Two-order-parameter description of liquid Al under five different pressures

In the present work, using the glue potential, the constant pressure molecular-dynamics simulations of liquid Al under five various pressures and a systematic analysis of the local atomic structures have been performed in order to test the two-order-parameter model proposed by Tanaka Phys. Rev. Lett. 80 5750 (1998) originally for explaining the unusual behaviors of liquid water. The temperature dependence of the bond order parameter Q₆ in liquid Al under five different pressures can be well fitted by the functional expression Q₆/1−Q₆=Q₆₀ exp(ΔE−PΔV/k_BT) which produces the energy gain ΔE and the volume change upon the formation of a locally favored structure: ΔE=0.025 eV and ΔV=−0.27 (Å)³. ΔE is nearly equal to the difference between the average bond energy of the other type I bonds and the average bond energy of 1551 bonds (characterizing the icosahedronlike local structure); ΔV could be explained as the average volume occupied by one atom in icosahedra minus that occupied by one atom in other structures. With the obtained ΔE and ΔV, it is satisfactorily explained that the density of liquid Al displays a much weaker nonlinear dependence on temperature under lower pressures. So it is demonstrated that the behavior of liquid Al can be well described by the two-order-parameter model.