An ion-beam-induced metastable phase in magnesium aluminate spinel (MgAl2O4) has been examined in detail using transmission electron microscopy. Single crystals of MgAl2O4 with (111) orientation were irradiated with 180-keV Ne+ ions at 120 K to fluences of 1016 and 1017/cm2. Selected-area electron diffraction patterns obtained from the irradiation-induced damaged layer revealed that all even hkl reflections (e.g., 222) possess strong intensity, while all odd hkl reflections (e.g., 111) are weak. The features of these diffraction patterns corresponded to those of the rocksalt (NaCl) structure, suggesting that an ordered spinel to disordered rocksalt structural phase transformation in MgAl2O4 was induced by ion beam irradiation.
This paper reviews the nature of defects and optical centers in magnesium aluminate spinel. The identification of absorption and luminescence bands was used to investigate the influence of UV and electron irradiation on optical center formation. Crystals examined were nominally pure MgO-nAl 2 O 3 crystals with compositions in the series n = 1.0, 1.5, 2.0, and 2.5. MgOؒAl 2 O 3 crystals doped with transition-metal ion dopants were also studied to determine the influence of impurities on the nature and concentration of lattice defects in spinel. Results strongly suggest that absorption bands at 3.78 and 4.15 eV, and luminescence bands near 5 eV, may be attributed to optical centers on antisite defects. It is also apparent that through the introduction of specific amounts of Mn and Cr ions, the concentration of optical centers (which are related to defects such as cation antisites in the spinel structure) can be considerably changed.
Ion-beam-induced microstructures in magnesium aluminate
(MgAl2O4) spinel have been examined using transmission
electron microscopy (TEM). Irradiations were performed at cryogenic
temperature (~120 K) on MgAl2O4 spinel
single-crystal surfaces with (111) orientation, using 180 keV neon
(Ne+) ions to ion fluences ranging from 1016 to
1017 Ne+ cm-2. Cross-sectional TEM observations
indicated that the MgAl2O4 spinel transforms first into a
metastable crystalline phase and then into an amorphous phase under
these irradiation conditions. On the basis of selected-area
electron diffraction and high-resolution TEM, we concluded that
Ne-ion-beam irradiation induces an ordered spinel-to-disordered
rock-salt-like structural phase transformation. Atomistic structures
of amorphous MgAl2O4 were also examined on the basis of
atomic pair distribution functions. We compared the experimentally
obtained results with previous theoretically calculated results for
the metastable and amorphous phases of MgAl2O4, and
discussed the validity of the proposed ion-beam-induced structural
changes in MgAl2O4 spinel.
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