Intermediate-valent cerium in Ce _2Ru _4Mg _(17) and a group-subgroup scheme for La _9Ru _4In _5 and Ce _9Ru _4Ga _5

摘要

Ce _2Ru _4Mg _(17) was synthesized by high-frequency melting of the elements in a sealed tantalum ampoule. This magnesium-rich compound crystallizes with a new tetragonal structure type: I4? 2m, a = 986.75(8), c = 1008.7(1) pm, wR2 = 0.0513, 909 F ~2 values and 34 variables. The striking structural motifs in the Ce _2Ru _4Mg _(17) structure are slightly bent CeRu _2 units with short Ce-Ru distances of 231 pm and additionally a short Ce-Ce distance of 307 pm. These features are a direct consequence of the cerium valence. The CeRu _2 units are embedded in a magnesium-rich matrix with a broad range of Mg-Mg distances (291-361 pm). Temperature-dependent magnetic susceptibility data show intermediate-valent behavior of the cerium atoms (0.23(5) μ _B per Ce atom) and no magnetic ordering down to 3 K, indicative of almost tetravalent cerium in Ce _2Ru _4Mg _(17)· The ceriumrich gallide Ce _9Ru _4Ga _5 shows an unusually short Ce-Ru distance of 237 pm for the Ce2 position as a result of an intermediate cerium valence. The structural distortions are discussed on the basis of a group-subgroup scheme for La _9Ru _4In _5 (space group I4/mmm) and the superstructure variant Ce _9Ru _4Ga _5 (space group I4mm).