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, Ce₃Ru₄Ge₁₃ and CeRuGe₃, have been studied, as well as the compound Y₃Co₄Ge₁₃, which contained a nonmagnetic rare earth as the reference material. Combined x-ray and neutron-diffraction studies of CeRuGe₃ and an x-ray study of Ce₃Ru₄Ge₁₃ show that we are dealing with internally distorted (probably 3D modulated) crystals with variable composition, i.e., Ce_4-xRu₄Ge_12+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 Ce_4-xRu₄Ge_12+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 Ce_4-xRu₄Ge_12+x is a heavy fermion with γ varying from 428 to 592 mJ/mol Ce³⁺K² 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 CeRuGe₃ is a spin-glass system below ~5 K, which is consistent with the specific features of its crystal structure. Y₃Co₄Ge₁₃ displays normal metallic behavior.