Figure 4: Interpretative representation of the anomalous electrical conductivity pattern of highest purity MgO single crystals, supposedly a model insulator. Three temperature windows are identified, below 250°C, between 250-450°C and 450-700°C, each associated with different states of peroxy defects in the MgO matrix, designated as I, II and III. The dissociation of the defect III uniquely controls the electric conductivity of MgO through the release of positive hole charge carriers, h•, here shown as O• states.