Beevers-Ross sites

The disposition of the atoms in the conduction plane itself is identical to that in (3-alumina (Fig. 6.10b). However, the local environment of the conduction planes is different in the two phases because the oxygen ions on both sides of the conduction plane are superimposed in (3-alumina and staggered in (3"-alumina (Fig. 6.KM). This means that Beevers-Ross and anti-Beevers-Ross sites are not present. Instead the sodium sites, which have a tetrahedral geometry, are called Beevers-Ross-type (BR-type). The mid-oxygen (mO) sites, located midway between the BR-type sites, also have a different geometry in the two phases. 

Barium titanate (BaTi03), strength after indentation, 185 Barrel indents, 44-126 BeBg Knoop hardness, 303 Beevers-Ross sites, 280 Berkovich hardness anisotropy, 94 diamond, 11, 39-40 equation, 11... 

The )5-aluminas are described in some detail in Chapter 2, only a few specific features are noted here. In the "-aluminas, the spinel blocks are stacked in such a way that the energetically equivalent sites occupied by Na" ions are ideally just half-filled in the -aluminas the spinel blocks are stacked so as to distinguish two types of Na" -ion sites of different potential energy, the Beevers-Ross (BR) and anti-Beevers-Ross (aBR) sites. In the Na-O planes, the shortest bottleneck distance 2.7 A is just a little greater than the sum of the ionic radii, 2.4 A, at room temperature, so a small value of AH can be anticipated. The discovery of fast Na -ion conductivity in the Na j5-aluminas (Yao and Kummer, 1967 Kummer and Weber, 1967) led to the invention of the Na/S battery that triggered extensive interest in the solid-electrolyte problem. 

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