Synthesis, crystal structure, and ionic conductivity of tunnel structure phosphates, RbMg1−xH2x(PO3)3·y(H2O)

Abstract

A new proton conductor, RbMg_1−xH_2x(PO₃)₃·yH₂O, was synthesized by the coprecipitation method followed by sintering at 540 K. The range of solid solution for RbMg_1−xH_2x(PO₃)₃·yH₂O was 0.00 < x < 0.18 and the highest conductivity of 5.5 × 10⁻³ S cm⁻¹ was observed at 443 K for the composition of x = 0.11. The structure of RbMg_1−xH_2x(PO₃)₃·yH₂O was determined from 298 to 553 K by Rietveld refinement with multi-profile analysis using X-ray and neutron diffraction data. The framework structure is composed of PO₄ tetrahedra connected with each other by corner-sharing to form spiral PO₄ chains along the c-direction. One-dimensional tunnels are formed between these spiral chains, in which water molecules are located. The water molecules form one-dimensional spiral chains and are connected to the spiral PO₄ chain by hydrogen bonding. The introduction of acidic, hydrophilic head groups (–PO₃H) into the PO₄ framework by the formation of a solid solution provides binding sites for water and an environment for the efficient diffusion of protons. One-dimensional proton diffusion, similar to proton channels in biological systems, could be explained by the vehicle mechanism of H₃O⁺ along the one-dimensional water chain.