Phase Distribution, Composition, and Disorder in Y₂(Hf,Sn)₂O₇ Ceramics: Insights from Solid-State NMR Spectroscopy and First-Principles Calculations

A NMR crystallographic approach, combining ⁸⁹Y, ¹¹⁹Sn and ¹⁷O NMR spectroscopy with X-ray diffraction and first-principles calculations has been used investigate the number and type of phases present, and the local structure and disorder in Y₂Hf_2–xSn_xO₇ ceramics. Although a phase change is predicted with increasing Hf content, NMR spectra clearly show the presence of a significant two-phase region, with a Sn-rich pyrochlore and relatively Hf-rich defect fluorite phase coexisting for much of the compositional series. A single-phase pyrochlore is found only for the Sn end member, and a single defect fluorite phase only for x = 0 to 0.2. A solid-solution limit of ∼10% is seen for the substitution of Hf into Y₂Sn₂O₇, although no evidence is seen for any cation ordering or antisite disorder in this phase. In the defect fluorite phase there is preferential ordering of oxygen vacancies around Sn, which is only ever seen in a six-coordinate environment. The remaining vacancies are more likely to be associated with Hf than with Y, although this distinction is less apparent at higher Sn concentrations. To acquire ¹⁷O NMR spectra samples were postsynthetically exchanged with ¹⁷O₂(g), although high temperatures (>900 °C) were required to ensure uniform enrichment of different chemical species. Although these ¹⁷O NMR spectra confirm the formation of mixed-metal materials and the presence of two phases, more quantitative analysis is hindered by the overlap of signals from pyrochlore and defect fluorite phases. In all cases, DFT calculations play a vital role in the interpretation and assignment of the NMR spectra, and in understanding the local structure and disorder in these complex multiphase materials.