The magnetic and thermodynamic properties of the complete Ln<Subscript>2/3</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subs cript>O<Subscript>12</Subscript> series were investigated. Here Ln stands for the lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. All the samples investigated crystallize in the space group <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$Im\bar{3}$</EquationSource> </InlineEquation> with lattice constants that follow the lanthanide contraction. The lattice constant of the Ce compound reveals the presence of Ce<Superscript>4+</Superscript> leading to the composition Ce<Subscript>1/2</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subs cript>O<Subscript>12</Subscript>. From magnetic susceptibility and electron-spin resonance experiments it can be concluded that the copper ions always carry a spin S=1/2 and order antiferromagnetically close to 25 K. The Curie-Weiss temperatures can approximately be calculated assuming a two-sublattice model corresponding to the copper and lanthanide ions, respectively. It seems that the magnetic moments of the heavy rare earths are weakly coupled to the copper spins, while for the light lanthanides no such coupling was found. The 4f moments remain paramagnetic down to the lowest temperatures, with the exception of the Tm compound, which indicates enhanced Van-Vleck magnetism due to a non-magnetic singlet ground state of the crystal-field split 4f manifold. From specific-heat measurements we accurately determined the antiferromagnetic ordering temperature and obtained information on the crystal-field states of the rare-earth ions. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2011
