ANNUAL REPORT - MTA SzFKI

defined coordination sphere was found, particularly at higher concentrations. The
orientation of water molecules around the cations may be characterized by a broad
distribution of Rb . . .O–H angles, peaking around the tetrahedral angle. This indicates that
cations have a weak tendency to occupy a position close to one of the lone electron pairs
of the O atoms. It was also found that in the rather vaguely defined hydration shell of
chloride ions, six (or somewhat more) water molecules may surround each anion.
However, on average, only about three of them point straight towards the anion with one
of their H atoms, whereas all other hydrogens are turned away from the ion. The six
neighbouring water molecules of the hydration shell can thus be separated into about three
hydrogen-bonding and three (or four) non-H-bonding water molecules.
Chalcogenide glasses. — The structure of sputtered amorphous Ge 2 Sb 2 Te 5 was studied by
Te-, Sb- and Ge K-edge EXAFS, X-ray diffraction and, for the first time, neutron
diffraction. The five datasets were modelled simultaneously by the reverse Monte Carlo
simulation technique. The results obtained are thus consistent with all of the five
measurements. Within the experimental errors the coordination number of Te, Sb and Ge
is 2, 3 and 4, respectively. Thus, in contrast with the findings of some recent works on
amorphous Ge 2 Sb 2 Te 5 all atoms satisfy the 8-N rule. Besides Te-Ge and Te-Sb bonds
present in the crystalline phases Ge-Ge and Sb-Ge bonding was also found to be
significant.
The atomic structure of the binary AsSe, ternary (AsSe) 80 Ag 20 , (AsSe) 85 I 15 and quaternary
(AsSe) 65 (AgI) 35 glasses has been studied with the X-ray and neutron diffraction. The local
order was also probed with EXAFS at the Ag, As, I and Se K-edges. All experimental data
were modelled simultaneously by the reverse Monte Carlo technique. Ag and I modify the
structure of the host matrix (AsSe) in opposite ways. Ag atoms are preferentially
covalently bonded to Se. Addition of I to AsSe decreases the connectivity of the matrix.
The average coordination number in amorphous (AsSe) 85 I 15 = 2.18 ± 0.2. I atoms are
covalently bonded to As atoms.
Metallic glasses. — Short range order of amorphous Mg 60 Cu 30 Y 10 was investigated by X-
ray and neutron diffraction, Cu and Y K-edge X-ray absorption fine structure
measurements and by the reverse Monte Carlo simulation technique. We found that Mg-
Mg and Mg-Cu nearest neighbour distances are very similar to values found in crystalline
Mg 2 Cu. The Cu-Y coordination number is 1.1±0.2 and the Cu-Y distance is ~4% shorter
than the sum of atomic radii suggesting that attraction between Cu and Y plays an
important role in stabilizing the glassy state. Thermal stability and structure evolution
upon annealing were also studied by differential scanning calorimetry and in-situ X-ray
powder diffraction. The alloy shows a glass transition and three crystallization events, the
first and dominant one at 456 K corresponding to eutectic crystallization of at least three
phases: Mg 2 Cu and most likely cubic MgY and CuMgY.
Borosilicate glasses. — The network structure of multi-component borosilicate based
waste glasses with composition (65-x)SiO 2 .·xB 2 O 3·25Na 2 O·5BaO·5ZrO 2 , x=5-15mol% (host
glass) was studied by high-Q neutron diffraction. For data analyses both the direct sine-
Fourier transformation and reverse Monte Carlo simulation was applied. Several atomic
partial correlation functions are displayed in Figure 2. It was established that the Si-O
network consists from tetrahedral SiO 4 units with characteristic first neighbour distances r Si-
O=1.60 Å and r Si-Si =3.0 Å. The boron surrounding contains two well-resolved B-O distances
at 1.40 and 1.60 Å and, both 3- and 4-fold coordination are present. A chemically mixed
network structure is proposed including [4] B-O- [4] Si and [3] B-O- [4] Si chain segments. The O-O
and Na-O distributions suggest partial segregation of silicon and boron rich regions. The
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