The structure of PbI₂polytypes 2H and 4H: a study of the 2H-4H transition

The structures of 2H PbI 2 , 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 D m of 5.0(1) Mg m -3 versus a crystallographic density D x of 6.09 Mg m -3 . The density of the polytype 12R grown by the Bridgman method is D m =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 PbI 2 , M r =461.01, a 2H =4.5580(5) AA, c 2H =6.986(3) AA, V 2H =125.69(8) AA -3 , Z=1, D x2H =6.09 Mg m -3 , Mo K alpha , lambda =0.71061 AA, mu =45.2 mm -1 and F(000)=188. For 4H PbI 2 , a 4H =4.55 (1) AA, c 4H =13.962(5) AA, V 4H =250.80(9) AA -3 , Z=2 and F(000)=376.