Luminescent, optical and electronic properties of Na₂Mo₂O₇ single crystals

Luminescent, optical and electronic properties of Na₂Mo₂O₇ single crystals grown by two different procedures, the conventional Czochralski method and the low-temperature gradient Czochralski technique, are presented. The band structure calculations were performed in generalized gradient and local density approximations and allowed to determine the composition and structure of the valence and conduction bands. It is shown that the bottom of the conduction band is formed by the 4d states of Mo in octahedral oxygen coordination while the contribution of 4d states of Mo in tetrahedral oxygen coordination starts at 0.7 eV above the bottom of the conduction band. The optical bandgap of the crystals was estimated from absorption spectra to E_g^opt = 3.2 eV. The absorption band observed below the fundamental absorption edge at 370 nm is connected with the presence of oxygen vacancies. The luminescence spectrum is represented by a single emission band at 1.82 eV (T = 10 K), which is ascribed to the excitons self-trapped at oxyanionic complexes. The origin of charge carrier traps was studied using the combination of the methods of electron paramagnetic resonance and thermostimulated luminescence. Several electron and hole trapping centers were revealed including self-trapped holes and F⁺ centers. The influence of trapping centers on the excitation energy transfer to the emission centers is discussed.