From a 3D protonic conductor VO(H2PO4)2 to a 2D cationic conductor Li4VO(PO4)2 through lithium exchange

A new phase, Li4VO(PO4)2 was synthesized by a lithium ion exchange reaction from protonic phase, VO(H2PO4)2. The structure was determined from neutron and synchrotron powder diffraction data. The exchange of lithium causes a stress, leading to a change in the dimensionality of the structure from 3D to 2D by the displacement of oxygen atoms. Thus, Li4VO(PO4)2 crystallizes in P4/n space group with lattice parameters a=8.8204(1) Å and c=8.7614(2) Å. It consists of double layers [V2P4O18]∞ formed by successive chains of VO6 octahedra and VO5 pyramids with isolated PO4 tetrahedra. The lithium ions located in between the layers promote mobility. Furthermore, the ionic conductivity of 10−4 S/cm at 550 °C for Li4VO(PO4)2 confirms the mobility of lithium ions in the layers. On the other hand, VO(H2PO4)2 exhibits a conductivity of 10−4 S/cm at room temperature due to the presence of protons in tunnels.