The infrared spectrum of OH-compensated defect sites in C-doped MgO and CaO single crystals

Abstract

The IR spectra of OH-compensated point defects in MgO (and CaO) single crystals of various purity grades were reinvestigated. Three distinct groups of IR bands appear in the O-H stretching region: A, B and C around 3550 cm −1 (3650 cm −1), 3300 cm −1 (3450 cm −1) and 3700cm −1 (3750cm −1). They are assigned as follows: band A to the fully compensated, band B to the half compensated and band C to the overcompensated cation vacancies, [OḢV” catHȮ] ×, [OḢV” cat], and [OḢOḢV” catHȮ ] ̇ , respectively. Upon cooling to 80 K the band A shows a complex behavior partly due to the formation of Ha molecules by charge transfer and concommittant O − formation: [Ȯ (H 2)” catȮ] ×. The O − represent defect electrons or positive holes in the O 2− matrix. Bands A and B show a characteristic multiplet splitting which is caused by local lattice strains coming from carbon atoms on near-by interstitial position. The intensity ratios between the multiplet components remain constant regardless of temperature pretreatments up to 1470 K, but strong variations of the integral intensities are observed. These are caused by the highly mobile C atoms entering and leaving reversibly the cation vacancy sites as a function of temperature and of the quenching speed. When the C atoms push the H 2 molecules onto interstitial sites, an H-H stretching signal appears around 4150cm −1.