Crystal Structure and Physical Properties of Uranium-Copper Oxyphosphide UCuPO

Single crystals of uranium- and thorium-copper oxyphosphide, UCuPO and ThCuPO, have been grown by an iode-transport method. The crystallographic parameters for UCuPO and ThCuPO are determined at room temperature by single crystal X-ray diffraction. These materials crystallize in a tetragonal ZrCuSiAs-type structure. UCuPO is confirmed to be a semimetallic antiferromagnet with a Néel temperature of T_N= 220 K, while ThCuPO shows non-magnetic metallic behavior. The specific heat measurements at temperatures much lower than T_N indicate that the density of states (DOS) for both compounds is quite low, and that the 5f component of DOS is fully gapped due to antiferromagnetic ordering in UCuPO. The magnetic susceptibility for a single crystal of UCuPO shows a large uniaxial anisotropy along the c axis in the paramagnetic state. The resistivity measurements for UCuPO under high pressures up to 2.45 GPa indicate a slight increase of the Néel temperature. When compared with the data for the reference material ThCuPO, the semimetallic conductivity may come mainly from Cu 4s-P 3p bands with low DOS, while the ordered and paramagnetic moments seem to be carried only by localized 5f electrons.