The electrical conductivity o of MgO single crystals shows a sharp increase at 500-800øC, in particular of Osurface, generally attributed to surface contamination. Charge Distribution Analysis (CDA), a new technique providing information on fundamental properties that was previously unavailable, allows for the determination of surface charges, their sign and associated internal electric field. Data on 99.99% purity, arc-fusion grown MgO crystals show that mobile charge carders start to appear in the bulk of the MgO crystals between 200 and 400øC when o (measured by conventional techniques) is in the 10 -14 to 10 -16 •-lcm-1 range. Above 500øC, as o increases to 10 -6 to 10-7 •-lcm-1, more charges appear giving rise to a strong positive surface charge supported by a strong internal field. This indicates that charges are generated in the bulk and diffuse to the surface by an internally controlled process. On the basis of their positive sign they are identified as holes (defect electrons). Because of the low cation content of these very pure MgO crystals, these holes cannot be associated with transition •netal impurities. Instead, they are associated with the 0 2-sublattice, e.g. consist of O-states or positive holes. This conclusion is supported by magnetic susceptibility data showing the appearance of 1000 +_500 ppm paramagnetic species between 200-500øC. The magnetic data are consistent 2with strongly coupled, diamagnetic O-pairs below 200-500øC, chemically equivalent to peroxy anions, 0 2 , 2and probably associated with cation vacancies in the MgO matrix. The fom•ation of 02 in arc-fusion grown MgO crystals is very unexpected because of the highly reducing growth conditions. Their presence implies an 2-plus H2 molecules. internal redox reaction involving dissolved "water" by which OH-pairs convert to 0 2 This redox conversion is supported by mass spectroscopic measurements of the H2 release from highly OH-doped, finely divided MgO and by wet-chemical analysis of its oxidant concentration. Paper number 93IB01327. 0148-0227/93/93IB 013 27505.00 model ionic conductor. It was also thought that in order to fully understand the conductivity of MgO, knowledge accrued from the study of the alkali halides only needed to be extended to higher temperatures [Davies, 1963; Wuensch, 1983; Wuensch et al., 1991; Tttller, 1985; Dieckmann, 1989]. Over the years the conductivity of MgO has been studied extensively by many investigators as a function of temperature, pressure, oxygen fugacity, and various transition metal impurity concentrations [LempickiSempolinski et al., 1980; Sempolinski and Kingesy, 1980]. The predominantly ionic conduction in pure MgO above 800øC has been confirmed. Electronic contributions, when observed, have generally been discussed in terms of electrons or detect electrons (holes) moving among transition metal impurity cations. There is, however, a fundamental difference between halides and oxides (and silicates) which is often not tully appreciated 22,209 22,210 FREUND ET AL.' CHARGE DISTRIBUTION ANALYSIS...

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Magnetic susceptibility measurements of high purity MgO single crystals (<50-wt. ppm transition metals) by means of a vibrating-sample magnetometer shows an anomaly at 800 K. At the same temperature the electric conductivity increases anomalously, the static dielectric constant epsilon increases from 9 to approximately 150, a pronounced positive surface charge appears, and Fe2+ in the MgO matrix oxidizes to Fe3+. The data are consistent with O2(2-) (peroxy) defects, representing self-trapped, spin-paired positive holes at Mg2+ vacancy sites. Diamagnetic at low temperatures, the holes start to decouple their spins > 600 K, probably forming at first V0 centers (two O- at an Mg2+ vacancy), then V- centers (single O- at an Mg2+ vacancy), and releasing mobile O- states. These O- represent itinerant charge carriers on acceptor levels near the O 2p-dominated valence band and conduct by O- /O2- valency fluctuations. The O- concentration is of the order of 8 X 10(19) cm-3.

High-purity MgO exhibits an unexpected giant anomaly of the apparent static dielectric constant and a positive surface charge of the order of 5xl0 21 cm -3 in the top 15 nm. We postulate that the MgO matrix contains traces of peroxy defects, O2 2-, associated with Mg 2+ vacancies. Above »400°C the O2 2dissociates to vacancy bound Oand highly mobile O" states, which diffuse to the surface, giving rise to a high surface conductivity. PACS numbers: 73.25.4-i, 61.70.At, 77.20.+y