Electrochemical difference between single‐ and poly‐crystalline LaF 3 for a compact and low‐price fluoride ion sensor

Fluoride ion sensor electrodes that use polycrystalline LaF 3 prepared by low‐temperature sintering can reduce sensor size and fabrication costs; however, decreasing the sintering temperature deteriorates sensor performance. Further, it is unclear why the performance of polycrystalline LaF 3 prepared at low temperatures is inferior to that of the single‐crystal LaF 3 . Thus, in this study, we investigated factors that deteriorate sensor response under low sintering temperatures, such as solution penetration, fluorine conductivity, and ion exchange. To this end, LaF 3 was prepared by changing the sintering temperature, and the electrical characteristics and sensor performance were evaluated and compared with those of the single‐crystal LaF 3 . The sensor performances of LaF 3 sintered at 900 °C (−9.3 mV/dec) and 1000 °C (−5.8 mV/dec) were inferior to that of a single‐crystal LaF 3 (−49.9 mV/dec). LaF 3 prepared by sintering over 800 °C achieved fluorine removal and showed lower ion conductivity and ion exchange compared to that of the single‐crystal LaF 3 . LaF 3 sintered at 700 °C did not show a sensor response because of solution penetration into the through‐pore. Our results confirmed that solution penetration, ion conductivity, and ion exchange influence the sensor performance of LaF 3 . Finally, we discussed approaches to achieve high sensor performance with LaF 3 prepared by low‐temperature sintering. The proposed approaches are expected to contribute to expanding the utility of fluoride ion sensor electrodes with LaF 3 .