OPTICAL-PROPERTIES OF MGNB2O6 SINGLE-CRYSTALS - A COMPARISON WITH LINBO3

As-grown MgNb2O6 single crystals grown by the Czochralski method show a deep-blue colour due to the presence of broad optical absorption in the visible spectral region. The absorption of the samples is modified by thermal treatments in air and in vacuum. By heating in air the sam ples become transparent and by heating in a 1 x 10(-2) mbar vacuum a c omplex and broad absorption in induced. An energy band gap of 5.25 +/- 0.15 eV has been determined. Oxidized samples have an absorption edge at 4.20 eV (at 15 K) but some pre-edge absorption is also present. Th is latter absorption and the associated photoluminescence emission wit h a maximum at 2.5 eV are attributed to interstitial Nb ions or Nb ion s in Mg lattice sites. The optical absorptions induced by x-ray irradi ation and vacuum reduction have been discussed in terms of the defects introduced. An absorption band at 2.30 eV induced by x-ray irradiatio n is attributed to holes trapped in oxygen (O--bound small polarons). The absorption band at 1.69 eV has been attributed to an electron trap ped in Nb5+ ions (Nb4+ small polarons). Another absorption observed at 1.20 eV has been ascribed to a Mg-related electron trap centre. The p ossible presence of bipolaron centres and other electronic traps is di scussed. A comparison with the optical absorption bands observed in Li NbO3 crystals has been established.