Experimental verification of vacancy defects and their vital role on reddish Bi₄Ge₃O₁₂ single crystals

Abstract Reddish bismuth germanate (Bi4Ge3O12, BGO) crystals are found during the industrial large-size and -scale crystal growth by Bridgman method, which directly causes visible absorption. The reddish can be eliminated by a modified oxygen-annealing. Oxygen vacancy is further confirmed through electron paramagnetic resonance, X-ray photoelectron spectroscopy, Raman spectroscopy to demonstrate the existence of vacancy defects and elucidate the nature and role. Bi vacancy in reddish BGO single crystals is verified via scanning transmission electron microscopy, energy-dispersive spectroscopy, and high-angle annular dark-field scanning TEM. The essence of annealing and vacancy evolution is also discussed.