A taxonomy of apatite frameworks for the crystal chemical design of fuel cell electrolytes

Apatite framework taxonomy succinctly rationalises the crystallographic modifications of this structural family as a function of chemical composition. Taking the neutral apatite [La8Sr2][(GeO4)(6)]O-2 as a prototype electrolyte, this classification scheme correctly predicted that 'excess' oxygen in La9SrGe6O26.5 is tenanted in the framework as [La9Sr][(GeO4)(5.5)(GeO5)(0.5)]O-2, rather than the presumptive tunnel location of [La9Sr][(GeO4)(6)]O-2.5. The implication of this approach is that in addition to the three known apatite genera-A(10)(BO3)(6)X-2,X- A(10)(BO4)(6)X-2, A(10)(BO5)(6)X-2-hybrid electrolytes of the types A10(BO3/BO4/ BO5)(6)X-2 can be designed, with potentially superior low-temperature ion Conduction, mediated by the introduction of oxygen to the framework reservoir. (C) 2008 Elsevier Inc. All rights reserved.