We report specific heat ($C$) and magnetization ($M$) of single crystalline Ce$_4$Pt$_{12}$Sn$_{25}$ at temperature down to $sim$50mK and in fields up to 3T. $C/T$ exhibits a sharp anomaly at 180mK, with a large $Delta C/Tsim$30J/molK$^2$-Ce, which,
together with the corresponding cusp-like magnetization anomaly, indicates an antiferromagnetic (AFM) ground state with a Neel temperature $T_N$=180mK. Numerical calculations based on a Heisenberg model reproduce both zero-field $C$ and $M$ data, thus placing Ce$_4$Pt$_{12}$Sn$_{25}$ in the weak exchange coupling $J<J_c$ limit of the Doniach diagram, with a very small Kondo scale $T_Kll T_N$. Magnetic field suppresses the AFM state at $H^*approx$0.7T, much more effectively than expected from the Heisenberg model, indicating additional effects possibly due to frustration or residual Kondo screening.
We report the properties of two new isostructural compounds, U3Bi4Ni3 and U3Bi4Rh3. The first of these compounds is non-metallic, and the second is a nearly ferromagnetic metal, both as anticipated from their electron count relative to other U-based
members of the larger 3-4-3 family. For U3Bi4Rh3, a logarithmic increase of C/T below 3 K, a resistivity proportional to T^4/3, and the recovery of Fermi-liquid behavior in both properties with applied fields greater than 3T, suggest that U3Bi4Rh3 may be a new example of a material displaying ferromagnetic quantum criticality.
