Structure, ion-ion correlation and compensation mechanisms in β- and β″-alumina

Abstract

The crystal structures and ion-ion correlations are examined in several types of β - and β″ -alumina materials, (1) Mg 2+ compensated materials with general formula (Al 11- y Mg y O 16 )(M 1+ y O) with defects in the spinel blocks. Compositions p =1.45, 1.57, 1.62 (β) and 5/3 (β″) are investigated. The cations are only present on B.R. and m-O sites with a relation between both occupancies which corresponds to two kinds of cells occupied either with one or three ions. Local order indicates the existence of a a 3× a 3 strictly 2-D superstructure with a conerence length decreasing with ionic concentration, the stable configurations correspond to p =1 and 5/3 and, for intermediave concentration, competition between a tendency to complete phase separation driven by ion-ion correlations and a restoring coupling due to the average charge of the host lattice, leads to a partial phase separation with short coherence length. β″-alumina is stabilized for high ionic concentration by an antiferroelectric polarization of the host lattice. (2) Oxygen compensated materials corresponding to the general formula (Al 11 O 16 )(M 1+ x O 1+ x /2 ) with interstitial oxygen stabilized by the Roth-Frenkel defect. The imerstilial oxygen is found on x ≈5/6, distinct from the m-O position on 8/9. X-ray diffuse scattering reveals that both m-O ions and interstitial oxygen are ordered in triply occupied cells ( a 3× a 3 quasi 2-D supercell) with a large coherence length. (3) Mix-conipensated materials ▤ Al and interstitial oxygen. These materials correspond to the general formula (Al 11-z O 16 )(M 1+x+3z O 1+x/2 ) with the 1+x ions on B.R. and m-O sites and the additional 3z ions on a-B.R. sites. This compensation mechanism is stabilized by clusters of non-mobile ions and lowers the ionic mobility. The order is only local with short coherence length and with diffuse intensity due to in-plane correlations (2-D superstructure) and to correlations associated with the two compensation mechanisms. (4) Mix-compensated materials, Mg 2+ and interstitial oxygen. They correspond to the formula (Al 11-y O 16 )(M 1+y+x/2 O 1+x/2 ) and also exhibit the a 3× a 3 local 2-D superstructure but with small coherence length.