Non-Vegard behavior of the UxLa1-xS system

We report extended x-ray-absorption fine structure measurements performed at the La K edge and the U L_III edge of 11 samples in the U_xLa_1-xS system. All samples have the simple NaCl fcc crystal structure. The measurements reveal a complex local variation of the first- (NN) and second-neighbor (NNN) distances. The La-S distances d_LaS are systematically larger than the U-S distances d_US and almost concentration independent for x⩽0.5. Around this concentration d_LaS starts to decrease linearly with x. The study of the second-neighbor distances confirms a non-Vegard behavior of the lattice parameter evolution with La substitution. Near x∼0.5 the static disorder in the solid solution strongly increases.