Crystal structure and magnetic properties of new cubic quaternary compounds RT₂Sn₂Zn₁₈ (R = La, Ce, Pr, and Nd, and T = Co and Fe)

The new cubic quaternary intermetallic compounds RT₂Sn₂Zn₁₈ (R = La, Ce, Pr, and Nd, and T = Co and Fe) were synthesized by the mixture-metal flux method using Zn and Sn. The crystal structure was investigated by powder X-ray diffraction and with a four-circle X-ray diffractometer using single crystals. The space group of the compounds is Fd3m (No. 227). The rare-earth atom is at the cubic site which is the center of a cage composed of Zn and Sn atoms. The crystal structure is the same as the CeCr₂Al₂₀-type crystal structure except the atoms at the 16c site, i.e., the Zn atoms at the 16c site are completely replaced by Sn atoms, indicating that the compounds are crystallographically new ordered quaternary compounds. The lattice parameter a and the physical properties of the magnetic susceptibility χ, the magnetization M, and the specific heat C of these cubic caged compounds were investigated. LaCo₂Sn₂Zn₁₈ and LaFe₂Sn₂Zn₁₈ are enhanced Pauli paramagnets that originate from the Co and Fe itinerant 3d electrons. CeCo₂Sn₂Zn₁₈and CeFe₂Sn₂Zn₁₈ are also enhanced Pauli paramagnets that originate from both the 3d electrons and Ce 4f electrons. PrCo₂Sn₂Zn₁₈ and PrFe₂Sn₂Zn₁₈ are nonmagnetic materials with huge values of C divided by temperature, which indicates that the ground state of Pr ions is a non-Kramers' doublet. NdCo₂Sn₂Zn₁₈ and NdFe₂Sn₂Zn₁₈ are magnetic materials with the Néel temperatures of 1.0 and 3.8 K, respectively. All eight compounds have large magnetic moments of Co/Fe in the paramagnetic temperature region, and thus their magnetic moments are inferred to be magnetically frustrating owing to the pyrochlore lattice in the low-temperature region.