The 3R polymorph of CaSi sub(2)

The Zintl phase CaSi sub(2) 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 CaSi sub(2) in a hydrogen atmosphere (30 bar) at temperatures between 200 and 700 degree C transforms 6R-CaSi sub(2) quantitatively into 3R-CaSi sub(2). 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-CaSi sub(2). First principles density functional calculations reveal that 6R and 3R-CaSi sub(2) are energetically degenerate at zero Kelvin. With increasing temperature 6R-CaSi sub(2) stabilizes over 3R because of its higher entropy. This suggests that 3R-CaSi sub(2) should revert to 6R at elevated temperatures, which however is not observed up to 800 degree C. 3R-CaSi sub(2) may be stabilized by small amounts of incorporated hydrogen and/or defects. The common 6R form of CaSi sub(2) can be transformed quantitatively into 3R-CaSi sub(2) upon sintering in a hydrogen atmosphere.