Elastic soft mode and charge ordering of Yb4As3

The elastic constant of C-44 in Yb4As3 relating to the transverse ultrasonic mode shows a pronounced softening around the structural phase transition point at T-c = 292 K associated with the charge ordering of Yb3+ and Yb2+ states. The group theoretical analysis leads to the charge fluctuation mode with Gamma(5) symmetry coupled to the elastic strain epsilon(yz), epsilon(zx), epsilon(xy) of the soft C-44 mode. In a framework of the Landau phenomenological theory for the phase transition, the order parameter Q(yz) = Q(zx) = Q(xy) not equal 0 below T-c gives rise to the linear chain of Yb3+ ions among the body diagonal [111] direction as well as the trigonal distortion of epsilon(yz) = epsilon(zx) = epsilon(xy) not equal 0. Furthermore, this theory explains experimental observations of the elastic softening C-44 = C-44(0)(T-T-c(0))/(T-Theta) above T-c, the structural change from cubic phase of T-d(6) to trigonal phase of C-3 upsilon(6), and the first-order nature of the transition. [S0163-1829(99)00301-X].