Articles you may be interested inAbsence of exchange interaction between localized magnetic moments and conduction-electrons in diluted Er3+ gold-nanoparticles J. Appl. Phys. 115, 17E128 (2014) The low temperature magnetic circular dichroism ͑MCD͒ and electron paramagnetic resonance ͑EPR͒ spectra of Cu͑II͒ doped Cs 2 ZrCl 6 are reported. The Cu͑II͒ ion is incorporated as the square planar copper tetrachloride ion, CuCl 4 2− , which substitutes at the Zr͑IV͒ site in the Cs 2 ZrCl 6 lattice, with a complete absence of axial coordination. Both the EPR and MCD show highly resolved spectra from which it is possible to determine the superhyperfine coupling constants and excited state geometries respectively. The Franck-Condon intensity patterns suggest that there is a substantial relaxation of the host lattice about the impurity ion. For the lowest energy 2 B 1g ͑x 2 -y 2 ͒ → 2 B 2g ͑xy͒ transition, both the magnetic dipole allowed electronic origin as well as vibronic false origins are observed. The high resolution of the spectra allowed the accurate determination of the odd parity vibrations that are active in the spectra. The opposite sign of the MCD of the two components of the 2 E g ͑xz , yz͒ excited state allows this splitting to be determined for the first time. Accurate and unambiguous spectral parameters for the CuCl 4 2− ion are important as it has become a benchmark transition metal complex for theoretical electronic structure calculations.