Crystal chemistry of melilite [CaLa]2[Ga]2[Ga2O7]2 : a five dimensional solid electrolyte
White, Timothy John
Date of Issue2012
School of Materials Science and Engineering
Singapore Institute of Manufacturing Technology
Melilite-type [A2]2[BI]2[BII2O7]2 gallates are promising ion conducting electrolytes for deployment in solid oxide fuel cells. Single crystals of [CaLa]2[Ga]2[Ga2O7]2, grown in an optical floating zone furnace, were investigated using a combination of transmission electron microscopy and single crystal X-ray diffraction. Strong anisotropic displacements of oxygen arise from the structural misfit between the interlayer Ca/La cations and the [Ga]-[Ga2O7] tetrahedral layers. A model employing two-dimensional modulation achieves bond lengths and bond angles that preserve satisfactory bond valence sums throughout the structure. The melilite belongs to the tetragonal superspace group P4̅21m(α, α, 0)00s(α̅, α, 0)000, α = 0.2160(5), with a subcell metric of a = 7.9383(2) Å, c = 5.2641(3) Å, onto which modulation vectors are superimposed: q1 = α (a* + b*), q2 = α (−a* + b*). Both displacive (cation and anion) and occupational (cation) modulations contribute to incommensuration. The analysis of structural adjustments that accompany changes in temperature and composition provides assurance that the crystal chemical model is correct. By better understanding the flexibility of this modulated structure a rational approach toward crystallochemical optimization of electrolyte performance by enhancing oxygen mobility becomes feasible.
© 2012 American Chemical Society. This is the author created version of a work that has been peer reviewed and accepted for publication by Inorganic chemistry, American Chemical Society. It incorporates referee’s comments but changes resulting from the publishing process, such as copyediting, structural formatting, may not be reflected in this document. The published version is available at: [http://dx.doi.org/10.1021/ic300585t].