QUADRUPOLE-STRAIN INTERACTION IN RARE-EARTH HEXABORIDES

Temperature dependence of the elastic constants in a series of rare ea rth hexaborides has been investigated by means of the ultrasonic metho d. The remarkable softening of the transverse C44 mode in PrB6, NdB6, DyB6 and HoB6 has been discussed in connection with the cooperative Ja hn-Teller effect due to the CEF ground state degenerated by the quadru polar moment. The low temperature behavior of C44 in CexLa1-xB6 compou nds is presented to show the Kondo effect on the quadrupolar moment of GAMMA8 ground state. The anomaly of the bulk modulus C(B) in SMB6 is related to the valence fluctuation state in the system.