Exploring Route for Pyrophosphate-based Electrode Materials: Interplay between Synthesis and Structure

New sodium vanadium(III) hydrogenphosphate hydrate NaV(HPO4)(2)(H2O)(0.5)and sodium vanadium(III) hydrogenphosphate beta-NaV(HPO4)(2)were prepared in mild hydrothermal conditions. The crystal structures of NaV(HPO4)(2)(H2O)(0.5)[space groupCc, Z= 4,a= 8.46174(19) angstrom,b= 9.52583(19) angstrom,c= 8.69376(15) angstrom,beta= 110.9553(11) degrees,V= 654.41(2) angstrom(3)] and beta-NaV(HPO4)(2)[space groupC2/c, Z= 4,a= 7.8681(3) angstrom,b= 9.8451(3) angstrom,c= 8.5180(2) angstrom,beta= 107.626(2) degrees,V= 628.85(3) angstrom(3)] were solved and refined from X-ray powder diffraction data. Both compounds were used as precursors in a new route for the preparation of attractive cathode material beta-NaVP2O7. The formation of the structure motive providing fast sodium ion diffusion at the first synthesis stage and its further conservation upon stepwise dehydration was revealed. The oversized Na+-embedding channels are stabilized by site-coordinated water in NaV(HPO4)(2)(H2O)(0.5)structure. The topology resemblance and difference in known sodium vanadium(III) complex phosphates are discussed.