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 Pc, 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 Pc. 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 Pc. 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.
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