ANNUAL REPORT - MTA SzFKI

K. NEUTRON SCATTERING
L. Pusztai, M. Fábián # , P. Jóvári, I. Harsányi, L. Kőszegi, Gy. Mészáros, V. Mile # , Sz.
Pothoczki # , E. Sváb, L. Temleitner
Liquids. — Neutron and X-ray diffraction results obtained for liquid carbon dioxide, CO 2 ,
have been modelled by means of the reverse Monte Carlo method. Partial pair correlation
functions, centre–centre pair correlation functions and relative orientations of molecular
axes as a function of distances between molecular centres have been calculated from the
models. It was found that well defined orientational correlations exist in the liquid state.
Close to the critical point, these correlations seem to disappear, but further away from the
critical point, in the high density supercritical state, they reappear. Using large particle
configurations, the density fluctuations close to the critical point could be visualized, as
demonstrated by Figure 1.
(a)
18
g Si-O
(r)
12
6
Si - O
(b)
6
4
2
g O-O
(r)
O-O
(c)
6
g Si-Si
(r)
4
2
Si - Si
0
0 1 2 3 4 5 6
r(Å)
0
1 2 3 4 5 6
r(Å)
0
2 3 4 5 6
r(Å)
g Na-O
(r)
(d)
3 Na - O
2
1
0
1 2 3 4 5 6
The coherent static structure factor of water has been investigated by polarized neutron
diffraction. Polarization analysis allows us to separate the huge incoherent scattering
background from hydrogen and to obtain high quality data of the coherent scattering from
four different mixtures of liquid H 2 O and D 2 O. Consistency with existing partial radial
distribution functions, derived without the application of polarized neutrons, was checked
by incorporating them into our reverse Monte Carlo calculations. It appeared that the
partials from the literature are not entirely consistent with our new polarised neutron data.
Reverse Monte Carlo calculations have been carried out to model the structure of aqueous
RbCl solutions, as a function of salt concentration. Neutron and X-ray diffraction data
were combined with various constraints concerning the coordination of chloride anions. It
could be established that increasing ionic concentration causes increasing distortions to the
hydrogen bonded network of water molecules—that is, RbCl is a ‘structure breaker’,
although to a smaller extent than, for instance, LiCl. For the Rb cations a quite well
# Ph.D. student
r(Å)
(e)
3
g Si-Na
(r)
2
1
Si - Na
0
2 3 4 5 6
r(Å)
(f)
4
3
2
1
Na - Na
0
2 3 4 5 6
Fig. 1. Two-dimensional projections of atomic density maps for gaseous (g), near-critical and high density
supercritical fluid (sc) and normal liquid (l) CO 2 . The pictures were taken on 3–4 Å thick layers
(depending on the edge length of the simulation cell), containing roughly 240 molecules.
g Na-Na
(r)
r(Å)
51