Ultrasonic Study of the Filled Skutterudite Compounds under Hydrostatic Pressure

We have developed a clamped piston-cylinder high pressure cell and performed ultrasonic measurements on the filled skutterudite compounds under hydrostatic pressures. Technical details and recent advances in the researches were reviewed by showing one of the compounds SmRu4P12 which exhibits a metal-insulator transition at TMI = 16.5 K and an additional transition at TN = 14 K. A pronounced elastic softening toward the additional temperature was induced in the application of pressure. Present results demonstrated that ultrasonic measurements on the filled skutterudite compounds under hydrostatic pressure are a powerful method to explore and elucidate a phase transition associated with a multipolar ordering.