Anisotropic oxide ion conduction in melilite intermediate temperature electrolytesElectronic supplementary information (ESI) available: Structural information from single crystal X-ray diffraction can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen (Germany), on quoting the depository number CSD-428196. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ta05132g

Electrolytes with oxide ion conductivities higher than 10 −2 S cm −1 at moderate temperatures (∼500-900 °C) offer the possibility for solid oxide fuel cells to operate with less maintenance. This study of [A 1+ x B 1− x ] 2 [Ga] 2 [Ga 2 O 7+ x /2 ] 2 (0 ≤ x ≤ 0.5) (A = La, Nd; B = Ca, Sr) layered-melilite found that in large single crystals intralayer oxide ion conduction is dominant. This anisotropic behavior arises by relaxation about the interstitial oxygen through changes in the interlayer A and Ga coordination, and at 850 °C conductivities are ∼0.008 S cm −1 along the c direction and ∼0.036 S cm −1 perpendicular to the c axis. It is found that the ionic conductivity can be optimized by increasing the number of interstitial oxygen and reducing the size of interlayer cations. Melilite, a tetrahedral-layered structure, shows strong anisotropic oxygen ion conduction at intermediate temperatures, which can be controlled by increasing the number of interstitial oxygen and reducing the size of interlayer cations.