Alkaline-earth oxides single crystals

EPR Parameters for V and V° Centers in Alkaline-Earth Oxide Single Crystals... 

Extensive study has been devoted to paramagnetic defects that are formed on high-surface area alkaline earth oxides, particularly magnesium oxide. The work carried out by Wertz el al. 187, 188) and Henderson and Wertz 139) on bulk defects formed in MgO single crystals has been quite valuable in the identification of the surface defects. Both the bulk and surface defects may be divided into two classes those in which an electron... 

In the previous section we summarized the chemical evidence that oxide ions in a state of low coordination can act as electron donors. At the same time, spectroscopic evidence has been accumulated which shows that highly dispersed alkaline-earth oxides have optical absorption bands that are not present in the pure single crystal. This is surprising at first because the energy required for electronic excitation of bulk MgO corresponds to a frequency in the vacuum ultraviolet. In order to understand this we must look at the absorption process more closely. 

In relation to the surface defects on the group II alkaline earth oxides, EPR has been instrumental in unraveling the electronic structure of the defects on both polycrystalline and well defined single crystal surfaces. These trapped electron centers can be formed in a number of different ways. The most convenient means on powders is by exposure of the alkaline earth oxide, such as MgO, to hydrogen atoms [22]. Spontaneous ionization of the H atoms occurs with the subsequent formation of excess electrons on the surface ... 

Electronic Transport, General Description. Magnesium Oxide (MgO). Electrical transport measurements on alkaline earth oxides encounter several difficulties, such as high resistance at low temperatures, a strong influence of surface layers, and high-temperature thermionic emission. The partly contradictory results depend considerably on the purity and nature of the samples (pressed porous powders, sintered samples, polycrystals, and single crystals) and on the experimental conditions. 

Surface structure of alkaline earth oxides was investigated using UV absorption and luminescence spectroscopies. High surface area MgO absorbs UV light and emits luminescence, which is not observed with MgO single crystal. UV absorption, corresponds to the following electron transfer process involving surface ion pairs. 

Salt-inclusion solids described herein were synthesized at high temperature (>500°C) in the presence of reactive alkali and alkaline-earth metal halide salt media. For single crystal growth, an extra amount of molten salt is used, typically 3 5 times by weight of oxides. The reaction mixtures were placed in a carbon-coated silica ampoule, which was then sealed under vacuum. The reaction temperature was typically set at 100-150 °C above the melting point of employed salt. As shown in the schematic drawing in Fig. 16.2, the corresponding metal oxides were first dissolved conceivably via decomposition because of cor-... 

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