Thermal and Magnetic Properties of the Metallic Delafossite Triangular-Lattice Antiferromagnet PdCrO₂

The metallic delafossite antiferromagnet PdCrO₂ consists of triangular-lattice layers of Pd and CrO₂ alternating along its c-axis. While the Pd layers are metallic and host very high conductivity, the CrO₂ layers are Mott-insulating and generate antiferromagnetic (AF) order at T_N = 37.5 K. Much remains to be understood regarding the thermal and magnetic behavior, however. We address this here through wide-temperature-range X-ray diffraction (XRD), magnetic susceptibility, heat capacity, and synchrotron inelastic X-ray scattering (IXS) on high-quality chemical vapor transport (CVT) PdCrO₂ single crystals, combined with density functional theory (DFT) calculations. Surprisingly, PdCrO₂ is found to exhibit a large and unusual c-axis negative thermal expansion up to 300 K, while the a-axis shows typical behavior. As a result, the transverse acoustic phonon mode along the c-axis shows little softening across T_N, from 20 to 300 K. Specific heat data allow accurate separation of phononic, electronic, and magnetic contributions, in good agreement with DFT. Also surprising, high-purity CVT-grown PdCrO₂ has T_N = 38.1 K, slightly higher than in the literature, with no bifurcation of ZFC and FC magnetization curves. This is in contrast to prior work, suggesting that certain aspects of previously reported magnetic behavior could be extrinsic.