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The magnetic and thermodynamic properties of the completeLn$_{2/3}$Cu$_3$Ti$_4$O$_{12}$ series were investigated. Here $Ln$ stands forthe lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. %Mostof the compounds were prepared as single phase polycrystalline powder %withoutany traces of impurities. Marginal amounts of %impurities $(< 2%)$ weredetected $Ln=$ Gd, Er, and Tm. %Significant amounts of impurity phases werefound for $Ln=$ Ce and Yb. All the samples investigated crystallize in thespace group $Im\bar{3}$ with lattice constants that follow the lanthanidecontraction. The lattice constant of the Ce compound reveals the presence ofCe$^{4+}$ leading to the composition Ce$_{1/2}$Cu$_3$Ti$_4$O$_{12}$. Frommagnetic susceptibility and electron-spin resonance experiments it can beconcluded that the copper ions always carry a spin $S=1/2$ and orderantiferromagnetically close to 25\,K. The Curie-Weiss temperatures canapproximately be calculated assuming a two-sublattice model corresponding tothe copper and lanthanide ions, respectively. It seems that the magneticmoments of the heavy rare earths are weakly coupled to the copper spins, whilefor the light lanthanides no such coupling was found. The $4f$ moments remainparamagnetic down to the lowest temperatures, with the exception of the Tmcompound, which indicates enhanced Van-Vleck magnetism due to a non-magneticsinglet ground state of the crystal-field split $4f$ manifold. Fromspecific-heat measurements we accurately determined the antiferromagneticordering temperature and obtained information on the crystal-field states ofthe rare-earth ions. The heat-capacity results also revealed the presence of asmall fraction of Ce$^{3+}$ in a magnetic $4f^1$ state.

On the magnetism of Ln{2/3}Cu{3}Ti{4}O{12} (Ln = lanthanide)