A new perovskite phase Li2xCa0.5−xTaO3: Li+ ion conductivity and use as pH sensor

Several compounds of a new solid solution Li2xCa0.5−xTaO3 (0.05≤x≤0.25) with perovskite structure have been synthesised by classical solid state reaction. It has been found that synthesis at 1773 K followed by a quenching in air was necessary to obtain a pure perovskite phase. The cell parameters and their variation with composition x have been determined through X-ray diffraction patterns analysis. Scanning electron microscopy showed the presence of grains of 10 μm size and of aggregates of 50–100 μm thick. Impedance spectroscopy in the frequency range from 10 MHz to 1 Hz and in the temperature range from 348 to 573 K has been carried out to determine the electrical properties. A bulk dc-conductivity slightly higher than 10−6 S cm−1 has been found at 473 K. 7Li nuclear magnetic resonance relaxation times, T1 and T1ρ, vs. inverse of temperature have been measured to investigate the microscopic ionic motions of Li+ ions. Two motions of Li+ ions, a short and a long range one, are evidenced by these experiments. Furthermore, the Galvani potential of the ionic conductive membrane electrode, when put in contact with an aqueous solution, depends on the pH of the solution. The fast response on pH variation enables it then to be used as pH sensor in aqueous solutions.