Ground State and Optical Excitations in Compounds with Tetragonal CuF₆^4– Units: Insight into KAlCuF₆ and CuFAsF₆

Abstract

It has been argued that AAlCuF 6 (A = K, Cs) and CuFAsF 6 are the only known crystals that exhibit compressed CuF 6 4− units due to the Jahn−Teller effect. However, no grounds for this singular behavior have yet been reported. By means of first-principles calculations on such compounds and the isomorphous compounds involving Zn 2+ ions instead of Cu 2+ , we prove that neither the ground state nor the equilibrium geometry of CuF 6 4− complexes in KAlCuF 6 and CuFAsF 6 is the result of a Jahn−Teller effect. In contrast, it is shown that the internal electric field, E R (r), created by the rest of the lattice ions upon the localized electrons in the complex, plays an important role in understanding this matter as well as the d−d transitions of these two compounds. The energy of an optical transition is shown to involve two contributions: the intrinsic contribution derived for the isolated CuF 64− unit at equilibrium and the extrinsic contribution coming from the E R (r) field. Aside from reproduction of the experimental d−d transitions observed for KAlCuF 6 , it is found that in CuFAsF 6 the b 1g (x 2 − y 2 ) → a 1g (3z 2 − r 2 ) transition is not the lowest one due to the stronger effects from the internal field. Interestingly, the intrinsic contribution corresponding to that transition can simply be written as β(R eq − R ax ) where R eq and R ax are the equatorial and axial Cu 2+ −F − distances and β = 2.7 eV/Å is the same for all systems involving tetragonal CuF 6 4− units and an average metal− ligand distance close to 2.03 Å. This shows the existence of a common point shared by the Jahn−Teller system KZnF 3 :Cu 2+ and other non-Jahn−Teller systems such as KAlCuF 6 , CuFAsF 6 , K 2 ZnF 4 :Cu 2+ , and Ba 2 ZnF 6 :Cu 2+ . Although most Jahn−Teller systems display an elongated geometry, there are however many Cu 2+ compounds with a compressed geometry but hidden by an additional orthorhombic instability. The lack of that instability in KAlCuF 6 and CuFAsF 6 is also discussed.

Key Role of Deep Orbitals in the dx2–y2–d3z2–r2 Gap in Tetragonal Complexes and 10Dq

“…Compressed tetragonal CuF 6 4– units are formed in KAlCuF 6 or CuFAsF 6 pure compounds − in addition to Cu 2+ -doped crystals. ,− By contrast, in CuF 2 or A 2 CuF 4 (A = K, Na), the tetragonally compressed CuF 6 4– units undergo an additional orthorhombic distortion, favored by the existence of adjacent complexes which share F – ligands. ,, An orthorhombic instability also takes place in copper Tutton salts and in NH 4 Cl:CuCl 4 (H 2 O) 2 2– . , …”

Crystal Structures of Hexafluoridoantimonate(V) Salts of d‐block Metals in Oxidation State +2

“…As in the case of CuFAsF 6 , [11] the [CuF 6 ] 4− octahedra in CuFSbF 6 are considerably compressed with two shorter Cu−F bonds (2×1.8410(9) Å) within the [CuF 5 ] n 3n− chains and four longer Cu−F(Sb) bonds (4×2.120(2) Å). A recent study suggested that the local distortion of the CuF 6 octahedra in CuFAsF 6 is unlikely to be due to a Jahn‐Teller effect [14] . In contrast, it was shown that the internal electric field, created by the rest of the lattice ions upon the localized electrons in the complex, plays an important role in understanding this matter [14] …”

“…A recent study suggested that the local distortion of the CuF 6 octahedra in CuFAsF 6 is unlikely to be due to a Jahn‐Teller effect [14] . In contrast, it was shown that the internal electric field, created by the rest of the lattice ions upon the localized electrons in the complex, plays an important role in understanding this matter [14] …”

“…A recent study suggested that the local distortion of the CuF 6 octahedra in CuFAsF 6 is unlikely to be due to a Jahn-Teller effect. [14] In contrast, it was shown that the internal electric field, created by the rest of the lattice ions upon the localized electrons in the complex, plays an important role in understanding this matter. [14] Based on X-ray powder diffraction data (collected at ambient temperature) it was claimed that AgFSbF 6 and AgFBiF 6 are isotypic and that they crystalize differently than AgFRuF 6 or isotypic AgFAF 6 (A=As, Ir) salts.…”

“…[14] In contrast, it was shown that the internal electric field, created by the rest of the lattice ions upon the localized electrons in the complex, plays an important role in understanding this matter. [14] Based on X-ray powder diffraction data (collected at ambient temperature) it was claimed that AgFSbF 6 and AgFBiF 6 are isotypic and that they crystalize differently than AgFRuF 6 or isotypic AgFAF 6 (A=As, Ir) salts. [12] Crystal structure of determination of AgFSbF 6 on a single crystal (at 150 K) showed that it crystalizes isotypically to other AgFAF 6 salts (A=As, [15] Au, [16] Ir [12] ).…”