Novel Si Networks in the Ca/Si Phase Diagram under Pressure

Abstract

The rich Ca/Si binary phase diagram is explored here theoretically and over a range of pressures, through four stoichiometries: Ca2Si, CaSi, CaSi2, and CaSi3. The calculations are calibrated by recovering the known P = 1 atm structures (even as they suggest metastable alternatives for some) and by predicting correctly the range of pressures at which CaSi3, which has a positive heat of formation at P = 1 atm, can be synthesized. Ca2Si, in accord with its Zintl compound nature, features isolated Si4– ions then chains until decomposition. CaSi, which begins as a Zintl phase (albeit metallic) chain structure evolves to rings and zeolitic-channel structures, while CaSi2 explores a variety of triply connected Si polytypes to Laves phases with increasing pressure. CaSi3 has a calculated narrow range of stability of 8–18 GPa. Interestingly, all the phases studied are calculated to decompose to the elements at P ∼ 250 GPa. This unusual behavior is traced to the special compressibility of elemental Ca and the backward charge transfer from Si to Ca. All the phases studied are metallic, except that Ca2Si is a semiconductor at 1 atm.