Structure and thermodynamics of calcium rare earth silicate oxyapatites, Ca2RE8(SiO4)6O2 (RE = Pr, Tb, Ho, Tm)

Andrew C. Strzelecki; Yang Ren; Saehwa Chong; Brian J. Riley; Hongwu Xu; John S. McCloy; Xiaofeng Guo

doi:10.1007/s00269-022-01187-5

Structure and thermodynamics of calcium rare earth silicate oxyapatites, Ca₂RE₈(SiO₄)₆O₂ (RE = Pr, Tb, Ho, Tm)

Andrew C. Strzelecki, Yang Ren, Saehwa Chong, Brian J. Riley, Hongwu Xu, John S. McCloy, Xiaofeng Guo^*

^*Corresponding author for this work

Department of Physics

Research output: Journal Publications and Reviews › RGC 21 - Publication in refereed journal › peer-review

20 Citations (Scopus)

Abstract

Calcium rare earth silicate oxyapatites, (Ca₂RE₈(SiO₄)₆O₂), are of interest as components of glass-ceramic nuclear waste forms. To assess their long-term behavior in a geologic repository, it is essential to determine their structure and thermodynamic stability at relevant conditions. In this work, we performed detailed structural and thermodynamic investigations on Ca₂Pr₈(SiO₄)₆O₂, Ca₂Tb₈(SiO₄)₆O₂, Ca₂Ho₈(SiO₄)₆O₂, and Ca₂Tm₈(SiO₄)₆O₂ by high energy synchrotron powder X-ray diffraction combined with Rietveld analysis and high temperature oxide melt drop solution calorimetry. Enthalpies of formation from constituent oxides (ΔH_f,ox) were determined to be - 765.1 ± 22.8 kJ/mol for Ca₂Pr₈(SiO₄)₆O₂; - 638.9 ± 20.5 kJ/mol for Ca₂Tb₈(SiO₄)₆O₂; - 643.3 ± 10.3 kJ/mol for Ca₂Ho₈(SiO₄)₆O₂; and - 403.2 ± 5.1 kJ/mol for Ca₂Tm₈(SiO₄)₆O₂. These thermodynamic parameters were used in assessing the thermochemical stability of these phases in the presence of water vapor from room temperature to 600 K, as encountered in the subsurface environments of a geological repository.

Original language	English
Article number	13
Journal	Physics and Chemistry of Minerals
Volume	49
Issue number	5
Online published	3 May 2022
DOIs	https://doi.org/10.1007/s00269-022-01187-5
Publication status	Published - May 2022

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Research Keywords

Calcium rare earth silicate oxyapatites
Crystal structure
Enthalpy of formation
Ceramic waste forms
Lanthanide geochemistry
NUCLEAR-WASTE FORMS
CRYSTAL-STRUCTURE
APATITE
ENTHALPIES
ND
SUBSTITUTION
CALORIMETRY
BRITHOLITE
MECHANISMS
POLYHALITE

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@article{d9b63eff8a624981b92e47416fa8949f,

title = "Structure and thermodynamics of calcium rare earth silicate oxyapatites, Ca2RE8(SiO4)6O2 (RE = Pr, Tb, Ho, Tm)",

abstract = "Calcium rare earth silicate oxyapatites, (Ca2RE8(SiO4)6O2), are of interest as components of glass-ceramic nuclear waste forms. To assess their long-term behavior in a geologic repository, it is essential to determine their structure and thermodynamic stability at relevant conditions. In this work, we performed detailed structural and thermodynamic investigations on Ca2Pr8(SiO4)6O2, Ca2Tb8(SiO4)6O2, Ca2Ho8(SiO4)6O2, and Ca2Tm8(SiO4)6O2 by high energy synchrotron powder X-ray diffraction combined with Rietveld analysis and high temperature oxide melt drop solution calorimetry. Enthalpies of formation from constituent oxides (ΔHf,ox) were determined to be - 765.1 ± 22.8 kJ/mol for Ca2Pr8(SiO4)6O2; - 638.9 ± 20.5 kJ/mol for Ca2Tb8(SiO4)6O2; - 643.3 ± 10.3 kJ/mol for Ca2Ho8(SiO4)6O2; and - 403.2 ± 5.1 kJ/mol for Ca2Tm8(SiO4)6O2. These thermodynamic parameters were used in assessing the thermochemical stability of these phases in the presence of water vapor from room temperature to 600 K, as encountered in the subsurface environments of a geological repository.",

keywords = "Calcium rare earth silicate oxyapatites, Crystal structure, Enthalpy of formation, Ceramic waste forms, Lanthanide geochemistry, NUCLEAR-WASTE FORMS, CRYSTAL-STRUCTURE, APATITE, ENTHALPIES, ND, SUBSTITUTION, CALORIMETRY, BRITHOLITE, MECHANISMS, POLYHALITE",

author = "Strzelecki, {Andrew C.} and Yang Ren and Saehwa Chong and Riley, {Brian J.} and Hongwu Xu and McCloy, {John S.} and Xiaofeng Guo",

year = "2022",

month = may,

doi = "10.1007/s00269-022-01187-5",

language = "English",

volume = "49",

journal = "Physics and Chemistry of Minerals",

issn = "0342-1791",

publisher = "Springer Verlag",

number = "5",

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TY - JOUR

T1 - Structure and thermodynamics of calcium rare earth silicate oxyapatites, Ca2RE8(SiO4)6O2 (RE = Pr, Tb, Ho, Tm)

AU - Strzelecki, Andrew C.

AU - Ren, Yang

AU - Chong, Saehwa

AU - Riley, Brian J.

AU - Xu, Hongwu

AU - McCloy, John S.

AU - Guo, Xiaofeng

PY - 2022/5

Y1 - 2022/5

N2 - Calcium rare earth silicate oxyapatites, (Ca2RE8(SiO4)6O2), are of interest as components of glass-ceramic nuclear waste forms. To assess their long-term behavior in a geologic repository, it is essential to determine their structure and thermodynamic stability at relevant conditions. In this work, we performed detailed structural and thermodynamic investigations on Ca2Pr8(SiO4)6O2, Ca2Tb8(SiO4)6O2, Ca2Ho8(SiO4)6O2, and Ca2Tm8(SiO4)6O2 by high energy synchrotron powder X-ray diffraction combined with Rietveld analysis and high temperature oxide melt drop solution calorimetry. Enthalpies of formation from constituent oxides (ΔHf,ox) were determined to be - 765.1 ± 22.8 kJ/mol for Ca2Pr8(SiO4)6O2; - 638.9 ± 20.5 kJ/mol for Ca2Tb8(SiO4)6O2; - 643.3 ± 10.3 kJ/mol for Ca2Ho8(SiO4)6O2; and - 403.2 ± 5.1 kJ/mol for Ca2Tm8(SiO4)6O2. These thermodynamic parameters were used in assessing the thermochemical stability of these phases in the presence of water vapor from room temperature to 600 K, as encountered in the subsurface environments of a geological repository.

AB - Calcium rare earth silicate oxyapatites, (Ca2RE8(SiO4)6O2), are of interest as components of glass-ceramic nuclear waste forms. To assess their long-term behavior in a geologic repository, it is essential to determine their structure and thermodynamic stability at relevant conditions. In this work, we performed detailed structural and thermodynamic investigations on Ca2Pr8(SiO4)6O2, Ca2Tb8(SiO4)6O2, Ca2Ho8(SiO4)6O2, and Ca2Tm8(SiO4)6O2 by high energy synchrotron powder X-ray diffraction combined with Rietveld analysis and high temperature oxide melt drop solution calorimetry. Enthalpies of formation from constituent oxides (ΔHf,ox) were determined to be - 765.1 ± 22.8 kJ/mol for Ca2Pr8(SiO4)6O2; - 638.9 ± 20.5 kJ/mol for Ca2Tb8(SiO4)6O2; - 643.3 ± 10.3 kJ/mol for Ca2Ho8(SiO4)6O2; and - 403.2 ± 5.1 kJ/mol for Ca2Tm8(SiO4)6O2. These thermodynamic parameters were used in assessing the thermochemical stability of these phases in the presence of water vapor from room temperature to 600 K, as encountered in the subsurface environments of a geological repository.

KW - Calcium rare earth silicate oxyapatites

KW - Crystal structure

KW - Enthalpy of formation

KW - Ceramic waste forms

KW - Lanthanide geochemistry

KW - NUCLEAR-WASTE FORMS

KW - CRYSTAL-STRUCTURE

KW - APATITE

KW - ENTHALPIES

KW - ND

KW - SUBSTITUTION

KW - CALORIMETRY

KW - BRITHOLITE

KW - MECHANISMS

KW - POLYHALITE

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