Phase Relations, Resistivity, and Electronic Structure of Cesium at High Pressures

Phase relations among Cs II, Cs III, Cs IV, and liquid have been determined by resistance and differential-thermal-analysis techniques. The triple-point coordinates among Cs II-III-liquid and Cs III-IV-liquid are (42.5 kbar, 88°C) and (43.9 kbar, 98°C), respectively. The volume and entropy changes have been evaluated for the melting of Cs II, III, and IV from the measured slopes and the known volume changes for the Cs II-III and Cs III-IV transitions and assuming negligible $\Delta S$ for Cs II-III transition. The resistivity of liquid Cs as a function of pressure has been measured to 50 kbar at 220°C. The resistance rises markedly but smoothly in the 20-40-kbar range and increases by a factor of 6 at 40 kbar. The striking increase in the resistance with pressure strongly suggests that the $6s-5d$ "electron collapse" which is presumed to occur abruptly at the Cs II-III transition might be occurring continuously in liquid Cs over a broad pressure range. In terms of a continuous "electronic collapse" in the liquid phase, the anomalous fusion behavior of Cs becomes readily understandable. Some metastability phenomena encountered in the freezing of Cs in the interval 42-44 kbar are described.