Structures of δ-PdCl₂ and γ-PdCl₂: Phases with Negative Thermal Expansion in One Direction

Phase up to reality: In the tetramorphic PdCl2 system, three of the four phases show a negative thermal expansion in one direction. The two high‐temperature phases, α‐PdCl2 and δ‐PdCl2 (see picture; left), contain planar ribbons of edge‐connected PdCl4 squares. The low‐temperature phase, γ‐PdCl2 (right), has corrugated layers of corner‐connected PdCl4 squares. It is a link between the ribbon structures (α and δ) and the cluster structure (β) which shows a normal thermal behavior.