Paramagnetic Defect Centers at the MgO Surface. An Alternative Model to Oxygen Vacancies

Abstract

On the basis of embedded cluster calculations, we propose a new model for the structure of paramagnetic color centers at the MgO surface usually denoted as F(S)(H)(+) (an electron trapped near an adsorbed proton). These centers are produced by exposing the surface of polycrystalline MgO to H(2) followed by UV irradiation. We demonstrate that properties of H atom absorbed at surface sites such as step edges (MgO(step)) and reverse corner sites (MgO(RC)), formed at the intersection of two step edges, are compatible with a number of features observed for F(S)(H)(+). Our calculations suggest that (i) H(2) dissociates at the reverse corner site heterolytically and that there is no barrier for this exothermic reaction; (ii) the calculated vibrations of the resulting MgO(RC)(H(+))(H(-)) complex are fully consistent with the measured ones; (iii) desorption of a neutral H atom from the diamagnetic precursor requires UV light and leads to the formation of stable neutral paramagnetic centers at the surface, MgO(step)(H(+))(e(-))(trapped) and MgO(RC)(H(+))(e(-))(trapped). The computed isotropic hyperfine coupling constants and optical transitions of these centers are in broad agreement with the existing experimental data. We argue that these centers, which do not belong to the class of "oxygen vacancies", are two of the many possible forms of the F(S)(H)(+) defect center.

First Evidence of a Single‐Ion Electron Trap at the Surface of an Ionic Oxide

“…The reaction is exothermic, and the computed value for DE (À0.66 eV) is close to that measured using microcalorimetry (À0.5 eV). [16] The MgÀH stretching frequency is found at ñ = 1446 cm À1 and a bending motion at ñ = 852 cm À1 ; both values are typical for hydride species. [18] The OÀH group vibrates at ñ = 3624 cm À1 .…”

“…This model suffers from several drawbacks, such as the high formation energy of the oxygen vacancy and the need to compensate negative charge to maintain electroneutrality. In recent years several alternative models have been proposed, which range from low-coordinated vacancies at steps and corners, [14] to neutral divacancies, [15] or morphological defects, [16] all of which involve an array of cations that act as an electron trap. However, recent theoretical calculations [17] surprisingly predicted that single low-coordinate cations on MgO can act as potential electron-trapping sites.…”

Interactions of hydrogen with amorphous hafnium oxide

“…At a closer look, the picture is more nuanced as it depends on the character and concentration of electron trapping sites in a particular system. For example, atomic hydrogen does not react with terraces at the MgO (001) surface, but donates electrons to three-coordinated Mg sites at corners and kinks at that surface, creating new [Mg + − O-H]-type centers [35,36]. The recent study of amorphous silica has concluded that, unlike in α quartz, atomic H can break some strained Si-O bonds and form a new thermodynamically stable defect, termed hydroxyl E center [37,38].…”

Properties of Alkali Metal Atoms Deposited on a MgO Surface: A Systematic Experimental and Theoretical Study

“…For the sake of completeness we will also consider the behavior of hydrogen atoms adsorbed on the same surface because hydrogen may be regarded as the "first alkali metal". [6] The behavior of both alkali metal atoms [2][3][4][5]7] and hydrogen [8][9][10][11][12][13][14] adsorbed on MgO has been investigated previously, but a direct comparison has never been made. In addition, a comprehensive picture is often hampered by the complex morphology of the polycrystalline MgO surface used.…”

“…[15] The behavior of atomic hydrogen is very different and has been discussed in detail in a series of papers. [8][9][10][11][12][13][14] Herein, some account of the reactivity of hydrogen atoms is given, which will be restricted to the factors that distinguish them from the Group Ia metals. Finally, the trends in the chemical bonding and EPR properties of the alkali metal atoms going from the lighter (lithium) to the heavier (cesium) atoms are discussed.…”