Critical cluster size for mixed valence in small matrix-isolated Sm clusters

From $L_3$ absorption measurement, information on the valence and size of small matrix-isolated Sm clusters is obtained by analyzing the x-ray-absorption near-edge structure and the extended x-ray-absorption fine structure (EXAFS). Depending on the mean cluster size, which is a function of the atomic concentration (Sm to rare-gas atom ratio), every valence between 2.0 and 3.0 is reproducibly achievable. At very low Sm concentrations the samples have the atomic valence; at a critical concentration c≊1:100 a steep rise of the valence up to v=2.6 is found. Then, with increasing metal concentration, the Sm-cluster valence approaches the valence of the solid. From EXAFS analysis, a mean cluster size of 13 atoms for clusters with v=2.6 follows. The mixed valence of the Sm atoms is explained by the pressure exerted on the Sm cluster by the distorted rare-gas lattice (the rare gas is Ne, Ar, or Kr).