Polytypism and Packing-Dependent Colossal Positive and Negative Thermal Expansion in a 2D Layered Cerium-Based Coordination Polymer

A two-dimensional layered coordination polymer [Ce₂(H₂O)₂Prop₆]_∞ (Prop^– = C₂H₅COO^–) has been crystallized in the form of two polytypes (α and β). Their structural features have been studied by means of variable-temperature single-crystal (VT-SCXRD) and powder (VT-PXRD) X-ray diffraction along with pair distribution function analysis of total X-ray scattering data (PDF). As revealed from VT-SCXRD and PDF data, α and β polytypes possess a similar local structure of layers but differ in the arrangement of layers. Single crystals and powders of [Ce₂(H₂O)₂Prop₆]_∞ exhibit highly anisotropic thermal expansion, which is packing-dependent and drastically differs for α and β polytypes. Unit cell parameters for α demonstrate a nonlinear temperature dependence with colossal positive (α₃^CTE = +899 MK^–1) and negative (α₁^CTE = −427 MK^–1) linear coefficients of thermal expansion (CTEs) in the 190–210 K range. The temperature dependence of unit cell parameters for β demonstrates no colossal CTEs in the 100–300 K range. Periodic DFT-D calculations have shown the weakening of interlayer interactions (cohesive energies for α and β are 149 and 144 mJ/m² at 100 K, and 111 and 129 mJ/m² at 300 K, respectively) and relaxation of stressed layers upon heating.