抄録
This study aimed to determine the intrinsic bulk resistivity of neodymium silicate Nd9.33(SiO4)6O2 single crystals with an apatite-type structure and to clarify the origin of impedance features observed during oxide ion conductivity measurements. The c-axis oxide ionic conductivity of neodymium silicate Nd9.33(SiO4)6O2 single crystals with an apatite-type crystal structure was measured using AC two-probe and DC four-probe methods. In the Nyquist plot obtained from the AC two-terminal method, an arc was observed at high frequencies, passing through the origin below 350 °C, while a tail appeared at low frequencies. The resistance value corresponding to the intersection with the Z’ axis on the right-hand (low frequency) side of the arc was found to closely match the resistance measured by the DC four-probe method. This indicates that the arc represents the bulk resistance of the single crystal. As the temperature increased, the arc disappeared, and the tail separated into a new arc and a tail. These arcs and tails were attributed to the ionic transfer resistance on the surface of single crystal and the charge transfer resistance between the single crystal and electrodes. The results also revealed that the AC two-probe method is affected by the resistance of the platinum wires used in measurement, whereas the DC four-probe method provides a more accurate assessment of intrinsic resistivity. The novelty of this work lies in its direct comparison of AC and DC measurement techniques for a single crystal apatite-type oxide, offering new insight into the distinction between bulk and interfacial resistive behaviors and establishing a more reliable approach for evaluating oxide ion conduction in rare-earth silicate materials.
