Structural, magnetic, and superconducting properties of caged compounds ROs₂Zn₂₀ (R = La, Ce, Pr, and Nd)

The electrical resistivity, magnetization, and specific heat of the caged compounds ROs₂Zn₂₀ (R = La, Ce, Pr, and Nd) have been measured to study their structural, magnetic, and superconducting properties. These measurements indicate that the compounds undergo structural transitions at T_s = 151, 109, 87, and 62 K, respectively. The decrease in T_s along with the lanthanoid contraction suggests that the high-temperature phase is more stable for a smaller lattice volume. Analysis of the lattice specific heat of LaOs₂Zn₂₀ reveals that the Zn atom at the 16c site encapsulated in the R₂Zn₁₂ cage vibrates at a low energy of 3 meV. For CeOs₂Zn₂₀, the small magnetic susceptibility with a broad maximum indicates the valence-fluctuating state of the Ce ions. In PrOs₂Zn₂₀, the crystalline electric field ground state of the 4f² state of the Pr³⁺ ion remains in a non-Kramers doublet at T > T_s, which is lifted by symmetry lowering of the Pr site at T < T_s. Thereby, the quadrupolar degrees of freedom are quenched, avoiding the long-range quadrupole order. PrOs₂Zn₂₀ and LaOs₂Zn₂₀ exhibit superconducting transitions at 0.06 and 0.07 K, respectively. (author)