Interstitial oxide ion conductivity in the layered tetrahedral network melilite structure

Abstract

High-conductivity oxide ion electrolytes are needed to reduce the operating temperature of solid-oxide fuel cells. Oxide mobility in solids is associated with defects. Although anion vacancies are the charge carriers in most cases, excess (interstitial) oxide anions give high conductivities in isolated polyhedral anion structures such as the apatites. The development of new families of interstitial oxide conductors with less restrictive structural constraints requires an understanding of the mechanisms enabling both incorporation and mobility of the excess oxide. Here, we show how the two-dimensionally connected tetrahedral gallium oxide network in the melilite structure La(1.54)Sr(0.46)Ga(3)O(7.27) stabilizes oxygen interstitials by local relaxation around them, affording an oxide ion conductivity of 0.02-0.1 S cm(-1) over the 600-900 degrees C temperature range. Polyhedral frameworks with central elements exhibiting variable coordination number can have the flexibility needed to accommodate mobile interstitial oxide ions if non-bridging oxides are present to favour cooperative network distortions.

Reversible nano-structuring of SrCrO3-δ through oxidation and reduction at low temperature

“…4d. It is interesting to note that a similar cooperative mechanism for fast ion conduction has been found in other oxide materials with tetrahedral moieties, for example, gallium-based oxides 25,26 and Si/Ge apatites 27 , where the common features are the presence of open 2D tetrahedral channels and the structural flexibility for local relaxation, as well as changes in cation coordination.…”

The luminescence of NaxEu3+(2−x)/3MoO4scheelites depends on the number of Eu-clusters occurring in their incommensurately modulated structure

“…where A MD,0 ¼ 14.65 s À1 is the spontaneous emission probability for the magnetic dipole (2) total integrated intensity of Eu 3+ emission to the integrated 5 D 0 / 7 F 1 transition. In our case, however, the refractive indices are not known, so that we have proceeded differently, using the equations to determine n values: first, s rad data were calculated from the Q Eu Eu and s obs experimental values for each composition which, in turn, allowed estimating refractive indices from eqn (3).…”

The mechanism of oxide ion conductivity in bismuth rhenium oxide, Bi28Re2O49

“…Extensive experimental and computational work has led to the proposal of several general classes of mechanism for oxide ion migration: the vacancy mechanism; the interstitial and intersticialcy mechanisms; the cooperative mechanism and the variable coordination number mechanism. [9][10][11] The mechanism occurring in fluorite-related oxide ion conductors has been thought to proceed principally via vacancies, whilst some apatites and melilite-related compounds exhibit an interstitial mechanism. [9,12] La 1-x Ba 1+x GaO 4-x/2 related oxide ion conductors exhibit a cooperative mechanism which requires exchange of oxygen between GaO 4 groups, forming Ga 2 O 7 units as intermediates in the conduction process.…”

“…[9][10][11] The mechanism occurring in fluorite-related oxide ion conductors has been thought to proceed principally via vacancies, whilst some apatites and melilite-related compounds exhibit an interstitial mechanism. [9,12] La 1-x Ba 1+x GaO 4-x/2 related oxide ion conductors exhibit a cooperative mechanism which requires exchange of oxygen between GaO 4 groups, forming Ga 2 O 7 units as intermediates in the conduction process. [10] The crystal structure of α-La 2 Mo 2 O 9 was found to contain Mo in three different types of coordination environments (4-, 5-and 6-coordinate) and to undergo an order-disorder phase transition to the highly conducting β-La 2 Mo 2 O 9 polymorph.…”