Geometry of {001} Surfaces of Spinel (MgAl₂O₄): First‐Principles Simulations and Experimental Measurements

Abstract

The geometry of {001} surfaces of spinel (MgAl2O4) was investigated by first‐principles simulations within density‐functional theory (DFT). The calculations show that the structure relaxation or reconstruction occurs for a depth of about 0.5 nm. The (001)‐Mg surface shows moderate relaxations and the (001)‐Al surface shows reconstruction: a new surface was formed with half of the subsurface oxygen atoms moved to the top. The surface energy was calculated to be about 1.7 J/m2 for the relaxed (001)‐Mg surface and about 3.0 J/m2 for the relaxed (001)‐Al surface. The chemical composition of a freshly fractured (001) surface of a spinel single crystal was measured using Low Energy Ion Scattering. The measured Al/Mg ratio indicates that the (001)‐Mg surface is more stable, in good agreement with the calculations.

Noncontact atomic force microscopy imaging of atomic structure and cation defects of the polar MgAl2O4(100) surface: Experiments and first-principles simulations

“…1(b) and 1(c), respectively, creating an Mg or O 4 -Al 4 -O 4 -terminated surface. Both (100) terminations expose so-called polar surfaces when cleaved stoichiometrically, and previous theoretical studies have calculated the surface energy of these different terminations 7,[10][11][12]14,25 and also estimated the percentage of atoms that need to be removed in order to fulfill the polar stabilization criteria. 26 In a simple picture, the Mg 2 and O 4 Al 4 O 4 stacks can be regarded as a series of layers with alternating charge of equal magnitude analogous to a series of capacitors, each holding a certain charge density.…”

“…7 Magnesium aluminate (MgAl 2 O 4 ) is a ternary metal oxide with the spinel crystal structure, which defines a larger group of so-called spinel minerals, with the general formula A 2+ B 2 3+ O 4 2− . 8,9 Most previous results addressing the issue of surface structure were based on theory, [10][11][12][13][14][15] and only a very few experimental studies have been presented. Yanina et al used contact mode atomic force microscopy to investigate the stages of surface faceting and coarsening of MgAl 2 O 4 (100) samples exposed to very high temperatures.…”

“…Yanina et al used contact mode atomic force microscopy to investigate the stages of surface faceting and coarsening of MgAl 2 O 4 (100) samples exposed to very high temperatures. 14,16 Using lowenergy ion scattering (LEIS), van der Laag et al investigated the relative amount of Mg and Al atoms in the surface layer of cleaved MgAl 2 O 4 (100) and compared those findings with DFT simulations to show that fractured surfaces expose a surplus of Mg, reflecting an Mg-terminated surface. Atomic force microscopy (AFM) studies operated in the so-called noncontact mode (NC-AFM) have recently been successfully employed for atomic-scale studies of a range of clean metal oxide insulating surfaces, [17][18][19] such as α-Al 2 O 3 (0001) [20][21][22] or MgO.…”

Configurational and energy study of the (100) and (110) surfaces of the MgAl2O4spinel by means of quantum mechanical and empirical techniques

“…This finding is in agreement with the previous ones. [12][13][14] Always concerning the configurational analysis, it is advisable to treat the crystal surface as a crystallographic entity and not exclusively as a chemical system lacking of whatever symmetry. As a matter of fact, when a crystal face is studied, it is fundamental to specify the symmetry elements preserved, if any, in the different surface configurations, as well as the cell parameters of the 2D cell considered.…”

“…[12][13][14] In all of these works, only stoichiometric surfaces (that is surface reconstructed to cancel out the dipole moment) were taken into account and a restricted number of (100) surface configurations were analyzed; unfortunately, in these papers there are not sufficient information and detailed figures that allow to understand and analyze the structure of the surface configurations. In detail, van del Laag et al 12 have described only two surface configurations (Mg-and Al-terminated), whereas Fang et al 13 In this paper, we will deal with stoichiometric surfaces at 0K in vacuum and we will show that the number of surface configurations detected in the previous papers is not sufficient to give a correct description of the structural complexity of a crystal face of MgAl 2 O 4 spinel. Furthermore, we will also show that the surface energy value of the more stable Al-O-terminated configuration found in the present work is noteworthy lower than those previously published.…”