Diffraction studies of rare-earth phosphate glasses

Abstract

A series of lanthanide oxides was incorporated in a vitreous phosphate host network. Molar constituents of the glasses were typically (La2O3)(10)(RxOy)(10)(Al2O3)(5)(P2O5)(75). Each glass had a different lanthanide (R atom) from the series: La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er and the values of x and y depended on the valency of the rare-earth atom.Both X-ray and neutron diffraction were employed in examining their structures. The results indicate that the basic PO4 tetrahedral unit remains unaltered with an average P-O distance of 1.54 +/- 0.02 Angstrom and predominant Q(2) linkages to its neighbouring units so as to form a continuous network while accommodating the included lanthanides. In accordance with this model, the average distance of rare-earth (comprising La and a second type of R atom) to oxygen decreased from 2.44 to 2.26 A, a trend to be expected from the lanthanide contraction. The average oxygen coordination around the rare-earth was found to vary in the range of 6-8. With these average parameters, a small (74 atom) hand-built model was made to check the feasibility of constructing a continuous random network. Optical transmission measurements show all these glasses to absorb strongly in the UV region and to have marked absorption resonances in the visible region of 400-1000 nm except for the La, Ce, Eu, Tb containing glasses which have low or negligible absorption in the latter range.