Structural Phase Transition and Possible Valence Instability of Ce-4f Electron Induced by Pressure in CeCoSi

X-ray powder diffraction and electrical resistivity measurements were performed on the tetragonal compound CeCoSi under pressure to elucidate the phase boundary of the pressure-induced structural transition and the change in the 4 f electronic state. The temperature???pressure phase diagram was determined from the shift of the Bragg peaks and from the anomaly in the resistivity. The critical pressure, Ps - 4.9 GPa at 300 K, decreases to Ps - 3.7 GPa at 10 K. The decrease in Ps is caused not only by the decrease in volume of the unit cell but also by anisotropic change in the unit cell due to cooling. When crossing the boundary to the high-pressure phase, the resistivity shows a significant drop to exhibit a metallic temperature dependence. The results of this study strongly suggest that the structural phase transition can be ascribed to the valence instability of the Ce-4f electron.