High-Pressure Melting Curve of Zintl Sodium Silicide Na4Si4 by in Situ Electrical Measurements

The inorganic chemistry of the Na-Si system at high pressure is fascinating, with a large number of interesting compounds accessible in the industrial pressure scale, below 10 GPa. In particular, Na4Si4 is stable in this whole pressure range and thus plays an important role in understanding the thermodynamics and kinetics underlying materials synthesis at high pressures and high temperatures. In the present work, the melting curve of the Zintl compound Na4Si4 made of Na+ and Si-4(4-) tetrahedral cluster ions is studied at high pressures up to 5 GPa, by using in situ electrical measurements. During melting, the insulating Na4Si4 solid transforms into an ionic conductive liquid that can be probed through the conductance of the whole high-pressure cell, i.e., the system constituted of the sample, the heater, and the high-pressure assembly. Na4Si4 melts congruently in the studied pressure range, and its melting point increases with pressure with a positive slope dT(m)/dp of 20(4) K/GPa.