A STUDY CONCERNING PROBLEMS OF ORDER AND STABILITY IN THE LN2M12X7 PHASES (RARE EARTH-TRANSITION METAL-NONMETAL)

The method generally used for research into new phases with particular properties is systematic analysis of the components evident in the di fferent ternary diagrams (synthesis, characterization and determinatio n of the physical properties). In the case of the ternary phosphides a nd arsenides, the extreme complexity of the diagrams and the very larg e number of phases render this procedure extremely long and difficult. It is thus important to know whether, at least in the domain of compo sition, crystallochemical analysis could permit a less empirical appro ach to research into new compounds. With the purpose of testing the va lidity of this approach, we have prepared ten compounds, all of which crystallize in the hexagonal system. We have been able also to make co exist in the same Ln(2)M(12)X(7) structure the rare earth and chromium (or tungsten) in the form of a screening effect between the two eleme nts (Cr12P7 --> Ln(2)Fe(9)Cr(3)P(7)).