THE FORMATION OF THE L1(2) ORDERED STRUCTURE IN HYPOSTOICHIOMETRIC PD3MN ALLOYS CONTAINING INTERSTITIAL BORON

The ordering of the L1(2) structure in hypostoichiometric Pd3Mn alloys containing interstitial boron, i.e. Pd3+xMn1-xBy with x = 0.2 and 0.4 and y = 0.27 and 0.53, and for comparison in Pd3.2Mn0.8M0.27 with sub stitutional alloying elements M = Ag, Cu and Ni has been studied by X- ray, electron diffraction and electrical resistance measurements. The results clearly indicate the presence of the L1(2) (Cu3Au) type of sup erlattice in addition to small amounts of regions containing superimpo sed L1(2-s) and L1(2) superlattices. For alloys of Pd3.4Mn0.6By with y = 0.27 and Pd3.2Mn0.8M0.27 with M = Ag, Cu and Ni, however, this was not the case. It is suggested that the formation of the L1(2) Structur e in the alloys at high temperature requires the presence of the L1(2- s) Structure and an interstitial boron content greater than about y = 0.15. Thus the order transition L1(2-s) --> L1(2) in Pd3+xMn1-xBy is a ssociated with the preferential occupation of boron atoms at particula r octahedral sites of the L1(2-s) structure.