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

Investigating the phase relations of the system ${\mathrm{Th},\mathrm{U}}$-Pt-B at $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ the formation of two compounds has been observed: cubic ${\mathrm{ThPt}}_{3}\mathrm{B}$ with $Pm\overline{3}m$ structure as a representative of the perovskites, and tetragonal ${\mathrm{UPt}}_{3}\mathrm{B}$ with $P4mm$ structure being isotypic to the noncentrosymmetric structure of ${\mathrm{CePt}}_{3}\mathrm{B}$. The crystal structures of the two compounds are defined by combined x-ray diffraction and transmission electron microscopy. Characterization of physical properties for ${\mathrm{ThPt}}_{3}\mathrm{B}$ reveals a superconducting transition at 0.75 K and an upper critical field at $T=0$ exceeding 0.4 T. For nonsuperconducting ${\mathrm{UPt}}_{3}\mathrm{B}$ a metallic resistivity behavior was found in the entire temperature range; at very low temperatures spin fluctuations become evident and the resistivity $\ensuremath{\rho}(T)$ follows non-Fermi liquid characteristics, $\ensuremath{\rho}={\ensuremath{\rho}}_{0}+A{T}^{n}$ 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 ${\mathrm{ThPt}}_{3}\mathrm{B}$ 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 ${\mathrm{UPt}}_{3}\mathrm{B}$ the DFT results agree well with the experimental findings.