New insights into phase distribution, phase composition and disorder in Y₂(Zr,Sn)₂O₇ ceramics from NMR spectroscopy

Abstract

A combination of (89)Y and (119)Sn NMR spectroscopy and DFT calculations are used to investigate phase evolution, local structure and disorder in Y2Zr2-xSnxO7 ceramics, where a phase change is predicted, from pyrochlore to defect fluorite, with increasing Zr content. The ability of NMR to effectively probe materials that exhibit positional and compositional disorder provides insight into the atomic-scale structure in both ordered and disordered phases and, by exploiting the quantitative nature of the technique, we are able to determine detailed information on the composition of the phase(s) present and the average coordination number (and next-nearest neighbour environment) of the cations. In contrast to previous studies, a more complex picture of the phase variation with composition emerges, with single-phase pyrochlore found only for the Sn end member, and a single defect fluorite phase only for x = 0 to 0.6. A broad two-phase region is observed, from x = 1.8 to 0.8, but the two phases present have very different composition, with a maximum of 13% Zr incorporated into the pyrochlore phase, whereas the composition of the defect fluorite phase varies throughout. Preferential ordering of the anion vacancies in the defect fluorite phase is observed, with Sn only ever found in a six-coordinate environment, while remaining vacancies are shown to be more likely to be associated with Zr than Y. Our findings are then discussed in the light of those from previous studies, many of which utilize diffraction-based approaches, where, in most cases, a single phase of fixed composition has been assumed for the refinement procedure. The significant and surprising differences encountered demonstrate the need for complementary approaches to be considered for a detailed and accurate picture of both the long- and short-range structure of a solid to be achieved.

Fingerprint of local disorder in long range ordered isometric pyrochlores

“…We can now start our discussion with 17 O MAS NMR as it is a proven powerful tool to probe local order and disorder in crystalline pyrochlore 54 . The fully ordered stoichiometric pyrochlore structure possesses only two distinct crystallographic O sites; the presence of more than two O signals on the 17 O MAS NMR spectrum immediately signifies a local disorder at the atomic scale.…”

Unravelling the Mystery of Solid Solutions: A Case Study of 89Y Solid‐State NMR Spectroscopy

“…Incorporating other ions in fluorite‐type solids enables the facile modulation of properties. 89 Y SSNMR spectroscopy has shown great potential in uncovering the atomic‐scale structural information in various fluorite‐type solids …”

“…However, substitution can also induce structural changes, e. g., from pyrochlores to the defect fluorite structure, in which both cations and oxygen vacancies are randomly distributed. 89 Y SSNMR have been extensively used to study the ion substitution on cation sites of yttrium pyrochlores, providing fruitful information on cation ordering and structural transformations …”

“…( e ) Possible arrangements of Sn/Ti on the six next nearest‐neighbour B sites which surround the eight‐coordinated pyrochlore A site of Y 2 Ti 2− x Sn x O 7 . Adapted with permission . Copyright 2015, Royal Society of Chemistry.…”

Investigation of zeolitic imidazolate frameworks using 13 C and 15 N solid-state NMR spectroscopy

“…For nuclear sites that possess high local symmetry, resulting in small anisotropies, "amplification" experiments have also recently been developed [27], increasing the apparent magnitude of the interaction (by a user-defined scaling factor) and enabling more accurate measurement. The experiment used in this work, the CSA-amplified PASS approach [28,29], has been used successfully to investigate 1 H, 13 C, 31 P and 89 Y local environments in a range of materials, and correlations with local structural parameters, e.g., bond distances or point symmetry, demonstrated [29][30][31][32][33]. For ZIFs, solid-state NMR spectroscopy can provide information on the type and relative proportions of linkers present, and the number of crystallographically-distinct molecules present in the asymmetric unit.…”