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The crystal structure and magnetic properties of LaCo13-x-yFexSiy compounds were investigated by means of x-ray powder diffraction and magnetization measurements. The substitution of Si for Co induces an order-disorder transition from the cubic NaZn13-type to its tetragonal derivative structure, while the substitution of Fe for Co does not induce such a phase transition. After annealing treatment, the homogeneous range of the cubic phase is narrowed and that of the tetragonal phase is extended. From crystallographic and thermodynamic points of view, the stability of the cubic and the tetragonal phases is discussed. The measured magnetic moment of LaCo13-x-yFexSiy coincides well with the theoretical prediction based on the magnetic valence model. Within the framework of this model, LaCo13-x-yFexSiy compounds can be considered as strong ferromagnets and their magnetic moment can be theoretically predicted. The composition dependence of Curie temperature is discussed within the mean field approximation. At higher iron concentration the decrease of Curie temperature with Fe concentration might be attributed to the increase of the number of antiferromagnetically coupled Fe-Fe pairs and the decrease of the ferromagnetic coupling between transition metal atoms. (C) 1996 American Institute of Physics.