Ferroquadrupolar Ordering due to the Quasi-degenerate Quartet in the Trigonal Chiral Structure of DyNi^sub 3^Ga^sub 9

To clarify the origin of the phase transition at TN = 10 K 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 TN. 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 TN. Below TN, crystal symmetry will lower from the trigonal structure. Elastic softening below TN occurs in the other elastic moduli, reflecting that some kind of instability is still alive below TN. Elastic anomalies detected below TN suggest successive phase transitions.