High pressure behaviour and elastic properties of a dense inorganic–organic framework

The high pressure behaviour of a cubic dense inorganic–organic framework [DABCOH₂²⁺][K(ClO₄)₃] (DABCOH₂²⁺ = diazabicyclo[2.2.2]octane-1,4-diium) has been systematically studied via synchrotron X-ray powder diffraction, over the range of 0–3.12 GPa. The framework [DABCOH₂²⁺][K(ClO₄)₃] shows a more rigid response, with a bulk modulus of 30(1) GPa and an axial compressibility of 7.6(4) × 10⁻³ GPa⁻¹, compared with ZIF-8 and the dense hybrid solar cell perovskite CH₃NH₃PbI₃. Density functional theory calculations reveal that the structural change in [DABCOH₂²⁺][K(ClO₄)₃] is attributed to the contraction of the KO₁₂ polyhedra, which consequently results in the rotation of the perchlorate linkers and synergistic movement of the DABCOH₂²⁺ guest. Further extensive theoretical calculations of full elastic tensors give full mapping of the Young's moduli, shear moduli and Poisson's ratios of [DABCOH₂²⁺][K(ClO₄)₃], which are in the range of 31.6–36.6, 12.3–14.6 GPa and 0.2–0.32, respectively. The Young's and shear moduli of [DABCOH₂²⁺][K(ClO₄)₃] are larger than those of cubic MOF-5, ZIF-8 and CH₃NH₃PbI₃. In addition, the narrow range of Poisson's ratios in [DABCOH₂²⁺][K(ClO₄)₃] indicates its very isotropic nature in response to biaxial stress.