The 3R polymorph of CaSi2

highlights
• Quantitative and reproducible bulk synthesis of the rare 3R polymorph of CaSi2.
• Clarification of the energetic relation between 3R and conventional 6R form.
• 3R-CaSi2 is presumably stabilized by small amounts of incorporated hydrogen and/or defects.

The Zintl phase CaSi2 commonly occurs in the 6R structure where puckered hexagon layers of Si atoms are stacked in an AA′BB′CC′ fashion. In this study we show that sintering of CaSi2 in a hydrogen atmosphere (30 bar) at temperatures between 200 and 700 °C transforms 6R-CaSi2 quantitatively into 3R-CaSi2. In the 3R polymorph (space group R-3m (no. 166), a=3.8284(1), c=15.8966(4), Z=3) puckered hexagon layers are stacked in an ABC fashion. The volume per formula unit is about 3% larger compared to 6R-CaSi2. First principles density functional calculations reveal that 6R and 3R-CaSi2 are energetically degenerate at zero Kelvin. With increasing temperature 6R-CaSi2 stabilizes over 3R because of its higher entropy. This suggests that 3R-CaSi2 should revert to 6R at elevated temperatures, which however is not observed up to 800 °C. 3R-CaSi2 may be stabilized by small amounts of incorporated hydrogen and/or defects.

The common 6R form of CaSi2 can be transformed quantitatively into 3R-CaSi2 upon sintering in a hydrogen atmosphere.Figure optionsDownload as PowerPoint slide