Phase equilibria and thermodynamic modeling of the Ge-Te-Bi ternary system

Given the critical role of thermodynamic properties in material design, particularly their relationship with composition, structure and material stability, this study was dedicated to establishing an accurate thermodynamic database for the Ge-Te-Bi system, a key family of phase-change memory materials. The phase equilibria of the Ge-Te-Bi system were investigated through a combination of key equilibrated alloys, thermodynamic modeling and first-principles calculations. Twenty-eight ternary alloys were synthesized and analyzed using X-ray diffraction (XRD), electron probe micro-analysis (EPMA), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) to determine the isothermal sections at 673 and 573 K. The solubilities of Bi in the α-GeTe phase, and of Ge in the Bi 2 Te 3 , BiTe, Bi 4 Te 3 and Bi 7 Te 3 phases, were measured. The existence of the ternary phases τ 5 -Ge 5 Te 8 Bi 2 and τ 6 -Ge 4 Te 7 Bi 2 was confirmed in the Ge-Te-Bi system. Based on the experimental equilibria data from the literature and the present work, a thermodynamic description of the Ge-Te-Bi system was carried out by using the CALPHAD (CALculation of PHAse Diagrams) method. The associated solution model (Ge, GeTe, Te, Bi 2 Te 3 , Bi) 1 was employed to describe the liquid phase. A set of self-consistent thermodynamic parameters of the Ge-Te-Bi system was conclusively obtained. Most of the reliable experimental data were reproduced by the present thermodynamic modeling. This study is important for the development of phase-change memory materials with high thermal stability and excellent properties. • The phase equilibria data of the Ge-Te-Bi ternary systems are reviewed and the contradictions are elucidated. • First self-consistent thermodynamic database for the Ge-Te-Bi system is established via integrated CALPHAD modeling, first-principles calculations, and experimental validation. • Phase equilibria at 573 K and 673 K are experimentally determined, revealing significant solubilities of Bi in α-GeTe and Ge in bismuth tellurides. • Compounds τ₅ and τ₆ have been demonstrated to exist stably in the Ge-Te-Bi system.