Gold ruby glass doped with lanthanide oxides obtained by gamma irradiation

Andreia Ruivo 1,2,*, Carlos Queiroz 1, Joaquim Marçalo 3, Luísa Botelho 3 and António Pires de Matos 1,3 1 Unidade de Investigação VICARTE - Vidro e Cerâmica para as Artes, FCT/UNL, Campus de Caparica, 2829-516 Caparica, Portugal 2 REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte da Caparica, Portugal 3 Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal

The colour of red glasses with gold depends on the reducing conditions as well as on the striking temperature which determines the colloidal aggregation. In this work several gold ruby glasses were made by irradiating with gamma rays and further heating soda-lime silicate glasses containing different concentrations of gold, instead of using a reducing agent such as stannous oxide. The most intense red colour was obtained with 0.2% (all concentrations in w/w) of gold, with a heat treatment at 700 ºC during 120 minutes. Having in mind the preparation of luminescent gold ruby glass with different colours under UV light for application in artworks, cerium, europium and samarium oxides (CeO2, Eu2O3, Sm2O3) were added in several concentrations to the parent soda-lime silicate glass which had the following composition: 74% SiO2, 16% Na2O, 10% CaO, with added 0.2% HAuCl4. The samples were subjected to gamma irradiation from a 60Co facility (100 kGy) and further heated. The red colour could be achieved with the addition of any of the three lanthanide oxides. However, with Eu2O3, it was verified that when its concentration reached 6% the red colour was obtained just by heating, thus not recurring to gamma irradiation. With Sm2O3 the concentration needed to obtain the red colour without irradiation was 2%. Using CeO2 the results were peculiar because above 4% of the lanthanide oxide no red colour was obtained after heating the irradiated glasses. Probably CeO2 is quenching the formation of the defects produced by gamma radiation and/or at that concentration inhibits the reduction of gold. It is well known that CeO2 is added to lead glass used in the nuclear industry to avoid darkening when subjected to intense gamma radiation. The results obtained seem compatible with the standard reduction potentials in solution (E0/V referred to the standard hydrogen electrode) of Sm3+/Sm2+ (-1.55), Eu3+/Eu2+ (-0.35) and Ce4+/Ce3+ (+1.76) if an extrapolation might be insightful. For example: for values listed for Ag+/Ag0 (+0.7996 V), Cu+/Cu0 (+0.521 V) and Au+/Au0 (+1.68 V); no reduction to metals is expected by the addition of cerium. However, this is a rough extrapolation as we should consider the reduction potentials in glass melts. To our best knowledge, the only values published are: for the Ce4+/Ce3+ pair in soda-lime silicate glass, +0.08 (E0/V vs. ZrO2/pO2 = 0.21 bar)1; for Eu3+/Eu2+ in alkali borosilicate glass, -0.8 (E0/V at 1150 ºC and pO2 = 0.21 bar)2 and; for Sm3+/Sm2+ in molten LiCl+KCl eutectic, -2.0 (E0/V vs. Cl-/Cl2 reference system at 723 K)3. The order of the reduction potentials listed is consistent with the reduction potentials in solution indicated above. The effect of the investigated lanthanide oxides on the reduction of the gold ions to metal is discussed considering pertinent redox equilibria and the Ellingham diagrams for the lanthanide oxides. References [1] C. Rüssel, Redox Behavior and Electrochemical Behavior of Glass Melts, in Propeties of Glass-Forming Melts, Edited by L. D. Pye, A. Montenero and I. Joseph, Taylor and Francis, 2005, 27-55. [2] M. W. Medlin, K. D. Sienerth and H. D. Schreiber, J. Non-Crystalline Solids, 1998, 240, 193-201. [3] G. Gordoba, C. Caravaca, J. Electroanal. Chem., 2004, 572, 36-41.