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Issue 57, 2020
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Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series

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Abstract

Pyrochlore (A2B2O7) is an important, isometric structure-type because of its large variety of compositions and structural derivatives that are generally related to different disordering mechanisms at various spatial scales. The disordering is key to understanding variations in properties, such as magnetic behavior or ionic conduction. Neutron and X-ray total scattering methods were used to investigate the degree of structural disorder in the Ho2Ti2−xZrxO7 (x = 0.0–2.0, Δx = 0.25) solid solution series as a function of the Zr-content, x. Ordered pyrochlores (Fd[3 with combining macron]m) disorder to defect fluorite (Fm[3 with combining macron]m) via cation and anion disordering. Total scattering experiments with sensitivity to the cation and anion sublattices provide unique insight into the underlying atomic processes. Using simultaneous Rietveld refinement (long-range structure) and small-box refinement PDF analysis (short-range structure), we show that the series undergoes a rapid transformation from pyrochlore to defect fluorite at x ≈ 1.2, while the short-range structure exhibits a linear increase in a local weberite-type phase, C2221, over the entire composition range. Enthalpies of formation from the oxides Image ID:d0ra07118h-t1.gif determined using high temperature oxide melt solution calorimetry support the structural data and provide insight into the effect of local ordering on the energetics of disorder. The measured enthalpies of mixing are negative and are fit by a regular solution parameter of W = −31.8 ± 3.7 kJ mol−1. However, the extensive short-range ordering determined from the structural analysis strongly suggests that the entropies of mixing must be far less positive than implied by the random mixing of a regular solution. We propose a local disordering scheme involving the pyrochlore 48f to 8a site oxygen Frenkel defect that creates 7-coordinated Zr sites contained within local weberite-type coherent nanodomains. Thus, the solid solution is best described as a mixture of two phases, with the weberite-type nanodomains triggering the long-range structural transformation to defect fluorite after accumulation above a critical concentration (50% Ti replaced by Zr).

Graphical abstract: Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series

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Article information


Submitted
19 Aug 2020
Accepted
08 Sep 2020
First published
18 Sep 2020

This article is Open Access

RSC Adv., 2020,10, 34632-34650
Article type
Paper

Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series

D. L. Drey, E. C. O'Quinn, T. Subramani, K. Lilova, G. Baldinozzi, I. M. Gussev, A. F. Fuentes, J. C. Neuefeind, M. Everett, D. Sprouster, A. Navrotsky, R. C. Ewing and M. Lang, RSC Adv., 2020, 10, 34632 DOI: 10.1039/D0RA07118H

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