Minute crystal lattice misorientations, termed “subgrains”, degrade the optical homogeneity of Tl 3AsSe 3 (TAS) crystals. Three origins of the subgrain formation were identified: variations in the seeding process, thermal stresses, and local adhesion of molten TAS to its growth ampoule. Systematic experiments and improved diagnostic techniques led to defect control and to the growth of TAS crystals free from subgrains along their entire crystal length. Key results were (1) the achievement of high optical quality, (2) demonstration of 57% efficient second harmonic generation from CO 2 laser radiation and (3) a significant reduction in optical absorption.