The structures of 2H PbI2, grown from a gel, and of 4H PbI2, 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, a4H=4.55 (1) AA, c4H=13.962(5) AA, V4H=250.80(9) AA-3, Z=2 and F(000)=376.
Structural transformations between polytypes of a given material are expected to lead to lattice relaxations. Powder X‐ray diffraction of basic AX2 polytypes of CdI2, PbI2, SnS2 and SnSe2 showed these relaxations for the repetition unit along the stacking axis, conventionally the c axis. No variation of the lattice parameters were detected in the basal plane (001), except for CdI2 where small variations occur also for the a lattice parameter. The tensor of the spontaneous strain has its maximum component e3≲ 12 × 10−4 for SnS2. The powder diffraction pattern and lattice parameters of the phases of CdI2 (2H, 12R, 4H), PbI2 (2H, 12R), SnS2 (2H, 18R, 4H) and SnSe2 (2H, 18R) are given. JCPDS Diffraction File Nos. are: 40‐1468 for CdI2‐12H; 40–1469 for CdI2‐2H; 40‐1466 for SnS2‐18R, 40–1467 for SnS2‐2H; 40–1465 for SnSe2‐18R. The other polytypes studied in this paper have data in earlier sets of the PDF.
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