New Titanium-Vanadium Phosphates of Nasicon and Langbeinite Structures, and Differences between the Two Structures toward Deintercalation of Alkali Metal

Mixed metal phosphates of the general formula A_x TiV ^III(PO ₄) ₃, where A = Na, K, or Ba and (0 < x ≤ 3), have been synthesized. While the sodium phosphate, Na ₃TiV(PO ₄) ₃, adopts the NASICON structure, the potassium compound, K ₂TiV(PO ₄) ₃, crystallizes with the langbeinite structure. BaTiV(PO ₄) ₃ is dimorphic, adopting the NASICON structure at lower temperatures and the langbeinite structure at higher temperatures. Ba _1.5V ₂(PO ₄) ₃ and BaKV ₂(PO ₄) ₃, on the other hand, crystallize only with the langbeinite structure. Oxidative deintercalation of sodium from Na ₃TiV(PO ₄) ₃ readily occurs, yielding Na _xTiV(PO ₄) ₃ ( x ∼ 1.0) and retaining the NASICON framework, while a similar deintercalation of potassium does not occur from the langbeinite K ₂TiV(PO ₄) 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.