Origins of Phase Transitions in Valence Fluctuating YbPd

The cubic YbPd compound undergoes two first-order phase transitions at T-1 = 125 K and T-2 = 105 K. We report the experimental results of the X-ray diffraction (XRD) and electron diffraction of single crystals of YbPd at low temperatures. Below T-1, the (h00) Bragg peaks (h: an integer) are split into two subpeaks. Below T-2, the superlattice reflections of (n/2, 0, 0) (n: an odd number) in the XRD patterns and the superlattice spots of (1/2, 0, 0), (0, 1/2, 0), and (1/2, 1/2, 0) in the electron diffraction profile appear, which suggests the valence order of YbPd. We discuss two possible structures, assuming a tetragonal symmetry below T-1. The origins of the transitions at T-1 and T-2 are probably the band Jahn-Teller effect and the valence order, respectively.