Lattice parameters and spontaneous strain in AX ₂ 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 A X 2 polytypes of CdI 2 , PbI 2 , SnS 2 and SnSe 2 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 CdI 2 where small variations occur also for the a lattice parameter. The tensor of the spontaneous strain has its maximum component e 3 ≲ 12 × 10 −4 for SnS 2 . The powder diffraction pattern and lattice parameters of the phases of CdI 2 (2 H , 12 R , 4 H ), PbI 2 (2 H , 12 R ), SnS 2 (2 H , 18 R , 4 H ) and SnSe 2 (2 H , 18 R ) are given. JCPDS Diffraction File Nos. are: 40-1468 for CdI 2 -12 H ; 40–1469 for CdI 2 -2 H ; 40-1466 for SnS 2 -18 R , 40–1467 for SnS 2 -2 H ; 40–1465 for SnSe 2 -18 R . The other polytypes studied in this paper have data in earlier sets of the PDF.