Structural Instability and Magnetic Ordering in a Uranium-based Temary Compound

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The uranium-based ternary compound UNiSn undergoes a semiconductor-metal transition and an antiferromagnetic transition simultaneously at 43K.In addition, we have obtained a preliminary cvidence that the complex transition is accompanied by lattice deformation. In the present study, we investigated how these three instabilities affect each other. Our works are summarized as follows.(1) An X-ray diffractometer was modified so that crystal structures are analyzed in magnetic field upto 8 tesla at low temperatures down to 3.5K.(2) We measured specific heat, Mossbauer spectra, magnetic susceptibility, X-ray diffraction and elastic moduli of UNiSn around 43K.(3) We found that applied magnetic field separates structural transition from antiferromagnetic transition.(4) Partial substitution of Th for U separates the system into two phases with different Th concentration. The antiferromagnetic transition temperature decreases in both phases with increasing The concentration.(5) The quadrupole ordering is responsible for the lattice deformation from a high-temperature cubic structure to a low-temperature orthorhombic one. This suggests the nature of the doublet ground state of f electrons in the uranium-based compounds.The experiments were extended to other ternary compounds which contain uranium or cerium.