Study of the Long Period Ordered Alloy Cu_3Pt

The existence range of the one-dimensional long-period ordered alloy Cu_3Pt (L1_) on the equilibrium diagram has been determined by X-ray diffraction, single crystal specimens having been prepared and quenched at various temperatures. As the result it has become clear that there occurs a two-step transition L1_2→L1_→A1 by heating-up in this alloy system, and this transition has been confirmed by specific heat measurements. The half long-period M of L1_ phase has been measured from separations of split superlattice reflections. M increases with decreasing platinum content, and the behavior of this concentration dependence is well explained by the relation between M and e/a based on Sato-Toth\u27s idea. M decreases as the temperature approaches T_c, in the same way as in Cu_3AuII but contrary to the behavior in Ag_3Mg. Some argument is given on the temperature dependence of M and the degree of order in the long-period lattice of Cu_3Pt