[en] A low temperature method is described for the preparation of the new garnet compounds YCaLnGa₅O_l2 (Ln=Ce, Pr, Th). In this set of compounds (Ca²⁺ + Ln⁴⁺) replaces 2 Y³⁺ in the parent gallium based garnet Y₃Ga₅O_l2 in order to stabilize as effectively as possible the Ln⁴⁺ species in the eight-coordinate ''A'' site of the garnet structure. Characterization of the oxides by x-ray powder diffraction and thermogravimetric analysis is discussed with regard to the structural relationship of the substituted compound to the parent material. The tetravalent ions Pr⁴⁺ and Tb⁴⁺ exhibit increased thermal stability in reducing conditions as compared to the Ln⁴⁺ states in the fluorite (LnO₂) and perovskite (BaLnO₃) type structures. This result is discussed with reference to the complex crystal chemistry of these systems