New titanium-vanadium phosphates of nasicon and langbeinite structures, and differences between the two structures toward deintercalation of alkali metal

Abstract

Mixed metal phosphates of the general formula Ax TiV^III(PO₄)₃, where A = Na, K, or Ba and (0 < x ≤ 3), have been synthesized. While the sodium phosphate, Na₃TiV(PO₄)₃, adopts the NASICON structure, the potassium compound, K₂TiV(PO₄)₃, crystallizes with the langbeinite structure. BaTiV(PO₄)₃ is dimorphic, adopting the NASICON structure at lower temperatures and the langbeinite structure at higher temperatures. Ba_1.5V₂(PO₄)₃ and BaKV₂(PO₄)₃, on the other hand, crystallize only with the langbeinite structure. Oxidative deintercalation of sodium from Na₃TiV(PO₄)₃ readily occurs, yielding Na_xTiV(PO₄)₃ (x ~ 1.0) and retaining the NASICON framework, while a similar deintercalation of potassium does not occur from the langbeinite K₂TiV(PO₄)₃, revealing a difference in reactivity between the two structures: NASICON, being a skeleton structure with an interconnected interstitial space, facilitates mobility of alkali metal ions through the crystal, while langbeinite, being a true cage structure, does not permit a similar mobility.