NMR Investigations of the Protonic Transport Mechanism in Composed Materials on the Basis of Cesium Acid Sulfates and Phosphates

Abstract

The composite materials Cs(HSO4)1−x(H2PO4)x were investigated by X-ray phase analysis, differential scanning calorimetry, nuclear magnetic resonance (NMR) relaxation, pulsed field gradient NMR (PFG-NMR) and impedance spectroscopy. Three composite materials types x = 0.1 ÷ 0.3 mixture CsHSO4, α-Cs3(HSO4)2(H2PO4), β-Cs3(HSO4)2.5(H2PO4)0.5—compositions of area I; x = 0.4 ÷ 0.5 mixture α-Cs3(HSO4)2(H2PO4) and Cs2(HSO4)(H2PO4)—compositions of area II; x = 0.6 ÷ 0.9 mixture Cs2(HSO4)(H2PO4) and CsH2PO4—compositions of area III, were synthesized. The phase transition temperature from the low-to-high conductive phase for obtained composite materials is notably below (about 100 °C) than that for the individual components. The proton self-diffusion coefficients measured by PFG-NMR are lower than the diffusion coefficients calculated from proton conductivities data. The correlation times τd controlling the 31P–1H magnetic dipole–dipole interaction were calculated according to data of the spin–lattice relaxation on 31P nuclei. The self-diffusion coefficients estimated from the Einstein equation are in good agreement with the experimental self-diffusion coefficients measured by PFG-NMR. It confirms the fact that the proton mobility is caused by the rotation of PO4 anion tetrahedra.