Institute of Physical Chemistry of the Academy of Soienoes of the USSR, Moscow Effect of pRadiation on Mechanically Processed MgO Powder (Compressive and Shear Stress)I n mechanically processed MgO samples, y-irradiation results in the formation of F+ and V-centres. The higher the mechanically induced degree of distortion of the samples, the higher the concentration of F+ centres obtained by the irradiation. Our experiments suggest that the irradiation can be used as a probe in determining the concentration of the mechanically induced vacant anion and cation sites.Die y-Bestrahlung fiihrt in mechanisch bearbeiteten MgO-Proben zur Bildung von F+-und V--Zentren. J e hoher der mechanisch induzierte Storungsgrad der Proben ist, desto grol3er ist die durch die Bestrahlung erzielte F+-Zentren-Konzentration. Unsere Experimente gestatten die SchluSfolgerung, die Bestrahlung als Sonde fur die Bestimmung der Konzentration der mechanisch induzierten Anionen-und Kationenleerstellen zu benutzen.MgO powder activated mechanically in a vibration mill was exposed to a y-irradiation. The object of these investigations is to detect, by means of y-irradiation, defects induced by the mechanical activation. The defects are detected by means of EPR spectroscopy.As it has been shown by KRETZSCIWAR et al. in vibration-ground MgO (compressive and shear stress prevailing) mainly the ranges near the surface are highly distorted, whereas the defects are less in the interior of the crystal. MgO activated in this way was used as a starting material for the y-irradiation. ExperimentalStarting material: cleavage MgO (1)') containing the elements Fe, Mn, Cr, Ni (and V detected only by EPR) as impurities (Tab. 1, 2). The product was annealed over 4 hrs. a t 600 "C before being used. The mechanical activation was carried out in an LS 16 laboratory vibration mill (amplitude 2 mm) made by VEB KEFAMA, Katzhiitte, with a speed of 1430 rpm. Capacity of the porcelain vessels: 1.5 1; diameter of the porcelain balls: 10 mm; filling level: 45%. The quantity of porcelain does not exceed 2'7". Irradiation conditions: GUG-1202) unit, y-CosO, at -196 "C, Dose rate: 3.4-4.5 Mrad/ hr. The EPR spectroscopy was carried out with en ERS-210 of the Centre of Scientific Instrument Manufacture, Academy of Sciences of the GDR. The measurements were carried out in the X-band a t -196 "C; the g-factor was measured using a DPPH calibration 1) The product was supplied by VEB Kombinat Kali. 2, We are highly indebted to P. J. Glasunov, Institute for Physical Chemistry, Academy of Sciences of the USSR, under whose direction the y-irradiations were carried out..
As previous studies have shown (SPITSYN et al.; HENDERSON, WERTZ; STEINIKE et al. 1981, 1984 HENNIG et al.), mechanical activation of MgO in air results in the generation of anion and cation vacancies in the lettice, thus causing heterogeneous lattice distortions. These anion and cation vacancies are identified as F+-and V--centres, respectively, by EPR spectroscopy. F+-centres are anion vacancies filled with electrons, V--centres are cation vacancies filled with holes. The electrons and defect electrons filling the vacancies are generated essentially during y-irradiation. Since the effect of the ambient atmosphere on the generation of electronic and structural defects upon mechanical activation is known (BYSTRIKOV et al.), the studies of mechanical MgO activation in air were extended to the generation of above defects in an argon atmosphere. The results are compared with those obtained in air. ExperimentalThe MgO starting material used, the conditions of mechanical activation by vibration milling, the subsequent irradiation a t -196 "C, and the investigation methods employed were described by STEINIKE e t al. 1981. I n the present study the mechanical activation was carried out in an argon atmosphere; the argon was dried using silica gel and a molecular sieve. ResultsI n an argon atmosphere the mechanical activation yields also anion and cation vacancies.The F+-centre concentration is low (Fig. 1). It is, however, higher than the F+-centre concentration measured after activation in air. After y-irradiation the F+-centre concentration increases by more than one order of magnitude (Tab. 1). There is a characteristical dependence of the F+-centre Concentration on irradiation dose. The maximum centre concentration occurs at doses of 2 to 3 Mrad (Fig. 2); above this dose the concentration saturates. The observed kinetics of irradiation centre formation indicates filling of the anion vacancies produced by mechanical activation.The mechanism of F+-centre formation in argon is the same as in air. The ?+-centre concentrations are the same within one order of magnitude, with the higher concentration found for MgO activated in argon.Independent on the ambient atmosphere the maximum F+-centre concentration is to a good approximation linearly dependent on the relative lattice distortion
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