Polymorphism and crystal chemistry of Li2.5Ga0.5GeO4, an Li3PO4 analogue

Alastair D. Robertson, Anthony R. West

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

The structures of γ- and β-Li2.5Ga0.5GeO4 have been determined using Rietveld refinement of X-ray powder diffraction data. They are stoichiometric derivatives of γ- and β-Li3PO4. The arrangements of GeO4/PO4 tetrahedra are the same in each case, but the Li sites, especially Li(2), contain a disordered mixture of Li and Ga in γ- and β-Li2.5Ga0.5GeO4. Such cation disorder is unusual in a stoichiometric phase. The γ phase may be regarded as entropy-stabilised since it exists under equilibrium conditions only at high temperatures, ca. 1200°C, just below melting: the size mismatch between Li+ and Ga3+ ions and the likely unfavourable enthalpy of mixing associated with Li/Ga disorder is offset by the large entropy contribution to the Gibbs energy at high temperatures. A discussion of possible mechanisms for the β ⇋ γ transition is given.

Original languageEnglish
Pages (from-to)457-462
Number of pages6
JournalJournal of Materials Chemistry
Volume4
Issue number3
DOIs
Publication statusPublished - 1994
Externally publishedYes

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Crystal chemistry
Polymorphism
Entropy
Rietveld refinement
Gibbs free energy
X ray powder diffraction
Cations
Enthalpy
Melting
Positive ions
Ions
Derivatives
Temperature

Cite this

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title = "Polymorphism and crystal chemistry of Li2.5Ga0.5GeO4, an Li3PO4 analogue",
abstract = "The structures of γ- and β-Li2.5Ga0.5GeO4 have been determined using Rietveld refinement of X-ray powder diffraction data. They are stoichiometric derivatives of γ- and β-Li3PO4. The arrangements of GeO4/PO4 tetrahedra are the same in each case, but the Li sites, especially Li(2), contain a disordered mixture of Li and Ga in γ- and β-Li2.5Ga0.5GeO4. Such cation disorder is unusual in a stoichiometric phase. The γ phase may be regarded as entropy-stabilised since it exists under equilibrium conditions only at high temperatures, ca. 1200°C, just below melting: the size mismatch between Li+ and Ga3+ ions and the likely unfavourable enthalpy of mixing associated with Li/Ga disorder is offset by the large entropy contribution to the Gibbs energy at high temperatures. A discussion of possible mechanisms for the β ⇋ γ transition is given.",
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Polymorphism and crystal chemistry of Li2.5Ga0.5GeO4, an Li3PO4 analogue. / Robertson, Alastair D.; West, Anthony R.

In: Journal of Materials Chemistry, Vol. 4, No. 3, 1994, p. 457-462.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Polymorphism and crystal chemistry of Li2.5Ga0.5GeO4, an Li3PO4 analogue

AU - Robertson, Alastair D.

AU - West, Anthony R.

PY - 1994

Y1 - 1994

N2 - The structures of γ- and β-Li2.5Ga0.5GeO4 have been determined using Rietveld refinement of X-ray powder diffraction data. They are stoichiometric derivatives of γ- and β-Li3PO4. The arrangements of GeO4/PO4 tetrahedra are the same in each case, but the Li sites, especially Li(2), contain a disordered mixture of Li and Ga in γ- and β-Li2.5Ga0.5GeO4. Such cation disorder is unusual in a stoichiometric phase. The γ phase may be regarded as entropy-stabilised since it exists under equilibrium conditions only at high temperatures, ca. 1200°C, just below melting: the size mismatch between Li+ and Ga3+ ions and the likely unfavourable enthalpy of mixing associated with Li/Ga disorder is offset by the large entropy contribution to the Gibbs energy at high temperatures. A discussion of possible mechanisms for the β ⇋ γ transition is given.

AB - The structures of γ- and β-Li2.5Ga0.5GeO4 have been determined using Rietveld refinement of X-ray powder diffraction data. They are stoichiometric derivatives of γ- and β-Li3PO4. The arrangements of GeO4/PO4 tetrahedra are the same in each case, but the Li sites, especially Li(2), contain a disordered mixture of Li and Ga in γ- and β-Li2.5Ga0.5GeO4. Such cation disorder is unusual in a stoichiometric phase. The γ phase may be regarded as entropy-stabilised since it exists under equilibrium conditions only at high temperatures, ca. 1200°C, just below melting: the size mismatch between Li+ and Ga3+ ions and the likely unfavourable enthalpy of mixing associated with Li/Ga disorder is offset by the large entropy contribution to the Gibbs energy at high temperatures. A discussion of possible mechanisms for the β ⇋ γ transition is given.

U2 - 10.1039/JM9940400457

DO - 10.1039/JM9940400457

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JO - Journal of Materials Chemistry

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