Pressure-induced phase of sulfur-selenium

Crystals of a fibrous phase of sulfur-selenium obtained at 20 kilobars and 280 degrees C are trigonal, the most probable space groups being P3(1) and P3(2,) with a = 7.85, c = 4.62 +/- 0.01 A. The unit cell contains nine atoms, and the measured density of 3.20 g/cm(3) implies five sulfur and four Se atoms. The structure contains mixed atom helices of 1.54 A pitch and 0.91 A average radius.