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Transformation and phase separation of Mg2X (X = Si; Sn) solid solutions under Mg depletion and Mg excess
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DOI：10.1039/D6TA01411A.png)
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Abstract
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This study investigates the phase transformation in Mg2X (X = Si; Sn) solid solutions under Mg loss and Mg intake; in order to establish a mechanism for the observed demixing into Si-rich and Sn-rich Mg2X phases as well as further phases. Furthermore; it clarifies the influence of Mg availability on that process and assesses thermodynamic stability and phase boundaries (solubility limits) within the Mg-Si-Sn system. By annealing Mg2SixSn1-x samples at 600 °C under different Mg vapor pressure; we verify the central role of the Mg vapor pressure for the phase evolution and reveal that the system does not decompose spinodally. We furthermore identify contrasting behavior: driven by Mg loss; Mg-depleted samples exhibited the formation of Sn-rich Mg2X domains and Si precipitates; while the remaining matrix drifts toward the Si-rich limit of the miscibility gap; followed by formation of a liquid Sn-rich Sn-Mg melt. In contrast; under Mg-intake; the rapid infiltration of liquid Mg dissolving Sn from the matrix leads to the formation of a Mg-rich Mg-Sn melt; from which Sn-rich Mg2X precipitates. Consequently; the matrix again shifts toward the Si-rich boundary. We also clarify the limits of the incoherent miscibility gap in Mg2X; rationalize apparently contradicting reports on this and indicate potential pathways to improve thermoelectric performance through energy filtering as result of controlled phase separation. These findings provide crucial insight for optimizing Mg2(Si; Sn)-based thermoelectric materials by means of phase stabilization and phase engineering.
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