The LPSO structure with an extra order beyond stacking periodicity

Abstract

We have investigated a novel long-period stacking/order (LPSO) structure in a Mg75Al10Y15 alloy, based on electron diffraction, scanning transmission electron microscopy (STEM) observations and first-principles calculations. Fundamental lattice of the present LPSO structure is identified as one of the stacking polytypes of 10H-type, and the in-plane 6×(1210)hcp superlattice order is well developed as represented by the L12-type Al6Y8 cluster arrangement. We find that, across the stacking direction, the inter-cluster order tends to be further developed beyond its periodicity, showing an extra order which doubles the 10H-stacking periodicity. We have constructed the LPSO variant models including such extra inter-cluster order, and their formation energy comparison has confirmed that the extra order becomes indeed stable when the L12-Al6Y8 cluster contains interstitial atoms. The interstitial effects turn out to be prominent in favor of Y atom, as being consistent with the STEM observations.