Crystal Chemistry of Melilite
[CaLa]<sub>2</sub>[Ga]<sub>2</sub>[Ga<sub>2</sub>O<sub>7</sub>]<sub>2</sub>: a Five Dimensional Solid Electrolyte
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Abstract
Melilite-type [<i>A</i><sub>2</sub>]<sub>2</sub>[<i>B</i><sup>I</sup>]<sub>2</sub>[<i>B</i><sup>II</sup><sub>2</sub>O<sub>7</sub>]<sub>2</sub> gallates are promising
ion conducting electrolytes for deployment in solid oxide fuel cells.
Single crystals of [CaLa]<sub>2</sub>[Ga]<sub>2</sub>[Ga<sub>2</sub>O<sub>7</sub>]<sub>2</sub>, 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]-[Ga<sub>2</sub>O<sub>7</sub>] 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 <i>P</i>4̅2<sub>1</sub><i>m</i>(α,
α, 0)00<i>s</i>(α̅, α, 0)000, α
= 0.2160(5), with a subcell metric of <i>a</i> = 7.9383(2)
Å, <i>c</i> = 5.2641(3) Å, onto which modulation
vectors are superimposed: <i><b>q</b></i><sub><b>1</b></sub> = α (<i><b>a</b></i>* + <i><b>b</b></i>*), <i><b>q</b></i><sub><b>2</b></sub> = α (−<i><b>a</b></i>* + <i><b>b</b></i>*). 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
