Constitutional properties of the La-Cu-Mg system at 400 deg. C

The phase equilibria of the La-Cu-Mg system have been established at T = 400 deg. 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 LaCu6 deeply extends into the ternary field, solving about 20 at.% Mg. The ternary compounds La2Cu2Mg (τ1) and LaCuMg (τ2), 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: LaCuMg2 (τ3), LaCuMg3 (τ4) and LaCuMg4 (τ5). The diffraction pattern of LaCuMg2 has been indexed on the basis of the same crystal structure of the binary LaMg3, cubic BiF3-type. Seven more ternary phases were detected in different regions of the 400 deg. C isothermal section: LaCu2Mg (τ6), La8Cu82-87Mg10-5 (τ7), LaCu9Mg2 (τ8), La2CuMg9 (τ9), La15Cu41Mg44 (τ10), ∼LaCu2Mg2 (τ11) and ∼LaCu4Mg3 (τ12). The crystal structure of LaCu2Mg (τ6) was determined and refined by means of Rietveld's method, resulting hexagonal, hP8-GdPt2Sn type, space group P6 3/mmc. The LaCu9Mg2 (τ8) crystallizes in a new ordered version of the hexagonal CeNi3 structure type, space group P6 3/mmc