Andrea Di Cicco and Marco Minicucci
INFM, Dipartimento di Matematica e Fisica, Universita' di Camerino, via Madonna delle Carceri, 62032 Camerino (MC), Italy. URL: http://gnxas.unicam.it.
E-mail: dicicco@unixas.unicam.it
Beamlines: D42 (EXAFS 1) and D44 (EXAFS 2), DCI.
X-ray absorption measurements of solid and liquid CuBr in the T = 295 - 793 K range of temperature have been performed in the framework of a project concerning the determination of local two-body and three-body correlations in liquids.
Accurate short-range structural parameters were extracted using simultaneous and Cu and Br K-edge EXAFS refinement [1] in the framework of the multiple-scattering GNXAS method [2] for data-analysis. High-temperature measurements were collected using a furnace developed by us and now available at LURE. [3]
In solid CuBr we find an increasing asymmetry of the first-neighbour gCuBr(r) distribution function as a function of temperature.
The foot and the most probable value of the first-neighbour distribution are found to shift up to about 0.09 Angstroems toward shorter distances from 295 K to 670 K, while the long-range part of distribution broadens at high temperatures.
Results are discussed in ref. [4] taking into account previous neutron diffraction data. A complete account of our EXAFS experiment on liquid CuBr is reported in [5] and [4]. The short-range Cu-Br distribution is extracted showing evidence for nearly covalent bonding and improving previous neutron diffraction results. On the contrary, the local structure of molten salts like KBr [6] and RbBr [5] is in excellent agreement with models provided by molecular-dynamics simulation using simple ionic potentials.
The unique short-range EXAFS sensitivity and its complementarity with usual diffraction techniques for studying disordered systems are discussed in ref. [4-6].
[1] A. Di Cicco, ``Multiple-edge EXAFS refinement: short-range structure in liquid and crystalline Sn'', Phys. Rev. B 53, 6174 (1996).
[2] 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).
[3] A. Filipponi and A. Di Cicco, ``Development of an oven for x-ray absorption measurements under extremely high temperature conditions.'', Nucl. Instr. & Methods for Phys. Res. B 93, 302 (1994).
[4] M. Minicucci and A. Di Cicco, ``Short-range structure in solid and liquid CuBr probed by multiple-edge X-ray Absorption Spectroscopy.'' Phys. Rev. B 56, 11456 (1997).
[5] A. Di Cicco, M. Minicucci, and A. Filipponi, ``New advances in the study of local structure of molten binary salts'', Phys. Rev. Lett. 78, 460 (1997).
[6] A. Di Cicco, M. J. Rosolen, R. Tossici, R. Marassi, A. Filipponi, and J. Rybicki, ``Short-range Order in Solid and Liquid KBr Probed by EXAFS'', J. Phys.: Condens. Matter 8, 10779 (1996).