Magnetic instabilities and Kondo lattice behaviour of YbPt5B2, a new boride compound

Novel ternary platinum borides YbPt5B2, LuPt5B2 and YPt5B2 have been synthesized by arc melting of constituent elements and subsequent annealing at 1020 K. The unit cell of YbPt5B2 (space group C2/m, a=15.4982(6) Å, b=5.5288(3) Å, c=5.5600(3) Å, β=105.367°, single crystal X-ray diffraction data) is composed of two structural fragments alternating along the a axis and extending infinitively along the b axis: (i) condensed via common edges columns of face-fused boron filled and empty trigonal prisms, and (ii) zig-zag chains of trigonal prisms connected by common edges. YPt5B2 and LuPt5B2 were found to be isotypic (Rietveld refinement of powder X-ray diffraction data). YbPt5B2 exhibits two magnetic phase transitions at T ~ 8 K and T ~ 4 K as deduced from specific heat, magnetostriction, electric resistivity and magnetization data. Externally applied magnetic fields are responsible for further field induced phase transitions. The quite complex magnetic features appear to be the result of RKKY interaction and the Kondo effect in context of crystalline electric field effects. A magnetic phase diagram is established using specific heat, magnetostriction and magnetoresistance data. The nonmagnetic isotypes LuPt5B2 and YPt5B2 d are characterised by a simple metallic behavior.