Crystal structures and low-temperature behaviors of the heavy-fermion compounds CeRuGe3 and Ce3Ru4Ge13 containing both trivalent and tetravalent cerium.

Abstract

Two different samples of the Ce-Ru-Ge ternary system of nearly the same composition, ${\mathrm{Ce}}_{3}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{13}$ and ${\mathrm{CeRuGe}}_{3}$, have been studied, as well as the compound ${\mathrm{Y}}_{3}$${\mathrm{Co}}_{4}$${\mathrm{Ge}}_{13}$, which contained a nonmagnetic rare earth as the reference material. Combined x-ray and neutron-diffraction studies of ${\mathrm{CeRuGe}}_{3}$ and an x-ray study of ${\mathrm{Ce}}_{3}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{13}$ show that we are dealing with internally distorted (probably 3D modulated) crystals with variable composition, i.e., ${\mathrm{Ce}}_{4\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{12+\mathit{x}}$ and ranging from at least x=0 (1:1:3) to x=1 (3:4:13). We have measured the dc magnetic susceptibility, the ac susceptibility, the zero-magnetic-field heat capacity, the magnetic heat capacity in fields ranging from 0 to 9.85 T, and the electrical resistivity. The paramagnetic susceptibility and x-ray crystallography data indicate that the ${\mathrm{Ce}}_{4\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{12+\mathit{x}}$ alloys cotain both trivalent cerium and tetravalent cerium in a 1 to 3 ratio. As far as we are aware, this is the first time such a valence situation has been reported for a cerium compound. In addition, the heat-capacity results show that ${\mathrm{Ce}}_{4\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{12+\mathit{x}}$ is a heavy fermion with \ensuremath{\gamma} varying from 428 to 592 mJ/mol ${\mathrm{Ce}}^{3+}$${\mathrm{K}}^{2}$ at the 1:1:3 and 3:4:13 compositions, respectively. Zero-field and magnetic-field heat capacity, and ac magnetic susceptibility data suggest that ${\mathrm{CeRuGe}}_{3}$ is a spin-glass system below \ensuremath{\sim}5 K, which is consistent with the specific features of its crystal structure. ${\mathrm{Y}}_{3}$${\mathrm{Co}}_{4}$${\mathrm{Ge}}_{13}$ displays normal metallic behavior.