Synthesis and characterisation of a potential ceramic sulfide ion conductor based on the solid solution xCaS:Nd2S3, x = 0.7–1.0

Abstract

The effect of defect ion concentration on conductivity in the CaNd2S4 system has been investigated by varying the CaS:Nd2S3 ratio. Samples with the formula xCaS:Nd2S3 x = 0.7–1.0 have been prepared using solid state methods by high temperature synthesis in evacuated quartz tubes followed by annealing in hydrogen sulfide. The structures of the materials were determined using the Rietveld refinement of powder neutron diffraction data. The phases crystallise with a defect version of the cubic Th3P4 structure in the space group I-43d with cell parameter a ≈ 8.53 A. Temperature programmed oxidation (TPO) in a 5 Vol.% O2/Ar atmosphere show that the materials resist oxidation up to a temperature of approximately 680 °C. TPO also indicates that there is only one type of sulfide ion present in the system, based on the presence of only one peak in the TPO trace. In-situ impedance spectroscopy was carried out in an Ar and H2S/Ar atmosphere between 300 °C and 500 °C. Bulk conductivities, activation energies and time constants for ion hopping processes were determined. The sample, 0.9CaS:Nd2S3, was found to exhibit three impedance arcs in the Nyquist plot suggesting ionic conduction. The independence of the material’s bulk conductivity when exposed to an H2S/Ar atmosphere supports this statement. The xCaS:Nd2S3 sample with x ≤ 0.8 show strong electronic contributions to the overall conductivity. The total ionic conductivity for 0.9CaS:Nd2S3 of 1.09 × 10−6 S cm−1 measured at 500 °C, matches conductivity values reported using galvanic cells.