and undercooling. [8]

A deep insight on structural properties and phase transitions can be gained by EDXD, ADXD and x-ray density measurements performed in in-house laboratories or synchrotron radiation facilities. These measurements can give important information which nicely complement the above-mentioned XAS and nucleation rate results.
EDXD has been extensively used for determination of structures under high-pressure conditions, usually exploiting the high collimation of synchrotron radiation for measurements with diamond anvil cells (DAC) having sample dimensions of the order of 50 microns (see for example [9-11] and ref. therein). This technique has several advantages, like for example the absence of mechanical motion which facilitates the positioning of the sample, and can be used also with conventional or rotating anode x-ray sources in our home laboratories, using large-volume or DAC cells. In particular, DAC cells can be installed on our 4-circles diffractometer allowing us the study of single crystals, equation of states and metastable phases in an extended range of pressures. Measurements of the density under extreme conditions is also a very difficult task. The possibility of measuring the density by means of x-ray transmission data (see [12] and ref. therein) adds another useful piece of information to the structural data we intend to obtain under extreme conditions. In particular, first-order phase transitions can be easily identified by those density measurements.
However, no facilities for EDXD or x-ray density measurements under high-pressure conditions are present in Italy and we plan to further develop suitable experimental set-ups for such techniques in our laboratories.

The experimental program of the Camerino group, in the framework of the present project, is focused on the development and application of the above-mentioned techniques to liquid and glassy covalent substances under pressure, including therefore different activities mentioned below.

i) During the first year, we plan to perform accurate ADXD measurements under high-temperature conditions of model covalent systems (like GeO2, SiO2, Ge, Si and Ge-Si alloys) in their solid, liquid, undercooled liquid and amorphous densified states. These measurements will be performed using our powerful rotating anode source (18 KW) equipped with a Mo target. High-temperature measurements will be obtained using a modified version of the furnace originally developed for XAS [14] which allows us to collect diffraction pattern in a very wide angular range. Samples with different undercooling properties will be designed, produced, and characterized in our Camerino preparation laboratory. They will consist in dispersions of high-purity droplets for which we shall measure the size distribution, an important property affecting the undercooling limit (together with the sample container). A new W target x-ray source, suitable for EDXD measurements, will be installed for developing diffraction experiments under high-pressure conditions. In fact, we expect to install a portable and heatable large-volume high-pressure cell of the Paris-Edinburgh type, that can be used for x-ray measurements in the 0-10 GPa, 300-1600 K pressure and temperature ranges. Installation of the cell will require the testing of the 2 KBar oil pumping system, calibration of the pressures with suitable pressure markers, design of specific sample containers and heating elements. The cell will be designed to perform resistivity measurements as well as EDXD and x-ray density, using both rotating anode and conventional x-ray tube sources. The installation of the new large-volume cell on the rotating anode equipment will anyway require precise motorized positioning systems for heavy components. Our expertise in the usage of large-volume cells and manipulation of samples should allow us to complete the new system within one year.
The first x-ray experiments will be carried out on GeO2, Ge using a press already available within our group, working in the 0-10 GPa pressure range. The x-ray source will be positioned very near to the sample and WC anvils into the hydraulic press, and a collimator arm will be positioned at a fixed angle. A solid-state Ge detector, and a multi-channel analyzer connected to a computer will provide measurements of the EDXD spectra. Samples of densified amorphous GeO2 and Ge will be produced under different thermodynamic conditions using our large-volume press, to be studied using different techniques in collaboration with the other units.
XAS and SEXAD experiments will be carried out at synchrotron radiation facilities under high-pressure conditions on Ge and Ge-Si samples, depending on the beamtime available that particular year. XAS, SEXAD and diffraction data will be analysed using state-of-the-art data-analysis techniques (see for example GNXAS [1,15-17]) that we have developed or used in our previous extensive research activity.

Expected deliverables of the first year: 1) publication of the main results on scientific journals; 2) communication of the results at conferences and via website; 3) development of a new EDXD x-ray set-up with a W source; 4) development of new sample preparation techniques for high-pressure measurements of undercooled liquids/glasses.

ii) During the second year our plan is to complete accurate measurements of amorphous Ge, GeO2 (and possibly perform new experiments on Si, SiO2) where evidence for pressure-induced modifications, including metallization, have been found in previous works (see [18,19] for a-Ge). The combination of XAS, SEXAD, EDXD, ADXD and optical measurements should help in understanding structural and electronic transitions occurring in the amorphous solid (polyamorphism, see [20-22] for example).
In particular, we plan to install a suitable DAC cell on our 4-circles diffractometer, already equipped with a x-ray collimator. We thus intend to develop the performance of in-house ADXD and EDXD measurements in order to increase the limiting pressures up to about 80 GPa. We have the necessary expertise within our group[23,24] and this will require only the installation of the necessary equipments for gasket hole and indentation and for the measurements of the pressure through the ruby fluorescence.
Using these techniques, we also intend to study the occurrence of stable and metastable solid crystalline phases of Ge [25] and Si occurring in the 0-50 GPa pressure range for which there are no measurements at temperatures above the ambient temperature. The experiments at high pressures will be possibly extended to Si-Ge alloys and SiO2, GeO2 glasses.
The high-pressure high-temperature measurements performed with the Paris-Edinburgh cell will be extended to Ge-Si and Ge-Sn alloys in the impurity limit looking mainly at the liquid and undercooled liquid systems.
Samples of densified amorphous SiO2 and Si will be produced under different thermodynamic conditions using our large-volume press, to be studied using different techniques in collaboration with the other units.
Again, XAS and SEXAD experiments will be carried out at synchrotron radiation facilities under high-pressure conditions on these compounds, depending on the available beamtime. These measurements are intended to provide new information about non-equilibrium states following the effect of impurity atoms for increasing concentration.

Expected deliverables of the second year: 1) publication of the main results on scientific journals; 2) communication of the results at conferences and via website; 3) extension of the DAC XRD high-pressure set-up with the area detector ; 4) protocols for the production/XRD characterization of densified glasses under high pressures.


[1] A. Di Cicco et al., "Disordered matter under extreme conditions: x-ray diffraction, electron spectroscopy and electroresistance measurements", J. Non-Cryst. Solids 352, 4155 (2006); see also A. Di Cicco, A. Trapananti, S. Faggioni, and A. Filipponi, ``Is there icosahedral ordering in liquid and undercooled metals?'', Phys. Rev. Lett. 91, 135505-1 - 135505-4 (2003), highlighted on Phys. Rev. Focus 12, 10 (2003), http://focus.aps.org/story/v12/st10. More information can be obtained by visiting the www site http://gnxas.unicam.it and the relevant links.

[2] Y. Katayama, M. Mezouar, J. P. Iti'e, J. M. Besson, G.Syfosse, P.LeFevre, and A. Di Cicco, ``High-pressure high-temperature XAFS investigation of HgTe'',
J. de Physique IV Colloque C2, 1011 (1997).

[3] A. Di Cicco, ``Phase Transitions in Confined Gallium Droplets'', Phys. Rev. Lett. 81, 2942 (1998).

[4] A. Filipponi, A. Di Cicco, and S. De Panfilis, ``Structure of Undercooled Liquid Pd Probed by X-ray Absorption Spectroscopy'', Phys. Rev. Lett. 83, 560 (1999).

[5] A. Filipponi, M. Borowski, D. T. Bowron, S. Ansell, S. De Panfilis, A. Di Cicco, and J. P. Iti`e, ``An experimental station for advanced research on condensed matter under extreme conditions at the European Synchrotron Radiation Facility - BM29 beamline'' Rev. Sci. Instr. 71, 2422 (2000).

[6] L. Ottaviano, A. Filipponi, and A. Di Cicco, ``Supercooling of liquid metal droplets for x-ray-absorption-spectroscopy investigation.'', Phys. Rev. B 49, 11749 (1994).

[7] A. Di Cicco and A. Filipponi, ``Local g(r) properties in liquids probed by high-temperature EXAFS'', J. Non-Cryst. Solids 205-207, 304 (1996).

[8] A. Di Cicco, A. Trapananti, E. Principi, S. De Panfilis, and A. Filipponi, "Polymorphism and metastable phenomena in liquid tin under pressure", Appl. Phys. Lett. 89, 221912 (2006); E. Principi, M. Minicucci, A. Di Cicco, A. Trapananti, S. De Panfilis, R. Poloni, "Metastable phase diagram of Bi probed by single-energy x-ray absorption detection and angular dispersive x-ray diffraction", Phys. Rev. B 74, 064101 (2006)

[9] Y. Katayama, et al., ``Density measurements of liquids under high pressure and high temperature'', J. Synchrotron Rad. 5, 1023 (1998).

[10] A. San-Miguel, A. Polian, M. Gauthier, and J. P. Itie', ``ZnTe at high-pressure: XAS and x-ray diffraction studies'', Phys. Rev. B 48, 8683 (1993).

[11] O. Schulte and W. B. Holzapfel, ``Effect of pressure on the atomic volume of Ga and Tl up to 68 Gpa'', Phys. Rev. B 55, 8122 (1997).

[12] T. Kenichi, K. Kazuaki, and A. Masao, ``High-pressure bct-fcc phase transition in Ga'', Phys. Rev. B 58, 2482 (1998).

[13] V. V. Brazhkin, S. V. Popova, and R. N. Voloshin, High Press. Res. 15, 267 (1997).

[14] A. Filipponi and A. Di Cicco, ``Development of an oven for x-ray absorption measurements under extremely high temperature conditions.'', Nucl. Instr. and Methods for Phys. Res. B 93, 302 (1994).

[15] A. Filipponi, A. Di Cicco, and C. R. Natoli, ``X-ray absorption spectroscopy and n-body distribution functions in condensed matter (I): theory of the GNXAS data-analysis method.'', Phys. Rev. B 52, 15122 (1995).

[16] A. Filipponi and A. Di Cicco, ``X-ray absorption spectroscopy and n-body distribution functions in condensed matter (II): applications of the GNXAS data-analysis method.'', Phys. Rev. B 52, 15135 (1995).

[17] A. Di Cicco, ``Multiple-edge EXAFS refinement: short-range structure in liquid and crystalline Sn'', Phys. Rev. B 53, 6174 (1996).

[18] K. Tanaka, Phys. Rev. B 43, 4302 (1991).

[19] E. Principi, A. Di Cicco, F. Decremps, A. Polian, S. De Panfilis, and A. Filipponi, Phys. Rev. B 69, 201201 (R) (2004).

[20] P. H. Poole, T. Grande, C. Austen Angell, P. F. McMillan, Science 275, 322 (1997).

[21] P. F. McMillan, Nature 403, 151 (2000).

[22] I. Saika-Voivod, P. H. Poole, and F. Sciortino, Nature 412, 514 (2001).

[23] R. Bini, R. Ballerini, G. Pratesi, and H. J. Jodl, Rev. Sci. Instr. 68, 3154 (1997).

[24] A. Di Cicco, A. Filipponi, J. P. Itie', and A. Polian, Phys. Rev. B, 54, 9086 (1996).

[25] J. Crain, G. J. Ackland, and S. J. Clark, Rep. Prog. Phys. 58, 705 (1995).