Non-Vegard behavior of the UxLa1ÀxS system

We report extended x-ray-absorption fine structure measurements performed at the LaKedge and the ULIII edge of 11 samples in the UxLa12xS 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 distancesdLaSare systematically larger than the U-S distancesdUSand almost concentration independent forx<0.5. Around this concentrationdLaSstarts to decrease linearly withx. The study of the second-neighbor distances confirms a non-Vegard behavior of the lattice parameter evolution with La substitution. Nearx ;0.5 the static disorder in the solid solution strongly increases.