Large Differences in the Optical Spectrum Associated with the Same Complex: The Effect of the Anisotropy of the Embedding Lattice

Abstract

Transition-metal complexes with a well-defined geometry are usually considered to display almost the same properties independently of the system where they are embedded. Here we show that the above statement is not true depending on the anisotropy of the host lattice, which is revealed in the form of the electric field created by the rest of lattice ions over the complex. To illustrate this concept we analyze the origin of the surprisingly large differences in the d-d optical transitions of two systems containing square-planar CuF complexes, CaCuF, and center II in Cu-doped BaZnF, even though the Cu-Fdistance difference is just found to be 1%. Using a minimalist first-principles model we show that the different morphology of the host lattices creates an anisotropic field that red-shifts the in vacuo complex transitions to the 1.25-1.70 eV range in CaCuF, while it blue-shifts them to the 1.70-3.0 eV region in BaZnF:Cu. This particular example shows how the lattice anisotropy strongly alters the optical properties of a given transition-metal complex. This knowledge opens a new path to tune the spectra of this large family of systems.

Changing the Usual Interpretation of the Structure and Ground State of Cu2+-Layered Perovskites

“…As recently proved for the layered systems Ba 2 ZnF 6 :Cu 2+23 or K 2 ZnF 4 :Cu 2+56 , this effect is greatly due to the anisotropic internal electric field, E R (r), generated by the electrostatic potential, V R (r), felt by the electrons localized in the complex due to all lattice ions lying outside. It is worth noting that V R (r) can induce shifts up to ∼1 eV on optical transitions 64 and is responsible for the different color displayed by Cr 3+ -based gemstones. 57 The shape of the V R (r) potential for (CH 3 NH 3 ) 2 CdCl 4 :Cu 2+ is depicted in Figure 6.…”

“…Indeed, that field is greatly responsible for the different ground state of K 2 CuF 4 and La 2 CuO 4 as well as for the color of Cr 3+ -based gemstones and the Egyptian Blue pigment . As E R ( r ) induces shifts up to ∼1 eV on optical transitions, the properties of transition metal systems cannot, in general, be understood considering only the isolated complex. Despite these facts, the influence of this internal field in the interpretation of data from transition metal compounds is still often ignored …”

Ground State and Optical Excitations in Compounds with Tetragonal CuF64– Units: Insight into KAlCuF6 and CuFAsF6

“…Indeed, they can induce shifts up to a 0.4 eV on the b 1g ( x 2 – y 2 ) → a 1g (3 z 2 – r 2 ) transition energy, which thus cannot be explained by considering only the local distortion. Bigger shifts in that transition (up to 1 eV) have been reported for compounds containing square-planar CuF 4 2– or CuO 4 6– units.…”