MULTIPLATEAU ISOTHERMS RELATED TO A MULTIPHASE BEHAVIOR IN THE YFE2-D-2 SYSTEM

After hydrogenation at room temperature for x < 3.4 a mixture of YFe2 and YFe2Dx deuterides was obtained whereas for 3.4 less than or equal to x less than or equal to 3.6 a rhombohedral single phase compound is formed. In order to obtain single phase deuterides for x < 3.4, heat treatments at several temperatures were performed. It was found that e ither heat treatment or deuterium absorption at 408 K, allows us to ob tain single phase and crystalline YFe2Dx deuterides for some definite deuterium concentrations in the range 1.2 less than or equal to x less than or equal to 3.3. YFe2D1.2 is tetragonal (a = 11.981 Angstrom, c = 7.617 Angstrom). YFe2D1.75 and YFe2D1.9 are cubic but weak superstru cture lines imply a doubling of the cell parameter (a = 15.340 Angstro m and a = 15.378 Angstrom respectively). For 2.55 less than or equal t o x less than or equal to 2.9 C15 cubic deuterides are obtained (a = 7 .780 Angstrom to 7.828 Angstrom). For x greater than or equal to 3.3 r hombohedral deuteride is formed. The formation of deuterides with diff erent structures can explain the existence of several plateaus in the absorption isotherms of YFe2 measured at 373, 408 and 473 K. The struc tural deviations from the C15 structure are discussed in term of magne tic effects.