SPECIFIC-HEAT AND MAGNETIC-SUSCEPTIBILITY OF U1-XTHXNISN

The specific-heat C and magnetic susceptibility chi of U1-xThxNiSn wer e measured down to 1.5 K. Pure UNiSn (x = 0) shows an antiferromagneti c (AF) transition accompanied by a semiconductor-metal transition. The magnetic entropy S(m) for UNiSn abruptly increases at T(N) suggesting a first-order phase transition. In addition to an enhancement of the density of states at the Fermi level associated with the metallic phas e, the low-temperature specific heat suggests a magnon excitation in t he AF state. In the paramagnetic state, the x-independent S(m) (T) and chi(m) (T) are explained by a model based on the localized 5f electro ns in a crystalline electric field. Assuming the U4+ ion has a 5f2 con figuration, we determined the energy scheme of U4+, where the ground s tate is a nonmagnetic GAMMA3 doublet. The first-order AF transition, r eduction of the ordered moment, and enhancement of chi in the AF state are discussed.