On the Symmetry, Electronic Properties, and Possible Metallic States in NASICON-Structured A 4 V 2 (PO 4 ) 3 (A = Li, Na, K) Phosphates.

In this work, the electronic structure and properties of NASICON-structured A 4 V 2 (PO 4 ) 3 , where A = Li, Na, K were studied using hybrid density functional theory calculations. The symmetries were analyzed using a group theoretical approach, and the band structures were examined by the atom and orbital projected density of states analyses. Li 4 V 2 (PO 4 ) 3 and Na 4 V 2 (PO 4 ) 3 adopted monoclinic structures with the C2 space group and averaged vanadium oxidation states of V+2.5 in the ground state, whereas K 4 V 2 (PO 4 ) 3 adopted a monoclinic structure with the C2 space group and mixed vanadium oxidation states V+2/V+3 in the ground state. The mixed oxidation state is the least stable state in Na 4 V 2 (PO 4 ) 3 and Li 4 V 2 (PO 4 ) 3 . Symmetry increases in Li 4 V 2 (PO 4 ) 3 and Na 4 V 2 (PO 4 ) 3 led to the appearance of a metallic state that was independent of the vanadium oxidation states (except for the averaged oxidation state R32 Na 4 V 2 (PO 4 ) 3 ). On the other hand, K 4 V 2 (PO 4 ) 3 retained a small band gap in all studied configurations. These results might provide valuable guidance for crystallography and electronic structure investigations for this important class of materials.