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Explaining the optical spectrum of CrF2 and CuF2 model materials: role of the tetragonal to monoclinic instability

Abstract

The properties of MF2 (M = Cr, Cu) model compounds are usually interpreted assuming a Jahn-Teller effect leading to elongated MF64- units. By means of the analysis of experimental data and first-principles calculations on both the monoclinic P21/c structure and the parent rutile structure (tetragonal P42/mnm space group) we prove that such an assumption is not correct. It is shown that in MF2 compounds the MF64- complexes are actually compressed in the parent phase but along a different direction, a situation that is however hidden by an additional orthorhombic instability due to a negative force constant of b2g and b3g modes of the cell. Such distortion plays a key role for understanding the high experimental value of the lowest d-d transition energy, E1 = 1.23 and 0.93 eV for CrF2 and CuF2, respectively, when compared to the value E1 = 0.40 eV derived for the Jahn-Teller system KZnF3:Cu2+. Aside from reproducing reasonably the experimental values of spin allowed d-d transitions of both compounds, our first-principles calculations show the existence of an accidental degeneracy involving the yz and xy levels in the final P21/c structure. Moreover, the internal electric field of CrF2 and CuF2 is found to be much less anisotropic than in layered compounds like K2CuF4 and thus it has little influence upon the d-d transitions energy. The present results stress that the interpretation of experimental data through simple parameterized models can lead to wrong conclusions.

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Publication details

The article was received on 01 Apr 2019, accepted on 10 May 2019 and first published on 13 May 2019


Article type: Paper
DOI: 10.1039/C9CP01822K
Phys. Chem. Chem. Phys., 2019, Accepted Manuscript

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    Explaining the optical spectrum of CrF2 and CuF2 model materials: role of the tetragonal to monoclinic instability

    A. Jose Antonio and M. Moreno, Phys. Chem. Chem. Phys., 2019, Accepted Manuscript , DOI: 10.1039/C9CP01822K

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