Structure and Physical Properties of Ternary Rare-Earth Cobalt Bismuth Intermetallics (RE)₁₂Co₅Bi (RE = Y, Gd, Tb, Dy, Ho, Er, Tm)

Abstract

The rare-earth intermetallic compounds (RE)12Co5Bi (RE = Y, Gd, Tb, Dy, Ho, Er, Tm) were prepared by arc-melting and annealing at 600 degrees C. These compounds extend the previously known (RE)6M2+xX1-x (M = Co, Ni; X = Ga, In, Sn, Pb) series with the Ho6Co2Ga-type structure to X = Bi. The crystal structure of Ho12Co5Bi was refined by the Rietveld method from powder X-ray diffraction data obtained using synchrotron radiation (Pearson symbol oI36, orthorhombic, space group Immm, Z = 2, a = 9.37598(14) A, b = 9.37871(14) A, c = 9.85465(13) A). Unlike other Ho6Co2Ga-type compounds, the 2a site in Ho12Co5Bi is exclusively occupied by Co atoms. Four-probe electrical resistivity measurements on sintered polycrystalline samples of (RE)12Co5Bi indicated metallic behavior. Magnetic measurements revealed behavior ranging from frequency-dependent maxima in the ac susceptibility for Y12Co5Bi to possible ferrimagnetic ordering for Gd12Co5Bi to antiferromagnetic ordering with metamagnetic transitions for the remaining compounds. As confirmed by band structure calculations using Y12Co5Bi as a model compound, Y-Y and Y-Co interactions are the most important bonding components, but matrix effects are likely responsible for anomalously short Co-Co contacts in the structure.

Isothermal section of the Ho–Co–Bi system at 800°C and new ternary Mo5B2Si-type bismuthides R5Co2Bi (R=Gd, Ho, Dy)

“…The density of states (DOS) and its atomic projections (Fig. 5a) closely resemble those of the related compounds ''Y 12 Co 5 Bi'' [5] and ''Y 5 Ni 2 Sb'' [12]. Most of the Co and Bi orbitals are localized in energy from À12 to À9 eV.…”

“…The solubility of the third component in the binary phases was less than 1 at.%. Three ternary compounds have been previously reported: Ho 12 Co 5 Bi [5], Ho 6 CoBi 2 [6,8], and Ho 5 CoBi 2 (the precise composition was Ho 5 Co 0.9 Bi 2 ) [9]. Their existence has been confirmed, but Ho 5 CoBi 2 was found here to be an endmember of a solid solution Ho 5 Co x Bi 3Àx with a significant homogeneity range of 0.65 < x < 1.0, as revealed by microstructural analysis and variations in lattice parameters.…”

“…3a and b). Ho 12 Co 5 Bi adopts its own structure type [5], an ordered variant based on the Ho 6 Co 2 Ga-type structure. The new compound Ho 5 Co 2 Bi (as well as its rare-earth analogues R 5 Co 2 Bi (R ¼ Gd, Dy)) extends the family of ternary rare-earth bismuthides with composition R 5 M 2 Bi, which was previously restricted to M ¼ Ni, Pd, Pt [12,13].…”

“…Atomic parameters (valence shell ionization potentials H ii (eV) and orbital exponents z i ) were taken from literature values for related compounds [5,12] and are as follows:…”

“…On the other hand, increasing numbers of compounds have now been identified among the R-M-Bi systems, with M being limited largely to the first-row or later transition metals. In particular, several R-Co-Bi phases are now known, which display interesting electrical and magnetic properties: R 12 Co 5 Bi (R ¼ Y, Gd-Tm) [5], R 6 CoBi 2 (R ¼ Y, Ho, Er, Tm) [6][7][8], R 5 CoBi 2 (R ¼ Gd-Er) [9], and La 6 Co 13 Bi [10]. We present here the isothermal section of the ternary Ho-Co-Bi phase diagram at 800 C. In addition to clarifying the nature of the previously known phases in this system, we report the structure of a new bismuthide, Ho 5 Co 2 Bi.…”