THE CRYSTAL-STRUCTURE OF PARAREALGAR, AS4S4

Pararealgar, a polymorph of realgar (alpha-As4S4), crystallizes in the monoclinic space group P2(1)/c, with a = 9.909(2), b = 9.655(1), c = 8.502(1) Angstrom, beta = 97.29(1)degrees, V = 806.8(2) Angstrom(3). A single crystal of pararealgar was obtained by light exposure of realg ar. The structure was determined by direct methods and refined by leas t-squares technique to a final R index of 3.39%. The structure consist s of discrete covalently bonded As4S4 molecules, which are held togeth er by van der Waals forces. In each molecule, one As atom links 2As 1S, another links 35, and the other two link 1As + 28. Therefore this molecule differs from that of alpha-As4S4 but is the same as that foun d in As4S4(II). The difference between the structure of pararealgar an d As4S4(II) is due to different molecular packing.