Estrutura de vidros fosfatos ternários no sistema Pb-Al metafosfato
Abstract
In this work, the Pb-Al meta-phosphate glass system (1-x) Pb(PO3)2 xAl(PO3)3
was studied, with the aim of determining the structural effects induced in the glass network by
the substitution of Pb by Al. In the series of glasses encompassing the range 0 ≤ x ≤ 1 the
composition meta [O/P] = 3 was maintained. In this way, the resulting phosphate network will
be composed mainly by phosphate tetrahedra in condensation Q2 (chains or rings), simplifying
the formulation of structural models. The structure of this system was analyzed trough a
combination of Raman spectroscopy and Nuclear Magnetic Resonance (NMR). The structural
environments around the network former and modifiers species were analyzed by 31P, 27Al and
207Pb-NMR, quantifying the distribution of phosphate groups and determining the coordination
numbers of Al and Pb. For phosphate groups, several Q2 species were identified according to
the number of P-O-Pb and P-O-Al bonds established by each tetrahedron. Through the
quantitative analysis of the evolution of the population of these species with the concentration
of Al, it was possible to infer specific characteristics of organization of the Al coordination
polyhedra and phosphate tetrahedra. Phosphates share preferentially only a single corner with an
Al-O polyhedron, which is found with high coordination number, and the other corner with a
Pb-polyhedron. As x is increased, this medium range arrangement around Al is maintained
through the progressive decrease of the Al coordination number, up to a certain critical
concentration (x = 0.43). Above this concentration, the arrangement is not sustained and
phosphates sharing two corners with Al-polyhedrons appear. On the other hand, the
environments around Pb have high coordination numbers and a considerable degree of cornersharing
between Pb-polyhedra was determined. The dependence of Pb coordination number
with composition is determined by the quantity of available Onp, without apparent modification
in the degree of O-sharing. The behavior observed for the local order around the Al and the
arrangement of coordination polyhedra is similar to the observed in tow other meta-phosphates
(Ca-Al and Na-Al), pointing out to a possible general trend for ternary phosphate glasses.