Effects of valence and intermediate valence on the compressibility of the rare-earth hexaborides

The room temperature compressibilities of the rare-earth hexaborides LaB6, SmB6, EuB6, and GdB6 have been measured by single-crystal X-ray diffraction techniques under hydrostatic pressure conditions of 0-6 GPa. The bulk-moduli of the integral-valent MB6's have both valence dependent and independent contributions. The intermediate valence SmB6 is approximately 20 percent softer than the others and shows more curvature in the P-V data. This behavior indicates that as the Sm ion collapses under pressure its valence increases, but this process becomes increasingly difficult as the trivalent state is approached. (Auth.)