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, $T_{1} = 125$ K and $T_{2} = 110$ K, are found to decrease with increasing pressure. The magnetic ordering transition at $T_{\text{N}} \sim 1.9$ K decreases monotonically up to 1.7 GPa and disappears discontinuously at $P_{\text{c}}\sim 1.9$ GPa, where the structural phase transition at $T_{2}$ is also suppressed. At $P_{\text{c}}$, enhancement of the effective mass and residual resistivity are observed. Another probable magnetic phase transition, however, is found to take place at $T_{\text{ML}}\sim 0.3$ K and smoothly varies even when crossing $P_{\text{c}}$. Structural phase transitions at ambient pressure were also studied by a single crystal X-ray diffraction measurement. Below $T_{2}$, 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 $P_{\text{c}}$. These results indicate that one Yb-site, having larger valence configuration and thus smaller Kondo temperature, orders at $T_{\text{N}}$. The other phase transition at $T_{\text{ML}}$ seems to couple to the crystal structure for $T_{2} < T < T_{1}$. YbPd is a unique system exhibiting magnetic orderings in metallic charge-ordered Yb systems.