Interplay between charge and magnetic orderings in YbPd

The pressure-temperature phase diagram of an intermediate-valence compound YbPd was revealed via simultaneous ac-calorimetry and electrical resistivity measurements. Two successive structural phase transition temperatures, T ₁ =125K and T₂ =110 K, are found to decrease with increasing pressure. The magnetic ordering transition at TN ∼ 1:9K decreases monotonically up to 1.7 GPa and disappears discontinuously at Pc ∼ 1:9 GPa, where the structural phase transition at T₂ is also suppressed. At P_c, enhancement of the effective mass and residual resistivity are observed. Another probable magnetic phase transition, however, is found to take place at TML ∼ 0:3K and smoothly varies even when crossing P_c. Structural phase transitions at ambient pressure were also studied by a single crystal X-ray diffraction measurement. Below T₂, a tetragonal lattice distortion with doubling of the unit cell along the c-axis is observed, and thus there are two inequivalent Yb-sites below P_c. These results indicate that one Yb-site, having larger valence configuration and thus smaller Kondo temperature, orders at TN. The other phase transition at TML seems to couple to the crystal structure for T₂ < T < T1. YbPd is a unique system exhibiting magnetic orderings in metallic charge-ordered Yb systems.