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

Abstract

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 has been determined from the shift of the Bragg peaks and from the anomaly in the resistivity. The critical pressure, P s ∼ 4.9 GPa at 300 K, decreases to P s ∼ 3.6 GPa at 10 K. The decrease of P s is due not only to the decrease in volume of the unit cell but also to an anisotropic shrinkage by 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 valence instability of Ce-4 f electron.

Mean-field Study of Antiferromagnetic and Antiferroquadrupolar Orderings in Tetragonal CeCoSi

“…[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58] CeCoSi is one of the potential candidates with AFQ orderings in the tetragonal crystal structure, where the CEF levels consist of only the Kramers pairs. [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73] This material shows the signature of two ordered phases including a higherrank multipolar ordered phase, which is denoted as II and III phases in addition to a high-temperature paramagnetic phase (I phase); the low-temperature III phase was indicated to be characterized by an in-plane antiferromagnetic (AFM) spin configuration with ordering vector q = 0 and the II phase was suggested to be an AFQ phase, 60-62, 64, 70) although the order parameter of the II phase has not been identified yet. Since the q = 0 order described by the staggered-type alignment of the order parameters on two Ce ions [Ce (A) and Ce (B) in Fig.…”