Structural characterisation of rare earth rich glasses for nuclear waste immobilisation

Abstract

Local environments of rare earth ions are studied in a rare earth rich glass (Glass A wt%: 51.0SiO 2 -8.5B 2 O 3 -12.2Na 2 O-4.3Al 2 O 3 -4.8CaO-3.2ZrO 2 -16.0RE 2 O 3 with RE=Nd or La) developed for radioactive waste immobilisation. The aim is to determine the structural environment of rare earths in this glass according to their concentrations, and to study the influence of the peralkaline or peraluminous character of the glass on these environments. To achieve this objective, two series of glasses were prepared from Glass A. The first one contains variable amounts of rare earth oxide (from 0 to 30 wt% RE 2 O 3 ) and the second one is a series of peralkaline (R>50%) and peraluminous (R<50%) glasses, with R=([Na 2 O]+[CaO])/([Na 2 O]+[CaO] +[Al 2 O 3 ]) at fixed RE 2 O 3 contents (16 wt%). The coupling of characterisation methods such as EXAFS (extended x-ray absorption fine structure) spectroscopy at the neodymium L I I I -edge, optical absorption spectroscopy, Raman spectroscopy and 1 1 B, 2 9 Si and 2 7 Al MAS-NMR, enables discussion of several hypotheses concerning both the nature of rare earth neighbourhoods and the glassy network structure.