Stabilization of bcc Mg in Thin Films at Ambient Pressure: Experimental Evidence andab initioCalculations

Abstract

Recent experiments suggest that bcc Mg can be stabilized when grown together with bcc Nb in Mg/Nb multilayer films at ambient conditions. This finding is remarkable as (pure) bcc Mg has only been observed under very high pressures and is in fact (mechanically) unstable under room conditions. Density functional theory calculations were performed to gain insight into the stability of Mg in the bcc structure. Calculations of the thermodynamic, electronic and structural stability of bcc Mg show that this structure is in fact metastable under thin film conditions, when Mg grows epitaxially on bcc Nb, in agreement with experiments.

Room temperature deformation mechanisms of Mg/Nb nanolayered composites

“…These textures are consistent with prior microstructural analyses of fully BCC Mg/Nb composites. 42,46,47 In other studies, however, the 5 nm/5 nm Mg/Nb composite was not fully BCC. Instead, there, finer h to 2.5 nm (with Nb at the same h) or 1.5 nm (with Nb at a smaller h) was needed to create the fully BCC Mg/BCC Nb composite.…”

Morphological and mechanical stability of HCP-based multilayer nanofilms at elevated temperatures

“…The calculation results based on first-principles theory from Jona and Marcus revealed a martensitic transition of Mg from HCP structure to BCC structure at hydrostatic pressure lager than 500 kbar [35]. Moreover, the metastable BCC Mg were experimentally observed in the magnetron sputtered Mg/Nb multilayers with individual layer thickness of 5 nm, while subsequent density functional theory calculations demonstrated that the stabilization of BCC Mg deposited on BCC Nb substrate is analogous to the stabilization of BCC Mg under high pressures because of the mechanical constraints imposed on Mg [36,37].…”

Investigation of interfaces in Mg/Nb multilayer thin films

“…Thus bulk Nb exists in bcc form. Meanwhile bcc Mg can form under high pressure, since compression can stabilize the bcc Mg structure by lowering of its 3d band toward the Fermi level (E F ) [55,56,21].…”

“…The charge density and ELF contour of the unit cell were visualized by using VESTA [45]. [30,21]. Coherent interfaces with bcc structure were observed in Mg 5.6 nm/Nb 10.4 nm multilayers ( Fig.…”

“…multi-layered thin films dramatically decrease the hydrogenation temperature MgH 2 to $373 K. Detailed analysis of the structures of the hydrogenated films in combination with electronic structure calculations rationalized the dramatic de-stabilization of the magnesium hydride as a result of the formation of a metastable (orthorhombic) allotrope of MgH 2 , which is significantly less thermodynamically stable than the normal form of MgH 2 . In those and subsequent studies [21,22] the formation of metastable interfacial configurations involving bcc-like Mg and hcp-like Nb structures were observed depending on the thickness and composition (fraction of Nb/Mg) of the bilayer constituting the multilayer thin films.…”