Effect of substitutional hydride ions on the charge states of oxygen vacancies in thermochemically reduced CaO and MgO

Abstract

Different methods of thermochemical reduction of CaO and MgO crystals can yield dramatically different optical-absorption and emission properties. In general, thermochemical reduction produces oxygen vacancies and substitutional H (hydride) ions with the vacancies mainly in the twoelectron charge state (F center). Removal of protons before or during thermochemical reduction results in anion vacancies that are primarily in the one-electron F+ state. Data are presented to relate the effect of hydride concentration on the charge states of the anion vacancies. Photobleaching experiments permit identification of three absorption bands as due to electron donors in thermochemically reduced CaO. Finally, we provide experimental results which indicate that the intrinsic F and F centers in MgO or CaO crystals are not viable candidates for tunable lasers.

First principles calculations and experiment predictions for iodine vacancy centers in SrI2

“…The similarity of the F and F À energy levels in Fig. 3 despite the net charge difference is suggestive of the case of F þ and F centers in MgO addressed in a very recent first principles study [23] and earlier experiments [24]. In MgO (with divalent anion and cation), the absorption bands of the F þ center (one electron) and lattice-neutral F center (two electrons) are almost superimposed.…”

“…One important case for comparison is the one-and two-electron vacancy centers in MgO, termed F þ and F, respectively. These were the subject of a recent firstprinciples calculations [23] of optical spectra using the GW approach and the Bethe-Salpeter equation (BSE) aimed partly at elucidating the experimental observation [24] that the one-and two-electron centers in MgO have almost identical first optical absorption transitions, i.e., the optical binding energy of the second electron in the vacancy is almost the same as that of the first one. This might seem at first counter-intuitive in a static-lattice Coulomb potential picture.…”

Point-defect optical transitions and thermal ionization energies from quantum Monte Carlo methods: Application to theF-center defect in MgO

“…Despite its apparent simplicity, its properties as deduced from optical absorption and luminescence studies have proven somewhat ambiguous. Experimental characterization of the F-center in its neutral (F 0 ) and singly ionized (F +1 ) state has been complicated by their nearly identical optical absorption energies [25][26][27]. These energies have been corroborated by recent GW calculations [12]; however, these calculations also predicted optical emission energies that are substantially different from the assigned experimental values, causing the authors to suggest a reinvestigation of the experimental observations.…”

“…The F-center defect (oxygen vacancy) in MgO is a typical example of an intrinsic point defect in a binary ionic compound [22][23][24][25][26][27]. Despite its apparent simplicity, its properties as deduced from optical absorption and luminescence studies have proven somewhat ambiguous.…”

Theoretical study of bulk and surface oxygen and aluminum vacancies inα−Al2O3

“…In particular, the presence of oxygen vacancies in MgO and other alkaline-earth oxide single crystals has been established long ago and is one of the point defects most extensively studied. In the case of MgO, the oxygen vacancies are formed either by thermochemical reduction or by neutron irradiation: 34 these point defects are generally described as F centers, although different kinds of these centers exist depending on whether neutral atomic oxygen ͑F center͒, O Ϫ (F ϩ center͒, or O 2Ϫ (F 2ϩ center͒ is removed from the solid. In each case there are formally two, one, or zero electrons trapped in the cavity left by the removal of the oxygen species.…”