A La plasma is produced by evaporating La with an electron beam and ionizing the La particles in a RF discharge at a pressure of 10(-5) Pa. Low-energy ion implantations into soot containing fullerenes using the La plasma result in the formation of novel La-fullerene compound materials such as La-endohedral fullerene (La@C-n, ngreater than or equal to60), heterofullerene (LaCn-1, n>70), and dimetallofullerene (La-2@C-n, n>80). It is to be emphasized that in our experiment we have never observed La@C-82 and La-2@C-60, which are known to be formed in the conventional experiments using prevailing methods. Thus, our ion irradiation method could be useful for the overall understanding of the formation mechanism of endohedral metallofullerenes.