Electronic structure that gives rise to the optical properties of zinc, cadmium and silver hexacyanocobaltates(III)

Abstract

Transition metal hexacyanocobaltates(III) correspond to coordination polymers that present physical properties that can be used for technological applications. In this sense, properties like spin-crossover under different physical stimuli (light, temperature, pression, etc) or the colossal mechanical negative thermal expansion has been reported as astonishing properties of these materials (Goodwin et al., 2008; Goodwin et al., 2008; Avila et al., 2022) [1–3]. In this contribution, the electronic properties of hexacyanocobaltates(III) that contain Zn²⁺, Cd²⁺ and Ag¹⁺ as metal ions, with a closed d-shell electronic configuration, are studied by means of combined UV–Vis spectroscopy and ab-initio calculations. The influence of the outer metals (Zn, Cd, Ag) when forming the coordination polymers as well as the effect of the inner octahedral moiety [Co(CN)₆]^3- have on the macroscopic electronic properties is discussed. Metal to ligand charge transfer transitions that produce the optical behavior are also clarified. Furthermore, the origin of the band gap transitions for Zn and Cd hexacyanocobaltates(III) is reported for the first time and supported by ab-initio calculations. New optical band gap energy values are proposed from a combined Urbach and Tauc analysis. Finally, the effects of metal substitution in Prussian blue analogues and the structural phase shift towards a zeolite-like phase on the band gap are analyzed.