Elastic behavior around metal–insulator transition in PrRu4P12

Abstract Elastic properties of PrRu 4 P 12 have been investigated by means of the ultrasonic measurement. A clear bend was observed in elastic constants C 11 , ( C 11 - C 12 ) / 2 and C 44 at metal–insulator transition temperature T MI of 62.3 K. Furthermore, C 11 , and ( C 11 - C 12 ) / 2 exhibits a pronounced elastic softening towards low temperature in the temperature range down to 1.5 K. This fact suggests strongly that Pr 3 + Γ 3 non-Kramers doublet ground state is realized in PrRu 4 P 12 under the crystalline electric field (CEF) potential. This also suggests that an orbital degree of freedom still remains below T MI 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.