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]O2 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]O2, rather than the presumptive tunnel location of [La9Sr][(GeO4)6]O2.5. The implication of this approach is that in addition to the three known apatite genera—A10(BO3)6X2, A10(BO4)6X2, A10(BO5)6X2—hybrid electrolytes of the types A10(BO3/BO4/BO5)6X2 can be designed, with potentially superior low-temperature ion conduction, mediated by the introduction of oxygen to the framework reservoir.