A New Ordered Structure of the Off-Stoichiometric Laves Phase Having C15-Type Structure in Mg-Cu-Si System

Abstract

A new ordered structure has been found in a ternary Mg-Cu-Si Laves phase whose composition is around Mg(Cu_0.80Si_0.20)_2.50. The ordering between Cu and Si of Cl5-type Mg(Cu_0.81Si_0.19)_2.53 specimen was analysed by a single crystal X-ray diffraction method. It belongs to a space group P4₁ 32(or P4 ₃32) having a=6.9598Å contrary to the disordered structure of Fd3m. The ordered structure is characterized by the ordered 4-fold screw arrangements of Cu and Si atoms located on four tetrahedra in the unit cell leaving Mg atoms on 8c sites. The order parameter S= α_l−α₂ was found to be 0.74 in this specimen, where α_land α₂ are occupancy factors of Cu for 12d and 4a sites of the space group P4 ₁32, respectively. The order parameters were measured as a function of annealing temperature by using Warren’s method for quenched powder specimens of Mg(Cu _0.80 Si _0.20)_2.40. The maximum value of S reached to 0.75 at 200°C, which is a little lower than the expected value of perfect order, S=0.85. The order parameter then decreases continuously with increasing annealed temperature and become zero at about 600°C, indicating order-disorder transformation temperature.