Revision of the Ge–Ti phase diagram and structural stability of the new phase Ge4Ti5

Highlights: •New compound Ge4Ti5 found by experiments and by DFT ground state calculations. •Enthalpies of formation calculated for different Ge–Ti compounds. •Modifications of the Ge–Ti phase diagram suggested. -- Abstract: The binary phase diagram Ge–Ti was investigated experimentally by powder X-ray diffraction, scanning electron microscopy including EDX analysis, and differential thermal analysis. Total energies of the compounds GeTi3, GeTi2, Ge3Ti5, Ge4Ti5, Ge5Ti6, GeTi and Ge2Ti were calculated for various structure types employing electronic density-functional theory (DFT). Experimental studies as well as electronic calculations show the existence of a new phase Ge4Ti5 (Ge4Sm5-type, oP36, Pnma) which is formed in a solid state reaction Ge3Ti5 + Ge5Ti6 = Ge4Ti5. In addition, a significant homogeneity range was observed for the compound Ge3Ti5 and the composition of the liquid phase in the eutectic reaction L = Ge + Ge2Ti was found to be at significant higher Ge-content (97.5 at.% Ge) than reported in previous studies. Based on these new results, a modified phase diagram Ge–Ti is suggested. The zero-temperature lattice parameters and the formation enthalpies determined by DTF calculations were found to be in good agreement with experimental data