Superconductivity and spin fluctuations in the actinoid-platinum metal borides {Th,U}Pt3B

Investigating the phase relations of the system {Th,U}-Pt-B at 900C the formation of two compounds has been observed: cubic ThPt3B with Pm3¯m structure as a representative of the perovskites, and tetragonal UPt3B with P4mm structure being isotypic to the noncentrosymmetric structure of CePt3B. The crystal structures of the two compounds are defined by combined x-ray diffraction and transmission electron microscopy. Characterization of physical properties for ThPt3B reveals a superconducting transition at 0.75 K and an upper critical field at T=0 exceeding 0.4 T. For nonsuperconducting UPt3B a metallic resistivity behavior was found in the entire temperature range; at very low temperatures spin fluctuations become evident and the resistivity ρ(T) follows non-Fermi liquid characteristics, ρ=ρ0+ATn with n=1.6. Density functional theory (DFT) calculations were performed for both compounds for both types of structures. They predict that the experimentally claimed cubic structure of ThPt3B is thermodynamically not stable in comparison to a tetragonal phase, with a very large enthalpy difference of 25 kJ/mol, which cannot be explained by the formation energy of B vacancies. However, the presence of random boron vacancies possibly stabilizes the cubic structure via a local strain compensation mechanism during the growth of the crystal. For UPt3B the DFT results agree well with the experimental findings.