ANOMALOUS MAGNETIC TRANSITION IN UNISN

In concurrence with an antiferromagnetic transition at T-N similar or equal to 43 K, UNiSn undergoes a semiconductor-to-metal phase transiti on. The powder X-ray diffraction reveals that the complex transition i s accompanied also by a structural transition from a cubic to tetragon al symmetry. The temperature dependence of lattice parameters and that of Mossbauer spectra suggest that the transition is of first order an d is a cooperative phenomenon of the structural and magnetic instabili ties. However, an elastic modulus behaves as if the transition were of second order. This puzzle is solved by a scenario based on the crysta lline-electric-field for 5f(2) configuration in U4+ with the ground st ate of a non-magnetic doublet Gamma(3) in the cubic phase.