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

Devon L. Drey; Eric C. O'Quinn; Tamilarasan Subramani; Kristina Lilova; Gianguido Baldinozzi; Igor M. Gussev; Antonio F. Fuentes; Joerg C. Neuefeind; Michelle Everett; David Sprouster; Alexandra Navrotsky; Rodney C. Ewing; Maik Lang

doi:10.1039/d0ra07118h

Disorder in Ho₂Ti_2−xZr_xO₇: pyrochlore to defect fluorite solid solution series

Devon L. Drey, Eric C. O'Quinn, Tamilarasan Subramani, Kristina Lilova, Gianguido Baldinozzi, Igor M. Gussev, Antonio F. Fuentes, Joerg C. Neuefeind, Michelle Everett, David Sprouster, Alexandra Navrotsky, Rodney C. Ewing, Maik Lang

Research output: Contribution to journal › Article › peer-review

34 Scopus citations

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 Ho₂Ti_2−xZr_xO₇(x= 0.0-2.0, Δx= 0.25) solid solution series as a function of the Zr-content,x. Ordered pyrochlores (Fd3̄m) disorder to defect fluorite (Fm3̄m)viacation 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 atx≈ 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 etermined 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 ofW= −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).

Original language	English
Pages (from-to)	34632-34650
Number of pages	19
Journal	RSC Advances
Volume	10
Issue number	57
DOIs	https://doi.org/10.1039/d0ra07118h
State	Published - Sep 18 2020

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DESC0020321. This material is based upon work supported under an Integrated University Program Graduate Fellowship (D. L. D). E. C. O. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under contract number DE-SC0014664. The calorimetric work at UC Davis was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Award DE-FG02-03ER46053. A. F. F. thanks Mexican Conacyt, for the nancial support given to this work (Grant No. CB2013-01-221701). The research at ORNL's Spallation Neutron Source was sponsored by the Scientic User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This research used The X-ray Powder Diffraction beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0020321. This material is based upon work supported under an Integrated University Program Graduate Fellowship (D. L. D). E. C. O. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under contract number DE-SC0014664. The calorimetric work at UC Davis was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Award DE-FG02-03ER46053. A. F. F. thanks Mexican Conacyt, for the financial support given to this work (Grant No. CB2013-01-221701). The research at ORNL's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This research used The X-ray Powder Diffraction beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Funders	Funder number
DOE Office of Science
Mexican Conacyt	CB2013-01-221701
Office of Basic Energy Sciences	DE-FG02-03ER46053
Office of Science Graduate Student Research
SCGSR
Scientific User Facilities Division
Scientic User Facilities Division
U. S. Department of Energy
U.S. Department of Energy	DE-SC0014664
Office of Science
Basic Energy Sciences	DESC0020321
Workforce Development for Teachers and Scientists
Oak Ridge National Laboratory
Oak Ridge Institute for Science and Education
Brookhaven National Laboratory	DE-SC0012704

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10.1039/d0ra07118h

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title = "Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series",

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}m) disorder to defect fluorite (Fm{\=3}m)viacation 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 atx≈ 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 etermined 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 ofW= −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).",

author = "Drey, {Devon L.} and O'Quinn, {Eric C.} and Tamilarasan Subramani and Kristina Lilova and Gianguido Baldinozzi and Gussev, {Igor M.} and Fuentes, {Antonio F.} and Neuefeind, {Joerg C.} and Michelle Everett and David Sprouster and Alexandra Navrotsky and Ewing, {Rodney C.} and Maik Lang",

note = "Publisher Copyright: {\textcopyright} The Royal Society of Chemistry 2020.",

year = "2020",

month = sep,

day = "18",

doi = "10.1039/d0ra07118h",

language = "English",

volume = "10",

pages = "34632--34650",

journal = "RSC Advances",

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T1 - Disorder in Ho2Ti2−xZrxO7

T2 - pyrochlore to defect fluorite solid solution series

AU - Drey, Devon L.

AU - O'Quinn, Eric C.

AU - Subramani, Tamilarasan

AU - Lilova, Kristina

AU - Baldinozzi, Gianguido

AU - Gussev, Igor M.

AU - Fuentes, Antonio F.

AU - Neuefeind, Joerg C.

AU - Everett, Michelle

AU - Sprouster, David

AU - Navrotsky, Alexandra

AU - Ewing, Rodney C.

AU - Lang, Maik

PY - 2020/9/18

Y1 - 2020/9/18

N2 - 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 (Fd3̄m) disorder to defect fluorite (Fm3̄m)viacation 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 atx≈ 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 etermined 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 ofW= −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).

AB - 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 (Fd3̄m) disorder to defect fluorite (Fm3̄m)viacation 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 atx≈ 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 etermined 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 ofW= −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).

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JO - RSC Advances

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ER -

Disorder in Ho₂Ti_2−xZr_xO₇: pyrochlore to defect fluorite solid solution series

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