Ultrasonic investigation of field-dependent ordered phases in the filled skutterudite compound PrOs 4 As 12

Elastic constant measurements down to 0.48 K and in magnetic fields up to 11 T were performed on a single crystal of the filled skutterudite compound ${\text{PrOs}}_{4}{\text{As}}_{12}$. The longitudinal ${C}_{L111}={C}_{B}+4{C}_{44}/3$ mode shows Curie-type softening below $\ensuremath{\sim}25\text{ }\text{K}$, which is explained in terms of the quadrupole susceptibility modified by a crystalline electric field with a ${\ensuremath{\Gamma}}_{4}^{(2)}$ ground state. A detailed $H\text{\ensuremath{-}}T$ phase diagram for $H\ensuremath{\parallel}[111]$ is compiled with the elastic anomalies, which appear on the boundaries of two ordered phases at temperatures below 2.3 K and in fields below 3 T. The intermediate phase between 2 and 2.5 T assumes the aspect of antiferroquadrupolar ordering.

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