Transparent semiconducting β-Ga2O3 single crystals were grown by the Czochralski method from an iridium crucible under a dynamic protective atmosphere to control partial pressures of volatile species of Ga2O3. Thermodynamic calculations on different atmospheres containing CO2, Ar and O2 reveal that CO2 growth atmosphere combined with overpressure significantly decreases evaporation of volatile Ga2O3 species without any harm to iridium crucible. It has been found that CO2, besides providing high oxygen concentration at high temperatures, is also acting as a minor reducing agent for Ga2O3. Different coloration of obtained crystals as well as optical and electrical properties are directly correlated with growth conditions (atmosphere, pressure and temperature gradients), but not with residual impurities. Typical electrical properties of the n-type β-Ga2O3 crystals at room temperature are: ϱ = 0.1 – 0.3 Ωcm, µn,Hall = 110 - 150 cm2V-1s-1, nHall = 2 - 6 × 1017 cm-3 and EIonisation = 30 - 40 meV. A decrease of transmission in the IR-region is directly correlated with the free carrier concentration and can be effectively modulated by the dynamic growth atmosphere. Electron paramagnetic resonance (EPR) spectra exhibit an isotropic shallow donor level and anisotropic defect level. According to differential thermal analysis (DTA) measurements, there is substantially no mass change of β-Ga2O3 crystals below 1200 °C (i.e. no decomposition) under oxidizing or neutral atmosphere, while the mass gradually decreases with temperature above 1200 °C. High resolution transmission electron microscopy (HRTEM) images at atomic resolution show the presence of vacancies, which can be attributed to Ga or O sites, and interstitials, which can likely be attributed to Ga atoms. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)