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

A two-dimensional layered coordination polymer [Ce-2(H2O)(2)Prop(6)](infinity) (Prop(-) = C2H5COO-) has been crystallized in the form of two polytypes (alpha and beta). 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, alpha and beta polytypes possess a similar local structure of layers but differ in the arrangement of layers. Single crystals and powders of [Ce-2(H2O)2Prop(6)](infinity) exhibit highly anisotropic thermal expansion, which is packing-dependent and drastically differs for alpha and beta polytypes. Unit cell parameters for alpha demonstrate a nonlinear temperature dependence with colossal positive (alpha(3) (CTE) = +899 MK-1) and negative (alpha(1) (CTE) = -427 MK-1) linear coefficients of thermal expansion (CTEs) in the 190-210 K range. The temperature dependence of unit cell parameters for beta demonstrates no colossal CTEs in the 100-300 K range. Periodic DFT-D calculations have shown the weakening of interlayer interactions (cohesive energies for alpha and beta are 149 and 144 mJ/m(2) at 100 K, and 111 and 129 mJ/m(2) at 300 K, respectively) and relaxation of stressed layers upon heating.