We measured the electrical resistivity, magnetic susceptibility, magnetization, and specific heat of the quasicrystal approximants RCd 6 (R: rare earth, Y-Lu) with a body-centered cubic (bcc) crystal structure. Single crystals were grown by the Cd-self flux method and annealing method. We confirmed that the structural order-disorder transition is realized at about 160K when the lattice constant a is larger than 15.481 A in YCd 6, namely, for R = Pr, Nd, Sm, Gd, Tb, Dy, and Yb. At lower temperatures, RCd 6 compounds, except non-4f reference compounds YCd6 and LuCd 6, and a divalent compound YbCd 6, are found to order antiferromagnetically. We clarified that the structural order-disorder transition has a great influence on the magnetic ordering and transport properties. The Néel temperature of RCd 6 (R: Nd, Sm, Tb, and Dy) with the structural order-disorder transition is appreciably higher than the de Gennes scaling normalized by the Néel temperature of GdCd 6, while the the Néel temperature of RCd 6 (R: Ho, Er, and Tm) without the structural transition approximately follows the de Gennes scaling. Moreover, the electrical resistivity of RCd 6 with the structural transition decreases monotonically below the Néel temperature, while it increases below the Néel temperature and a large residual resistivity remains at low temperatures in RCd 6 without the structural transition. The contribution of an Einstein oscillator to the phonon specific heat is also discussed in YCd 6 and LuCd 6.
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