Selection and Preliminary Evaluation of Three Structures as Potential Solid Conductors of Alkali Ions Two Hollandites, a Titanate, and a Tungstate

摘要

Utilization of crystal-chemical criteria has suggested three structure types in which alkali ions may be mobile: (1)hollandites K(x)Mg(x/2)Ti(8-x/2)O16 and K(x)Al(x)Ti(8-x)O16 for 1.6 less than or equal to x less than or equal to 2.0 tungstate K2W4013; and (3) sodium hexatitante Na2Ti6O13. Each is a tunnel structure. An electrical screening procedure, previously tested on beta-alumina, has indicated high K(+) ion mobility in the hollandites and in the tungstate, but not in the hexatitanate. Specimens were polycrystalline disks near 90 percent of theoretical density. The ac conductivity calculated from dielectric and capacitance measurements has been attributed to ion mobility. This ac conductivity was up to 0.01/ohm-cm for hollandites and about 0.0001/ohm-cm for the tungstate, with approximate activation energies of 21 to 25 and 16 kJ/mole (5 to 6 and 4 kcal/mole), respectively. Electronic conduction and chemical reactivity have eliminated the tungstate from further consideration. The hollandites have been considered worthy of further development and evaluation. (Author)