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Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P134
Rare-earth site distribution in R(PO3)3 (R=La, Nd, Er, Yb) glasses by
Reverse Monte Carlo simulations
Uwe Hoppe 1
1 Universität Rostock, Institut für Physik, 18051 Rostock
(R2O3)x(P2O5)1−x glasses with x=0.25 possess comparably great fractions of rareearth
ions R 3+ . Determination of their mutual order is important to understand special
glass properties and is also a challenging work because the relevant distances are already
outside of the short-range order. Results of several approaches based on neutron
diffraction are presented in the literature. On the other hand, the R-R correlations
possess most weight in x-ray scattering data but extraction of the desired information
is difficult. The approach used is a combination of total x-ray and neutron diffraction
data by means of the Reverse Monte Carlo (RMC) method. Results of glasses with
four different R 3+ ions are presented where effects of the decrease of the ionic radius
in the order La, Nd, Er, and Yb become visible.
The RMC method generates three-dimensional atomic configurations which reflect
main features of the short-range order of the metaphosphate glasses due to use of
reasonable constraints in addition to the total x-ray and neutron structure factors.
Twofold corner-connected PO4 tetrahedra forming chain and cyclic structures surround
the R 3+ sites. The terminal oxygen corners of the PO4 tetrahedra coordinate the rareearth
ions. The main results for further analysis are the R-R partial structure factors
and pair distribution functions. The assumption of a random packing of R-centered
spheres with minimum separation of 0.63 nm reproduces the first sharp diffraction
peak. But for reproducing the detailed features of the R-R pair distributions the
description of the medium-range order of the R sites needs a more specific model.
Starting form the structure of the monoclinic crystal Yb(PO3)3 a hexagonal order of
R sites is suggested (c/a=0.87). This model yields broad R-R first-neighbour peaks (8
neighbours) and the second-neighbour peak lies in the right position (12 neighbours).
In addition to this model some short R-R distances (0.45 nm) occur. This peak reflects
R sites which share terminal oxygen neighbours. This effect is very small for glasses
with R=Yb but it increase with increasing radii of the rare-earth ions. The number of
neighbours correlates excellently with the R-O coordination numbers and the numbers
of the terminal oxygens which is available for coordination of an R site.