Melting of Tin at High Pressures

Introduction Information about the physical properties of materials at high pressures and temperatures is important for advancing our understanding of planetary interiors. Determining the melting curves of materials has attracted much interest in the planetary sciences. For example, the pressure dependencies of the melting curve and viscosity have been linked [1]. Furthermore, the accurate determination of melting curves may be important in studying the formation of a magma ocean in the early history of our planet [2]. Assessing the accuracy of melting at pressure requires a material for which melting has been characterized with different techniques. Sn fulfills these requirements. Its melting curve has been determined experimentally up to 8 GPa from static measurements [3-5]. Furthermore, its melting curve has been estimated from shock-wave experiments [6] and predicted from theory [7]. The melting curves from the static experiments agree to within 20°C up to the highest measured pressures (8 GPa) [3-5, 8]. Along the Sn-II/liquid boundary, the melting temperature increases with pressures up to 3.3 GPa, where it merges into a triple point at 310°C (Fig. 1) [3-5, 8]. The Sn-II/Sn-III phase boundary has a negative clapeyron slope [4, 5] and has been observed at 25°C between 9.8 GPa [9] and 10.0 ±0.6 GPa [10]. Up to at least 40 GPa at ambient temperature, no further phase transitions have been observed for Sn. Beyond this pressure, Sn-III (tetragonal) transforms to Sn-IV (bcc) [11]. In agreement with the static measurements of the melting curve, shock waves [6] and theory [7] predict an increase in the melting temperature of Sn with increasing pressure. However, due to the small pressure range of the static measurements, a detailed comparison of the melting curves at higher pressures is difficult.