Origins of phase transitions in valence fluctuating YbPd

The cubic YbPd compound undergoes two first-order phase transitions at T₁ = 125K and T₂ = 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 T₁, the (h00) Bragg peaks (h: an integer) are split into two subpeaks. Below T₂, 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 T₁ and T₂ are probably the band Jahn-Teller effect and the valence order, respectively.