MIXED ALKALI EFFECT AND STRUCTURE OF (K~+,Cs~+)-β-FERRITE

摘要

Mixed alkali effect in (K~+, Cs~+)-β-ferrite with β-alumina structure has been investigated. As well as β-alumina, this compound reveals ionic conduction in the alkali layers. In this study, the distribution of alkali ions in (K~+, Cs~+)-β-ferrite was determined by X-ray crystallography to discuss the mixed alkali effect. The structure determination was performed for K_(0.43)Cs_(0.90)Fe_(11)O_(17) with 0.3/0.9 in K~+/Cs~+ ratio, in which the conduction minimum (mixed alkali effect) had been reported previously. Crystallographic parameters were as follows; crystal system: hexagonal, space group: P6_3/mmc, and the lattice constants: a = 0.592210(10) nm, c = 2.413321(4) nm. When Cs~+ ions and K~+ ions were put in BR sites and in mO sites respectively, minimum R (R1 and wR2) was obtained. The conduction plane in a half unit cell was contained three K~+ ions (triplet cell) or one Cs~+ ion (single cell). This distribution was different from that obtained for K_(0.34)Cs_(0.82)Fe_(11)O_(17) with the same K~+/Cs~+ ratio, in which two K~+ ions were in a half cell (doublet cell). The difference was in the net amount of alkali ions. Because of no vacancy in the cell and surrounding heavy Cs~+ ions, K~+ ions in the triplet cells are deduced to be difficult to move.