Experimental study of lattice dynamics in LaB6 and YbB6

Lattice dynamics of La 11 B 6 was studied on the neutron triple axis spectrometers at HFIR. The acoustic and optical modes were measured in the high symmetry directions. The rotary motion of the B 6 molecule was measured only near the zone boundaries but shows no softening. Instead, very flat low frequency La modes were observed. The agreement with the elastic constants and the temperature dependence of specific heat is satisfactory. In YbB 6 , the elastic constants and specific heat were measured. Both results give the quite different feature from those of LaB 6 , no softening of C 12 and more softened flat low frequency Yb modes, due to the lack of the metallic bonds.