Ferroelectrics and superionic conductors

Abstract

I had been working at the same institute (ISSP) with Professor Nakamura for more than twenty years and during this time I was given invaluable instruction and stimulation by him. In the latter half of that period, I mainly studied the structure and dynamics of superionic conductors, apart from the research on proper ferroelectrics. In 1978, we found a new phase transition of Ag₃SI at low temperature from its superionic β-phase to the γ-phase with an ordered structure of the space group C₃ ⁴ – R3.^1–2 Because of the structure of γ-Ag₃SI, there was a possibility that γ-Ag₃SI showed ferroelectricity and so we tried to measure the polarization reversal by ac electric field. If it had been detected, Ag₃SI would have shown a ferroelectric-superionic phase transition. When we were repeating the measurements, Professor Nakamura came into our laboratory frequently and gave us stimulating encouragement. Unfortunately, the polarization reversal could not be measured because of a high coercive field at low temperature and the nature of the ionic conduction near the transition point, but we did find the pyroelectricity of γ-Ag₃SI.³ β-Ag₃SI has an anti-perovskite structure with a disordered arrangement of silver ions which is very similar to the structure of the highest temperature phase of CsPbCl₃, ⁴ Later we found⁵ that the low temperature transition of Ag₃SBr, having the same β-phase structure as Ag₃SI, resembled one of the transitions in CsPbCl₃.