First-Principles Thermodynamics Study of Spinel MgAl₂O₄ Surface Stability

Abstract

The surface stability of all possible terminations for three low-index (100, 110, 111) structures of spinel MgAl 2 O 4 was studied using a first-principles-based thermodynamic approach. The surface Gibbs free energy results indicate that the 100_AlO 2 termination is the most stable surface structure under ultrahigh vacuum at T = 1100 K regardless of an Al-poor or Al-rich condition. With increasing oxygen pressure, the 111_O 2 (Al) termination becomes the most stable surface in the Al-rich condition. The oxygen vacancy formation is thermodynamically favorable over the 100_AlO 2 , 111_O 2 (Al), and ( 111) structures with Mg/O connected terminations. On the basis of the surface Gibbs free energies for both perfect and defective surface terminations, 100_AlO 2 and 111_O 2 (Al) are the most dominant surfaces in Al-rich conditions under atmospheric conditions. This is also consistent with our previously reported experimental observation.

Li2O‐doped MgAl2O4 nanopowders: Energetics of interface segregation

“…The obtained surface energies were 1.51 and 1.17 J m −2 , respectively. As indicated by the calculations of Cai et al, the proportion of MgO surface termination significantly increases the surface energy, which can explain the higher surface energy observed by Mordekovitz and Hayun, because their measurements indicated the median energies. Both results are in accordance with our previous observations of the surface energy and the surface lixiviation in this study .…”

“…The surface stability of MgAl 2 O 4 was previously studied via density functional theory calculations and thermodynamic analysis . For a high Al chemical potential and an O 2 pressure of 0.21 atm, the (111) face was shown to be the dominant face for MgAl 2 O 4 particles with 111_O 2 (Al) termination with only Al 2 O 3 and a surface energy of 1.01 J m −2 .…”

“…The surface stability of MgAl 2 O 4 was previously studied via density functional theory calculations and thermodynamic analysis. 52 For a high Al chemical potential and an O 2 pressure of 0.21 atm, the (111) face was shown to be the dominant face for MgAl 2 O 4 particles with 111_O 2 (Al) termination with only Al 2 O 3 and a surface energy of 1.01 J m −2 . Mordekovitz and Hayun measured the surface energy of MgAl 2 O 4 via differential scanning calorimetry in Ar at 1350°C for two stoichiometries (MgO‧1.06Al 2 O 3 ) and Al excess (MgO‧1.21Al 2 O 3 ) powders.…”

Temperature‐Induced Atomic Reconstruction At Au/MgAl2O4 Interfaces

“…Figure 1c,d shows the monolayer-reconstructed interface from the two orthogonal directions [110] and [112]. [16] Based on the above monolayer models, there are two stable interface models corresponding to maximized energy reduction (see Figure 2e), i.e., the clean model with direct contact between Au and Al 3 -termination (Figure 2b), and the monolayer model that includes an overlayer with less gold occupancy on Mg-termination (Figure 2c). The oscillating contrast observed within this reconstructed monolayer suggests the presence of either other elements or less Au atoms between those bright dots within this monolayer.…”

“…The above simu lations avoid the complex issue of evaluating the change in elemental chemical potentials under various conditions. [16] Based on the above monolayer models, there are two stable interface models corresponding to maximized energy reduction (see Figure 2e), i.e., the clean model with direct contact between Au and Al 3 -termination (Figure 2b), and the monolayer model that includes an overlayer with less gold occupancy on Mg-termination (Figure 2c). The monolayer model includes an interfacial reconstruction with an oscillating number of gold atoms between alternating gold sites.…”