The antiferroquadrupole orderings of the ternary rare earth compounds HoB2C2 and DyB2C2 have been investigated by means of ultrasonic measurements. The transverse (C11-C12)/2, C44, C66 modes in HoB2C2 exhibit characteristic softening above Tc2 = 5.0 K, which consist with an E-doublet and a singlet for the ground state. Considerable elastic softening and ultrasonic attenuation in phase IV below Tc1 = 5.9 K of HoB2C2 indicates an enhancement of the quadrupole fluctuation with a relaxation rate \tau =7\times 10-9 sec. A softening of the transverse elastic constant C44 in DyB2C2 above TQ = 24.7 K indicates accidentally degenerated Kramers doublets of E1/2 and E3/2 for the ground state.
PACS numbers: 71.27.+a, 74.50.+r
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