Constitutional properties of the La–Cu–Mg system at 400 °C

Abstract

The phase equilibria of the La–Cu–Mg system have been established at T = 400 °C by means of light optical microscopy (LOM), scanning electron microscopy (SEM), electron probe microanalysis based on energy dispersive X-ray spectroscopy (EPMA-EDS) and X-ray powder diffraction (XRPD). Some binary phases of the subsystems La–Cu and La–Mg solve an appreciable amount of the third element, forming ternary solid solutions whose homogeneity ranges are at a constant La content: particularly the LaCu 6 deeply extends into the ternary field, solving about 20 at.% Mg. The ternary compounds La 2Cu 2Mg (τ 1) and LaCuMg (τ 2), previously reported in the literature, were confirmed in this work. Three more ternary phases were found to exist along the line corresponding to the Cu/La equiatomic ratio: LaCuMg 2 (τ 3), LaCuMg 3 (τ 4) and LaCuMg 4 (τ 5). The diffraction pattern of LaCuMg 2 has been indexed on the basis of the same crystal structure of the binary LaMg 3, cubic BiF 3-type. Seven more ternary phases were detected in different regions of the 400 °C isothermal section: LaCu 2Mg (τ 6), La 8Cu 82–87Mg 10–5 (τ 7), LaCu 9Mg 2 (τ 8), La 2CuMg 9 (τ 9), La 15Cu 41Mg 44 (τ 10), ∼LaCu 2Mg 2 (τ 11) and ∼LaCu 4Mg 3 (τ 12). The crystal structure of LaCu 2Mg (τ 6) was determined and refined by means of Rietveld's method, resulting hexagonal, hP8-GdPt 2Sn type, space group P6 3 / mmc. The LaCu 9Mg 2 (τ 8) crystallizes in a new ordered version of the hexagonal CeNi 3 structure type, space group P6 3 / mmc.