Ab initio investigations on the crystal structure, formation enthalpy, electronic structure, chemical bonding, and optical properties of experimentally synthesized isoreticular metal–organic framework-10 and its analogues: M-IRMOF-10 (M = Zn, Cd, Be, Mg, Ca, Sr and Ba)

Abstract

The equilibrium solid-state structure, electronic structure, formation enthalpy, chemical bonding, and optical properties of IRMOF-10 and its alkaline earth metal analogues M-IRMOF-10 (M = Cd, Be, Mg, Ca, Sr, Ba) have been investigated with density functional calculations. The unit cell volume and atomic positions were fully optimized with the GGA functional. This supplements the incomplete experimental structural parameters available for Zn-IRMOF-10. The calculated bulk moduli decrease monotonically from Zn to Cd, and from Be to Ba, and indicate that Zn-IRMOF-10 and its analogues are relatively soft materials. The estimated bandgap values are in the range 2.9 to 3.0 eV, indicating nonmetallic character. Importantly, the bandgaps within the M-IRMOF-10 series (containing a rather long 4,4′-biphenyldicarboxylate linker) are smaller than those within the M-IRMOF-1 series (shorter benzene dicarboxylate linker). The optical properties (dielectric function ε(ω), refractive index n(ω), absorption coefficient α(ω), optical conductivity σ(ω), reflectivity R(ω), and electron energy-loss spectrum L(ω)) of the M-IRMOF-10 series were computed. The observation of very small reflectivities over a wide energy range suggests possible uses in hybrid solar cell applications. The main characteristics of the optical properties are similar for the whole series although differences are seen in the details. An analysis of chemical bonding in the M-IRMOF-10 series reveals as might be anticipated that M–O bonds are largely ionic whereas C–O, C–H and C–C exhibit mainly covalent interactions. The BOP values of M–O decrease through the series when going from Zn to Cd, and from Be to Ba, i.e. the ionicity increases and the covalency decreases for the M–O bonds.