A Combined EPR and Quantum Chemical Approach to the Structure of Surface F_s⁺(H) Centers on MgO

Abstract

Fs +(H) color centers at the surface of MgO have been studied using a combined EPR and quantum chemical approach. Fs +(H) are paramagnetic excess electrons centers where the unpaired electron is trapped in a surface anion vacancy. They are formed at the surface of thoroughly dehydrated MgO (1073K) upon UV irradiation under hydrogen in parallel with the formation of minor fractions of different color centers. The whole EPR spectrum resulting from irradiation has been analyzed by simulation of the experimental profile. Fs +(H) centers are characterized by an axial g tensor and display a hyperfine interaction with a hydrogen nucleus (belonging to an hydroxyl group stabilized nearby the vacancy) and two distinct families of 25Mg nuclei characterized by a large (10.5 G) and a small (0.7 G) hyperfine coupling constant, respectively. Both EPR and ab initio calculations on clusters of ions converge in indicating that the features of the centers are due to the polarization of the electron density by the positive charge of the hydrogen in the OH group toward two (or possibly three) equivalent magnesium ions of the vacancy close to the OH group itself. The formation mechanism of the centers is strictly analogous to that occurring upon contact of low ionization energy metals with the surface of MgO leading to the formation of another type of color centers. Partially hydrated MgO samples also give rise to another family of paramagnetic center based on electrons trapped in anion vacancies. This finding indicates that the structure of the partially hydroxylated oxide and the mechanism of its dehydration are still open questions.

Surface-specific visible light luminescence from composite metal oxide nanocrystals

“…Excitation energies are also calculated by time-dependent density functional theory. This type of approach has proved extremely reliable for predicting the electronic, optical, and chemical properties of MgO nanopowders and a wide range of defects [11,12,24,30,31,[37][38][39]. In our implementation, Mg and O ions in the quantum cluster are described at the all electron level using a Gaussian 6-311G** basis set.…”

Ground- and excited-state properties ofM-center oxygen vacancy aggregates in the bulk and surface of MgO

“…While the ground state properties of F centers have been studied in some detail from a theoretical point of view, [15][16][17][18][19] few ab initio calculations have been reported on the excited states of F centers, or on the ground and excited state properties of F centers aggregates ͑e.g., the M center͒. Finocchi et al 20 have studied the interaction of surface oxygen vacancies on the MgO͑100͒ surface by means of periodic calculations in the framework of the density functional theory ͑DFT͒ within the local density approximation ͑LDA͒.…”

Geometric Characterization of a Singly Charged Oxygen Vacancy on a Single-Crystalline MgO(001) Film by Electron Paramagnetic Resonance Spectroscopy

“…The resonance field position is only slightly different to the 0 situation. This small difference in resonance positions already points at a small g-tensor anisotropy, which is generally observed for color centers on MgO [9,17]. To extract the principal components of the g tensor, one needs to resort to spectral simulations.…”