Li2B4O7 (LTB) single crystals are grown from stoichiometric and B2O3-enriched (1.6 and 2.4 mol %) melts. At large B2O3 excesses, the thermoluminescence intensity in the crystals is markedly higher, attesting to an increased density of structural defects. LTB crystals grown from B2O3-enriched melts may contain zones enriched in two types of defects. Defects of one type result from the breaking of bonds involving bridging oxygens, without significant changes in the position of neighboring atoms. Defects of the other type appear at large B2O3 excesses in the melt, which enable more significant changes in the structure of the boron-oxygen framework, leading to the formation of nanoscale regions differing in composition from LTB. The present results may be helpful in assessing the quality of LTB single crystals and in optimizing the crystal growth of LTB.