The structure and dielectric properties of compounds with the formula BA_x(Ti_8–xMg_x)O₁₆

Abstract

The preparation of a series of oriented aggregates of the non-stoichiometric phase Ba,(TiS-xMgx) 016 (1.14 > x > 0.67) is described. The crystal structure, determined by single crystal methods of X-ray analysis, discloses that the Ba ions are situated at interstitial sites in unidirectional tunnels formed by a framework of (Ti, Mg)O:! octahedra. Dielectric absorption in these compounds has a directional property and can' be interpreted in terms of the movement of Ba ions from eight co-ordinated sites to adjacent unoccupied positions of a similar kind. The energy barrier to the movement of the Ba ions is determined (0.17 ev). The magnitude of the dielectric dispersion is compared with that calculated from a simple model.

Incommensurate superlattice ordering in the hollandites Ba x Ti8−x Mg x O16 and Ba x Ti8−2x Ga2x O16

“…Single-crystal X-ray diffraction studies of the phase BaxMgxTia_xO~6, 0.67 < x < 1.14, showed it to be tetragonal with unit-cell parameters a~ = a 2 --10.110, c = 2.986A (Dryden & Wadsley, 1958). The fractional coordinates were essentially identical to those reported earlier for the hollandite group of minerals (Bystr6m & Bystr6m.…”

Structural relationships between β-gallia, rutile, hollandite, psilomelane, ramsdellite and gallium titanate type structures

“…Their fractional atomic coordinates are virtually identical with those found by Bystr6m & Bystr6m (1950), see Table 1. Dryden & Wadsley (1958) reported diffuse supplementary spectra, the positions and intensities of which were independent of chemical composition in the range 0.67 < x < 1.14. At composition x = 1, strong dielectric absorption was found parallel to the tunnel direction.…”

An analysis of the structural characteristics of hollandite compounds

“…Refined data from X-ray singlecrystal studies (i.e. Dryden & Wadsley, 1958;Cadre & Verschoor, 1978;Sinclair et al, 1980;Schmactel & Muller-Buschbaum, 1980;Post etal., 1982) are also included on these figures and Fig. 6 to present a better overall view of the behaviour of hollandites.…”

“…Ba, Cs) and B are smaller cations occupying octahedral sites such as A13+, Ti 3÷ and Ti 4+ (Kesson, 1983). They may be tetragonal [I4/m, a = 9.90 to 10.10/~ and c=2.9 to 3.0/~ (BystrSm & Bystr/Sm, 1950;Dryden & Wadsley, 1958;Sinclair, McLaughlin & Ringwood, 1980)] or monoclinic [I2/m, fl =90 to 91.5 ° (Cad6e & Verschoor, 1978;Post, Von Dreele & Buseck, 1982)]. The symmetry is usually monoclinic when the ratio of the cation radii RA/RB < 2.08 and tetragonal when this ratio >2.08 although this is by no means a general rule for all hollandites (Post et al, 1982).…”