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

Abstract

A NMR crystallographic approach, combining 89 Y, 119 Sn and 17 O 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 Y 2 Hf 2-x Sn x O 7 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 co-existing 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 Y 2 Sn 2 O 7 , 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 17 O NMR spectra samples were postsynthetically exchanged with 17 O 2 (g), although high temperatures (> 900 ºC) were required to ensure uniform enrichment of different chemical species. Although these 17 O 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 multi-phase materials.

Improved Understanding of Atomic Ordering in Y4SixAl2–xO9–xNx Materials Using a Combined Solid-State NMR and Computational Approach

“…Details of the isotropic chemical shift referencing are given in the Supporting Information, section S4. For 27 Al, 17 O, 29 Si, and 15 N there is a high degree of correlation ( R 2 ≈ 1), comparable with that obtained in the literature. − For 89 Y, our correlation is not quite as good; a higher degree of correlation has been achieved in the literature using a broader data set, but is sufficient for accurate assignment. , Structural parameters, atomic coordinates and unit cell parameters were obtained from experimental crystal structures in the literature. ,, Prior to the calculation of NMR parameters, geometry optimization of the models was performed by allowing the atomic coordinates to relax within either a fixed unit cell or the optimized position in a fully relaxed cell.…”

Effects of Lattice Evolution and Ordering on the Microwave Dielectric Properties of Tin-Modified Li3Mg2NbO6-Based Ceramics

“…The driving force of the phase transition induced by the compositional changes might be related to the Coulombic destabilization and lattice distortions. 18,19 However, there was scarcely any study focused on the relationship between the ordering and dielectric properties of the rock-salt ceramics, even though the cation ordering played a vital role in the quality factors of the ceramics with other structures, such as in perovskites. 20 In general, the AX (A = cation, X = anion) type of the rock-salt structure displays octahedral coordination for both cations and anions.…”

“…Moreover, the studies conducted by Bian et al also revealed the phenomena of solid solution formation and dielectric propertie enhancement in Li (3–3 x ) Mg 4 x Nb (1– x ) O 4 and Li 3 NbO 4 –Li 2 SnO 3 rock-salt systems. , The studies mentioned above indicated that the complex substitutions of the nonequivalent ions in some of the rock-salt structures could result in phase transitions in a solid solution system, and a range of excellent dielectric properties might be generated by the ordering transition and structural changes. The driving force of the phase transition induced by the compositional changes might be related to the Coulombic destabilization and lattice distortions. , However, there was scarcely any study focused on the relationship between the ordering and dielectric properties of the rock-salt ceramics, even though the cation ordering played a vital role in the quality factors of the ceramics with other structures, such as in perovskites . In general, the AX (A = cation, X = anion) type of the rock-salt structure displays octahedral coordination for both cations and anions.…”

Lattice Disorder and Oxygen Migration Pathways in Pyrochlore and Defect-Fluorite Oxides

“…However, this does not rule out the possibility of additional long-range ordering effects, including periodic modulations, which would merit further investigation. Further developments in RMC methods and the inclusion of other local structure datasets, such as extended X-ray absorption fine-edge structure, NMR, or diffuse scattering, will permit more complex modelling to deepen our understanding of defect-fluorite materials and identify rational strategies to tune their properties for catalysis, fast-ion conduction, and nuclear waste storage applications.…”

“…Many studies have addressed the local structures of pyrochlore and defect-fluorite oxides. − Spectroscopic techniques, such as nuclear magnetic resonance (NMR) and X-ray absorption near-edge structure, have provided insights into the disorder and cation coordination numbers (CNs). ,,,,,− However, these techniques are element-specific and only sensitive to average local structure information, so they can miss important larger-scale aspects of how different local structure domains interact with each other. The neutron pair distribution function (PDF) analysis is an excellent tool for studying the local structure of these materials as it provides a distribution of the local-scale information and it is sensitive to features in both the cation and anion sub-lattices. , A recent study, using the neutron PDF measured under ambient conditions, found the local structure of the defect-fluorite to be weberite-type .…”