Elastic behavior around metal-insulator transition in PrRu4P12

Elastic properties of PrRu4P12 have been investigated by means of the ultrasonic measurement. A clear bend was observed in elastic constants C11, (C11 - C12)/2 and C44 at metal-insulator transition temperature TM1 of 62.3 K. Furthermore, C11, and (C11 - C12)/2 exhibits a pronounced elastic softening towards low temperature in the temperature range down to 1.5 K. This fact suggests strongly that Pr3} F3 non-Kramers doublet ground state is realized in PrRu4P12 under the crystalline electric field (CEF) potential. This also suggests that an orbital degree of freedom still remains below TM! and a quadrupolar ordering has nothing to do with the metal insulator transition. The elastic property and 4f ground state of Pr ions in this system will be discussed from the view point of CEF effect.