Effect of long-range order on elastic properties of Pd0.5Ag0.5

The effect of long-range order on single-crystal elastic constants of Pd${}_{0.5}$Ag${}_{0.5}$ alloy has been investigated using first-principles electronic structure calculations. The lowest energy among the considered ordered, partially ordered, and disordered structures is found to be the L1${}_{1}$ layered structure, which is formed by alternate (111) Pd and Ag layers. The ordering effect is found to follow a clear trend: in contrast to the disordered phase, for which the ${K}_{a}$ and ${K}_{c}$ compressibilities are equal, the L1${}_{1}$ structure becomes less compressible along the $c$ axis than along the $a$ axis.