Structural characterization of Nd-doped calcium aluminosilicate glasses designed for the preparation of zirconolite (CaZrTi2O7) -based glass-ceramic

TL;DR

This study investigates the atomic structure of Nd-doped calcium aluminosilicate glasses to understand their crystallization into zirconolite, aiming to develop nuclear waste forms with controlled properties.

Contribution

It provides detailed structural insights into the local environments of Ti, Zr, and Nd in glasses designed for zirconolite crystallization, highlighting differences from the crystalline phase.

Findings

Zr occupies a well-defined 6-7-fold oxygen-coordinated site.

Nd is in a highly distorted 8-9-fold environment with strong Nd-Nd interactions.

The short-range order around Zr resembles zirconolite, indicating potential for nucleation.

Abstract

This work concerns glasses belonging to SiO2-Al2O3-CaO-ZrO2-TiO2-Nd2O3 system that lead to zirconolite crystallization in their bulk after nucleation + crystal growth thermal treatments and that could find application as nuclear waste form. The understanding of crystallization processes in glasses implied to investigate their structure. The environment around Ti, Zr (nucleating agents) and Nd was characterized for various Nd2O3 loadings. Electron spin resonance study of the small amount of Ti3+ occurring in glasses enabled to identify two types of sites for titanium. EXAFS showed that Zr occupied a quite well defined 6-7-fold site coordinated by oxygen at 2.20 {\AA}, with second neighbors that could correspond to Ca/Ti and Zr around 3.5 {\AA}. This short range order presents some similarities with zirconolite, which could predispose glass to zirconolite nucleation. Nd environment was probed by optical spectroscopies, ESR and EXAFS. Results showed that the environment around Nd was very constrained by the glassy network and significantly differed from the one in zirconolite. Nd occupied a highly distorted 8-9-fold coordinated site in glass with oxygen atoms at 2.53 {\AA}. No second neighbors were clearly identified by EXAFS around Nd, but the study of Nd optical fluorescence decays suggested a strong interaction between Nd ions.