Thermal Instability of β-Zn4Sb3: Insights from Transport and Structural Measurements

beta-Zn4Sb3 exhibits poor thermal stability, with zinc-rich specimens recently shown to be more stable. In this work, temperature-dependent transport measurements [Seebeck coefficient (S) and electrical conductivity (sigma)] between room temperature (RT) and 525 K were carried out on a zinc-poor specimen. The sequentially measured S and sigma data in the same measurement cycle show sharp changes in their absolute values between 450 K and 500 K during the heating cycle, which is not retraced back during cooling. A repeat measurement carried out on the specimen after similar to 1 month again shows the sharp changes in the absolute values between 450 K and 500 K, indicating reversibility of the process. Temperature-dependent synchrotron measurements were further carried out between RT and 525 K. Formation of elemental Sb was observed beyond 400 K. Between 450 K and 500 K, movement of zinc from lattice to interstitial position is observed, which is also accompanied by the onset of ZnSb formation. The overall zinc content within the beta-Zn4Sb3 phase is observed to increase with increasing temperature. These observations indicate that both the overall zinc content and the Zn-I/Zn-L ratio are crucial in stabilizing the beta-Zn4Sb3 phase.