Ferroquadrupolar ordering due to the quasi-degenerate quartet in the trigonal chiral structure of DyNi₃Ga₉

To clarify the origin of the phase transition at T_N = 10K in the trigonal chiral compound DyNi₃Ga₉, we performed ultrasonic measurements on single-crystalline samples. We found a significant elastic softening of the transverse modulus C₆₆ toward T_N. By analyzing the data using the strain susceptibility, the positive sign of the quadrupole-quadrupole coupling constant for C₆₆ was obtained. These results strongly suggest that ferroquadrupolar ordering will be dominant for the phase transition at T_N. Below T_N, crystal symmetry will lower from the trigonal structure. Elastic softening below T_N occurs in the other elastic moduli, reflecting that some kind of instability is still alive below T_N. Elastic anomalies detected below T_N suggest successive phase transitions.