First-Order Phase Transition in a New CaCu5-Related Antimonide, CePt5Sb

A new CaCu5 related antimonide, CePt5Sb, has been identified. This ternary compound undergoes a structural phase transition at about 80 K according to room- and low-temperature X-ray and neutron diffraction, and measurements of electrical resistivity, specific heat and magnetism. The room temperature phase forms a new rhombohedral structure, space group R3̅, a = 0.53535(2) nm, c = 3.10814(12) nm and consists of alternating blocks of CaCu5- and MnCu2Al-type fragments that extend along the c-axis. The low-temperature phase is monoclinic, space group Cm, a = 0.91821(5) nm, b = 0.53696(1) nm, c = 1.08064(6) nm, β = 107.40(1)°. The unit cells of both structures (orthohexagonal and monoclinic) are geometrically related via the transformation matrix a′ = –b′, b′ = –a, c′ = 1/3b – 1/3c. Bulk properties elucidate the phase transition being of first-order and evidence Kondo interactions at low temperatures.