Electron Channeling X-Ray Microanalysis for Cation Configuration in Irradiated Magnesium Aluminate Spinel
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spinel compounds. The characteristic x-ray emission is measured precisely as a function of incident-beam direction in the experiment, which we have termed high angular resolution electron channeling x-ray spectroscopy (HARECXS) to differentiate it from conventional electron channeling techniques [5]. The orientation dependent profiles of emitted x-ray intensity are measured and then compared to theoretical calculations [7-9] analyzed to determine the local lattice-ion configurations. In this work, the HARECXS technique is employed to investigate cation configuration in MgODnA120 3 with n=l.0 and 2.4 as well as disordering behavior induced by irradiation with 1 MeV Ne÷ ions at 870 K. EXPERIMENTALPROCEDURE Disk specimens of MgO'nAl 20 3 with n=l.0 (Union Carbide) and 2.4 (Nakazumi Crystal) were dimpled to 30Rim thickness and then ion-milled with 5 keV Ar÷ ions to electron transparency. These specimens were subsequently annealed at 1470 K for 48 hours to remove lattice defects produced by ion milling, and allowed to furnace cool. Single-crystalline MgO'nAl 20 3 bulk crystals were irradiated at 870 K with 1 MeVNe+ ions to a dose of about 2 dpa (-=4.5 x 1020 Ne÷/m2). The irradiated bulk specimens were mechanically thinned with a tripod polisher. The wedge shape cross sectional specimens allowed observation along Ne-ion beam direction. Argon plasma processing [10] was performed in order to suppress the hydrocarbon contamination on the specimens during the following HARECXS measurement. Experimental data was acquired using a Philips EM420T analytical electron microscope equipped with an LaB 6 electron source and an EDAX 9900 EDX system operating in the TEM mode at 120 kV. Two-dimensional angular resolution measurements of x-ray emission were carried out in a 128 x 100 pixel scan synchronized with incident beam rocking over an angular range of about 100 by 80 mrad between -4g and +4g (g=400) Bragg conditions, using customized computer control. The typical acquisition time for a complete two-dimensional scan was 18-24 hours. RESULTS AND DISCUSSION Fig 1 illustrates calculated HARECXS profiles of stoichiometric MgO'nAl 20 3 spinel crystals with various cation arrangements. These calculations were accomplished using a computer program developed by Rossouw et al, which takes account of dynamical scattering of incident electrons as well as the dechanneling and delocalization effects on induced x-ray emission [7,9]. Fifteen reflections in the 400 systematic row were considered in these calculations and the parameter k along the abscissa refers to the intersection of the Ewald sphere with the axis along 400 systematic reflections. In this nomenclature, k/g 4oo=1 corresponds to the exact Bragg condition for 400 reflection. The intensities of Mg-K, Al-K and O-K x-rays are given as a function of incident electron beam orientation, normalized by the values at the 5g (g=400) Bragg condition. The x-ray intensities drastically change with the beam orientation, especially in the central part of strong dynamical excitation of low order re
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