Experimental Determination of the Phase Equilibria of the La-Zn-Si System at 600 °C

The isothermal section at 600 °C of the La-Zn-Si system was established by using equilibrated alloys and the diffusion couple technique. Microstructural observation and phase identification were performed using optical microscopy, scanning electron microscopy with energy dispersive x-ray spectroscopy and a wave dispersive x-ray spectrometer, and x-ray powder diffraction. Seven ternary compounds were identified in this ternary system, and temporally designated as τ 1, τ 2, τ 3, τ 4, τ 5, τ 6 and τ 7. Two previously reported ternary compounds, viz. τ 1 and τ 2, were found to exist at 600 °C. EDS analysis revealed that the τ 1 phase contained Si from 2.4 to 6.1 at.% and La from 22.3 to 23.3 at.%. The compositional region of the τ 2 single-phase ranged from 28.3 to 40.7 at.% for Si and from 31.5 to 33.1 at.% for La. Two new ternary phases, τ 5 and τ 6, were identified. The τ 5 phase with the CeNiSi2-type was determined to have a composition range of 36.9-39.2 at.% Si for about 24 at.% La. The τ 6 phase with the ThCr2Si2-type was found to have a composition range of 18.9-19.7 at.% La and 27.4-30.6 at.% Si. The crystal structures of τ 3, τ 4, and τ 7 were not determined. The solubility of Si in LaZn13, LaZn11, La2Zn17, La3Zn22, and LaZn5 was negligible, and that of Si in LaZn4, LaZn2 and Zn in LaSi2, La3Si2 was determined as 5.1, 2.3, 16.2 and 3.8 at.%, respectively.