Lattice parameters and spontaneous strain in AX₂ polytypes: CdI₂, PbI₂ SnS₂ and SnSe₂

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.