The results of the study of quasibinary (lateral) and non-quasibinary polythermal sections in quadrilateral systems Y₂O₂S–Ga₂S₃–Tb₂O₂S show the prognosis of the established liquidity surface, where the nature of the chemical interaction is relatively simple. Thus, the Ga₂S₃–Y₂O₂S və Ga₂S₃–Tb₂O₂S sides of a quasi-triangular system are quasi-binary eutectic systems with a limited solubility based on both components. On the Y₂O₂S–Tb₂O₂S side, a continuous solid solution area is formed. Thus, in the Y₂O₂S–Ga₂S₃–Tb₂O₂S quasi-triangular system, crystallization ends not at the non-variant triple point, but at the low-temperature binary non-variant (eutectic e₁)

doi.org/10.32737/0005-2531-2023-1-55-63