Alkali Metal Doping of MgO:  Mechanisms of Formation of Paramagnetic Surface Centers

Abstract

The adsorption of alkali-metal atoms (M ) Li, Na, and K) on the surface of MgO has been studied by means of embedded-cluster DFT calculations. Alkali-metal atoms bind preferentially to the oxide anions with energies of the order of 1 eV. On these sites the ns valence electron remains localized on the alkali atom, but the substantial polarization (ns-np mixing) leads to major changes in the isotropic hyperfine coupling constants. In the presence of specific defect sites, like a bare oxygen vacancy, F S 2+ center, a net charge transfer occurs, with formation of F S + color centers. At higher coverages, once the F S 2+ centers have been saturated, a different process takes place. At specific neutral morphological defects, like a cationic reverse corner, the alkali atom valence electron is transferred to the surface with formation of M + (e -) trapped pairs. The computed properties of the unprecedented M + (e -) trapped pairs (hyperfine constants and optical transitions) are consistent with the experimental measures and show that the trapped electron and the adsorbed alkali-metal cation are separated by short distances.

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

“…A small unresolved signal is present in the experimental spectrum at approximately g = 2. This signal contains contributions from ionized alkali cations (< 1 %) that result from the adsorption on tiny amounts of surface oxygen vacancies (F centers) [4] or nearby residual surface OH À groups, [32] which are both capable of inducing spontaneous ionization of the metal. Contributions from small metal particles also leads to resonance absorption in the same region.…”

“…Last, but not least, alkali metals on MgO surfaces represent an excellent system to test the validity of theoretical methods and models to interpret and predict the properties of adsorbed atoms (in particular to reproduce observable EPR properties, such as g factors and hyperfine coupling constants (hfccs), in both isotropic, a iso , and dipolar, B tensor, forms). [2,4] The aim of this study was to systematically investigate the properties of alkali metal atoms deposited on a MgO surface to extend previous experiments by looking at the behavior of the heavier members of the group and exploring the interaction at various surface sites with respect to their site-specificity. 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".…”

“…[4] These minority sites have been tentatively identified as empty F 2 + centers, that is, strongly electron-deficient oxygen vacancies. After saturation of all of the "F 2 + " centers, slightly larger amounts of alkali metal lead to EPR signals characteristic of alkali metal atoms stabilized as monomeric entities at specific surface sites.…”

“…After saturation of all of the "F 2 + " centers, slightly larger amounts of alkali metal lead to EPR signals characteristic of alkali metal atoms stabilized as monomeric entities at specific surface sites. [2,4] At these sites the atoms are often stable at room temperature or above, which means the diffusion barriers for these atoms are very high. An additional increase in the amount of deposited metal results in the formation of tiny clusters and small aggregates.…”

Crystallinity of Li-doped MgO:Dy3+ nanocrystals via combustion process and their photoluminescence properties

“…The appearance of this strong band implies this sample bibulous when Li concentration is high. Pacchioni and co-workers have studied the deposition condition of alkali atoms on MgO surface by density functional theory (DFT) cluster model calculations [25]. They found at higher coverages small alkali aggregates begin to form.…”