Ultrasonic study of the filled skutterudite compound Pr1−xLaxFe4P12 (x = 0.05 and 0.15)

We have performed ultrasonic measurements on the filled skutterudite compound Pr1−xLaxFe4P12 (x = 0.05 and 0.15) to investigate the elastic properties and the 4f electric state of Pr ions. La-substitution causes a drastic change of the elastic constants compared with those of PrFe4P12, suggesting that the interplay between Pr ions plays a crucial role in the low temperature properties. Characteristic behavior was observed in the temperature dependence of elastic constants C11, (C11−C12)/2 and C44 at low temperatures. The elastic constants exhibit a steep decrease around a ferromagnetic transition. Furthermore, a characteristic dip was observed around 10 K, possibly due to the low-lying 4f levels of the Pr ion formed by a crystalline electric field effect (CEF). The results obtained could be explained reasonably by a pseudo-quartet ground state: Γ1–Γ4(1). We discuss the CEF ground state of Pr1−xLaxFe4P12 and the origin of the ferromagnetic transition induced by the La-substitution.

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