The structure and ordering of zirconium and hafnium containing garnets studied by electron channelling, neutron diffraction and Mössbauer spectroscopy

Garnets, A3B2C3O12, are an important group of minerals and have potential uses in the safe immobilisation of high-level nuclear waste. They have been found naturally to incorporate Zr, Ti and Fe, three elements of interest in the safe storage of nuclear waste. Kimzeyite, Ca3(Zr,Ti)2(Si,Al,Fe)3O12, is a naturally occurring garnet that contains Zr in a high percentage∼30 wt%. For such a material to be of potential immobilisation for nuclear waste the structure needs to be completely understood. Electron channelling studies have shown that the Zr/Ti cations are located on the Y-site, with the Al/Fe cations located on the Z-site. This work has investigated synthetic analogues of kimzeyite, Ca3(Zr,Hf)2(Al,Fe,Si)3O12, by neutron powder diffraction, using the C2 spectrometer at the Chalk River nuclear facility, coupled with 57Fe Mössbauer spectroscopy. Such work has allowed the structure of the synthetic material to be determined along with the distribution of cations across the X (CN=8), Y (CN=6), and Z (CN=4) sites. Results have shown that it is possible to synthesise Ca3(Zr,Hf)2(Al,Fe,Si)3O12 with a range of Al/Fe ratios containing Zr and Hf. The Mössbauer data has indicated the Fe is located on the Z site. The structural analyses show that the unit cell changes linearly as a function of composition, and analysis of the disorder indicates that the Zr, Hf reside on the Y site and the Al, Fe, and Si reside on the Z site.