Reinvestigation of trilithium divanadium(III) tris(orthophosphate), Li3V2(PO4)3, based on single-crystal X-ray data

The structure of Li3V2(PO4)3 has been reinvestigated from single-crystal X-ray data. Although the results of the previous studies (all based on powder diffraction data) are comparable with our redetermination, all atoms were refined with anisotropic displacement parameters in the current study, and the resulting bond lengths are more accurate than those determined from powder diffraction data. The title compound adopts the Li3Fe2(PO4)3 structure type. The structure is composed of VO6 octahedra and PO4 tetrahedra by sharing O atoms to form the three-dimensional anionic framework ∞ 3[V2(PO4)3]3−. The positions of the Li+ ions in the empty channels can vary depending on the synthetic conditions. Bond-valence-sum calculations showed structures that are similar to the results of the present study seem to be more stable compared with others. The classical charge balance of the title compound can be represented as [Li+]3[V3+]2[P5+]3[O2−]12.

The structure of Li 3 V 2 (PO 4 ) 3 has been reinvestigated from single-crystal X-ray data. Although the results of the previous studies (all based on powder diffraction data) are comparable with our redetermination, all atoms were refined with anisotropic displacement parameters in the current study, and the resulting bond lengths are more accurate than those determined from powder diffraction data. The title compound adopts the Li 3 Fe 2 (PO 4 ) 3 structure type. The structure is composed of VO 6 octahedra and PO 4 tetrahedra by sharing O atoms to form the three-dimensional anionic framework 1 3 [V 2 (PO 4 ) 3 ] 3À . The positions of the Li + ions in the empty channels can vary depending on the synthetic conditions. Bond-valence-sum calculations showed structures that are similar to the results of the present study seem to be more stable compared with others.
The title compound adopts the Li 3 Fe 2 (PO 4 ) 3 structure type. The general structural features of this compound are the same as reported previously (Patoux et al., 2003;Fu et al., 2010). However, as would be expected, the bond lengths found here from single-crystal diffraction data are more accurate than those reported previously from powder diffraction data. For example, the V-O distances (Table 1) reported by Kuo et al. (2008) range from 1.846 (3) to 2.258 (4) Å compared with 1.904 (2)-2.117 (2) Å here. Figure 1 shows the local coordination environment of the V and P atoms. In the structure, VO 6 octahedra are joined to PO 4 tetrahedra forming a [V 2 (PO 4 ) 3 ] unit. These units share a terminal oxygen atom to construct the anionic three-dimensional framework, ∞ 3 [V 2 (PO 4 ) 3 ] 3- (Fig. 2). The V-O distances are in good agreement with those calculated from their ionic radii (1.99 Å, Shannon, 1976), assuming a valence of +III for V.
The Li + ions in the empty channels are surrounded by four O atoms in distorted tetrahedral coordination sites. There are three crystallographically independent Li sites for this phase. It has been reported that the positions of the Li atoms can vary depending on the synthetic conditions while those of the V, P, and O atoms comprising the rigid framework remain intact (Yang et al., 2010). The Li positions found from the present single-crystal study are consistent with those reported by Patoux et al.(2003) and of a sample treated with microwave radiation at 1123 K for 3 min by Yang et al. (2010).
According to bond valence sum calculations (Adams, 2001)

Experimental
The title compound, Li 3 V 2 (PO 4 ) 3 , was prepared by the reaction of the elements with the use of the reactive halide-flux technique. A combination of the pure elements, Nb powder (Alfa Aesar 99.8%), V powder (STREM CHEMICALS 99.5%) and P powder (CERAC 99.5%) were mixed in a fused silica tube in a molar ratio of Nb:V:P = 1:1:3 and then LiCl (Sigma-Aldrich 99%) was added. The mass ratio of the reactants and the halide was 1:5. The tube was evacuated to 0.133 Pa, sealed, and heated gradually (150 K/h) to 1123 K, where it was kept for 12 h. The tube was cooled to room temperature at a rate of 3 K/h. The excess halide was removed with water and colourless block-shaped crystals were supplementary materials obtained. The crystals are stable in air and water. A qualitative X-ray fluorescence analysis of selected crystal indicated the presence of V, P, and O. The composition of the compound was determined by single-crystal X-ray diffraction.

Refinement
Although all the previous structural studies of Li 3 V 2 (PO 4 ) 3 have been performed in space group settings P<ι>112 1 /n<ι> or P<ι>12 1 /n<ι>1 of space group no. 14, we have chosen the standard setting, P<ι>12 1 /c<ι>1, for this and future studies. For the comparison between the different settings in this and the previous studies, the fractional coordinates transformed to the standard setting for the various entries in the ICSD (2012) can be used. The highest peak (0.58 e/Å-3) and the deepest hole (-0.49 e/ Å-3) are 0.68 Å and 0.77 Å from the atom O12 and P1, respectively.