Phase relationship of CsH₂PO₄-CsPO₃ system and electrical properties of CsPO₃

The dehydration behavior of the paraelectric phase of CsH ₂PO₄ was investigated by thermogravimetric and X-ray diffraction analyses, and then the phase diagrams of CsH₂PO ₄-CsPO₃ system were established. The relationship between the onset temperature of dehydration (T_dehy/K) and the partial pressure of water (pH₂O/atm) is log pH₂O=7.62(±1. 18) -4.42(±0.56)(1000/T_dehy) below 228°C. The thermodynamically stable phase just above Tdehy is the fully dehydrated product CsPO₃ (s), although partially dehydrated products transiently appeared in the course of the dehydration to CsPO₃. Such developments allowed us to complete the temperature-humidity phase diagram and to regard the composition-temperature phase diagram as the eutectic type. This paper also reports the phase transition and electrical properties of CsPO₃ examined by differential thermal analysis, ac impedance spectroscopy, and dc polarization measurement. The just-synthesized CsPO₃ showed the relatively high electrical conductivity in unhumidified Ar. Ionic conductivity as high as 5× 10^-4 S cm^-1 was observed on heating from 150 to 450°C. Such high ionic conductivity disappeared after first heating up to 620°C and was explained by the proton diffusion through the absorbed H₂O. At around 600°C, a high-temperature phase of CsPO₃ showed the electrical conductivity as high as 10^-3 S cm^-1. However, this conductivity was not purely ionic.