THE STRUCTURE OF PBI2 POLYTYPE-2H AND POLYTYPE-4H - A STUDY OF THE 2H-4H TRANSITION

The structures of 2H PbI2, grown from a gel, and of 4H PbI 2, obtained by thermal transformation of 2H at 423 K, have been determined by single-crystal X-ray analysis. The space groups are P3m1 for 2H and P3m1 for 4H. The occupancy of the atomic sites in the 2H and 4H structures is only about 80-85%, indicated by an observed density Dm of 5.0(1) Mg m-3 versus a crystallographic density Dx of 6.09 Mg m-3. The density of the polytype 12R grown by the Bridgman method is Dm=6.19(5) Mg m-3. The initial 2H and the transformed 4H structure have been refined to wR=0.021 and 0.049, respectively. A strong modulation of the occupancy of the Pb and I layers (for 2H the occupancies are 0.8 in both layers) along c has been derived for 4H. The occupancies vary between 0.7 and 0.9. The atomic layers with the highest occupancies in the 4H stacking are split into double layers with z coordinates differing by about 0.4 AA. For 2H PbI2, Mr=461.01, a2H=4.5580(5) AA, c2H=6.986(3) AA, V2H=125.69(8) AA-3, Z=1, Dx2H=6.09 Mg m-3, Mo Kalpha, lambda =0.71061 AA, mu =45.2 mm-1 and F(000)=188. For 4H PbI2, a 4H=4.55 (1) AA, c4H=13.962(5) AA, V4H=250.80(9) AA-3, Z=2 and F(000)=376.