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 PrOs₄As₁₂. The longitudinal C_L111=C_B+4C₄₄/3 mode shows Curie-type softening below ~25 K, which is explained in terms of the quadrupole susceptibility modified by a crystalline electric field with a Γ₄⁽²⁾ ground state. A detailed H-T phase diagram for H || [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.

Phys. Rev. B 77 (2008) 094435,

also on J. Phys. Soc. Jpn. 77 (2008) Suppl. A, pp. 225-228.

Figure : Magnetic field vs. Temperature (H-T) phase diagram of PrOs₄As₁₂ with fields (a) H || [111] and (b) H || [1-10]. Data points are determined by a feature of elastic anomalies; upward or downward in C_L111 vs. H (solid circles and squares), C_L111 vs. T (open circles and squares); minimum in C_L111 vs. H (times), C_L111 vs. T (plus). Error bars on the markers indicate roughly estimated transition width. Solid lines are guides to the eyes, separating the PM region (phase I), AFM region (phase III) and unknown ordered phase II and II'.