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 (α3CTE = +899 MK–1) and negative (α1CTE = −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.