Hole centres in magnesium oxide

When magnesium oxide crystals are exposed to ionizing radiation the electro n-loss (hole) centres are normally identified as O . (-) ions. In this pape r, I examine the EPR evidence for this, and compare the data with those for the hole centres in alkali- halides V-K centres). The latter are clearly s igma* radical anions, such as F-radical anion F-. The analogous centre in M gO is O-radical anion O3-, which does not seem to have been considered. The results compare well, suggesting that the O . (-) centres are really O-rad ical anion O3- radicals. In particular, the O-17 data for the oxygen centre and the F-19 data for F-2. (-) give similar estimates of the 2 s and 2p ch aracter of the orbitals on oxygen and flourine, suggesting that the spin-de nsity on oxygen is ca. 50%. The exception is the direction of the principal axes of these centres, which are quite different from each other. It is su ggested that the primary hole-centres in MgO migrate by electron transfer t o neighbouring cation vacancies where they are stable, and that at these si tes the (1,0,0) directions are most favourable for sigma -bonding. (C) 2001 Elsevier Science Ltd. All rights reserved.