Phase analysis and determination of local charge carrier concentration in eutectic Mg₂Si–Si alloys

Multiphase materials attract attention due to possible combination of various properties attributed to each phase. The phase diagram of Mg–Si system shows that solidification of a melt containing about 45 and 55 at.% of Mg and Si should result in formation of Mg₂Si and Si. Two alloys, Mg₄₅Si₅₅ and Mg₄₆Si₅₄ + 0.5 wt.% Cu have been synthesized and studied using XRD, SEM, and ²⁹Si NMR at 300 K, and the Seebeck effect, electrical resistivity, and thermal conductivity in the temperature range of 300–750 K have been measured. ²⁹Si NMR detects two distinct signals, at −177 and −80 ppm, in both materials, which are assigned to Mg₂Si and Si phases, respectively. Both phases are slightly nonstoichiometric and doped with Mg. Two phases also are found by XRD and electron microscopy. ²⁹Si NMR spin-lattice relaxation measurements in Mg₂Si and Si phases show at least two components, short and long, which can be attributed to different local carrier concentrations, high and low, respectively, reflecting a local electronic inhomogeneity in each phase. The carrier concentrations range between 0.6 × 10¹⁹ and 9 × 10¹⁹ cm⁻³. The Seebeck coefficient in both alloys is mostly determined by the Si phase, while the thermal conductivity is limited by the Mg₂Si phase with a lower value than that of the Si phase. By utilizing all characterization tools, we show how various experimental methods can be used as complementary methods to better understand the individual and combined properties of multiphase alloys. - Highlights: • Two distinct phases, Mg₂Si and Si, are found in Mg₄₅Si₅₅ and Mg₄₆Si₅₄ + 0.5 wt.% Cu alloys. • ²⁹Si NMR spin-lattice relaxation measurements demonstrate two relaxation components in each phase. • XRD, electron microscopy, and NMR have been demonstrated as complementary methods to study multiphase alloys