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

Abstract

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. Especially, Na4Si4 is stable in this whole pressure range, and thus plays an important role for 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 Si4 4tetrahedral 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 dTm/dp of 20(4) K/GPa.

Thermoelastic equation of state and melting of Mg metal at high pressure and high temperature

“…Similar method has been recently used for melting curve measurement of Na 4 Si 4 . 24 The knowledge of Mg melting curve also allowed to refine the pressure or temperature values in some cases.…”

A Review on Metastable Silicon Allotropes

“…Compared with the HPHP and direct HP methods, the phase transition pressure is lower than that in the chemical HP method under the same conditions, but the test results of binary system are not significantly different from the properties of the metastable phase prepared by the monolithic silicon system. Recently, Courac et al [ 68 ] carried out a detailed study of its high-pressure/high-temperature phase diagram and thermodynamics and kinetics behaviors, which contributes to the use of Na 4 Si 4 for high-pressure syntheses.…”

In Situ High-Pressure Synthesis of New Outstanding Light-Element Materials under Industrial P-T Range

“…In situ studies [ 97 ] (e.g., Figure 11 d) allowed to observe the following features of phase transformations in Na-Si system: (i) the Na-Si equilibrium phases are Na 4 Si 4 , Na 8 Si 46 (sI), and Na 30 Si 136 (sII-HP), stoichiometric compounds without solid solutions; (ii) sII-HP coexists with Si-I at “low” temperatures, while sI (or Na 4 Si 24 ) becomes stable at high temperature (if Si is not in excess); (iii) high Na content makes sII-HP stable up to melting temperatures and sII-HP melts congruently; (iv) co-existence of sI and sII-HP (without Si-I) is possible. Such observations are typical for in situ XRD probing of the synthesis; this information together with XRD and electrical in situ data on T 1 [ 99 ], T 4 [ 98 ], and T 6 [ 100 ], allowed drawing the experimental approximation of phase diagram in the 5–8 GPa range [ 98 ] ( Figure 11 e).…”