ANOMALOUS MAGNETIC TRANSITION AND LATTICE DEFORMATION IN UNISN

Powder X-ray diffraction and ultrasonic experiments were performed for polycrystalline UNiSn in a temperature range between 4.2 and 300 K. A discontinuous change has been found in the crystalline structure from cubic to tetragonal symmetry at the antiferromagnetic (AF) transition point T(N), reflecting the first-order nature of the AF transition. T his result can be understood by taking account of a CEF energy scheme of U4+ with a nonmagnetic doublet ground state GAMMA3 and a magnetic f irst excited state GAMMA4.