Furthermore, the average V-O distances in V(1a)O6 and V(1b)O6 octahedra are 1.92(8) Å and 2.04(8) Å, respectively, suggesting a possible charge ordering of the V 4+ and V 3+ in the crystal structure (see the details of V-O distances in Table ). In terms of V charge ordering, Model 2 compares well with the DFT data discussed below. Finally, the Na(1) site is unique, while the Na(2) site splits into four distinct Wyckoff positions, i.e., Na(2a1), Na(2a2), Na(2b1), and Na(2b2). The refined occupancy factors for Na(1), Na(2a1), Na(2a2), Na(2b1), and Na(2b2) are 0.98(3), 0.17 (7), 0.21 (7), 0.63 (11), and 0.68 (11), respectively, yielding a composition Na2.0(3)V2(PO4)3. Detailed structural information is listed in Table . Note that both monoclinic structural models for Na2V2(PO4)3 (P2/c and P21/c) have very similar cell parameters and can fully account for the existence of the (100) reflection at 2θ = 3.85°. However, subtle differences to distinguish between the two space groups are convoluted in the possible existence of [(0k0), k=2n+1] reflections for P2/c while they are forbidden for P21/c. A very low-intensity peak at ~5.5° (of the SXRPD pattern) could be assigned to the (010) reflection in the space group P2/c, although the same was also observed in Na3V2(PO4)3 and Na1V2(PO4)3 phases with the same intensity. This evidence suggests this peak may not originate from the phase reflection of NaxV2(PO4)3 but it is rather a noisy background (Figure ). For this reason, we believe that the P21/c model (Model 1) is more suitable, although the P2/c one (Model 2) cannot be completely excluded.