EPR Study of Cu²⁺ Doped (C_nH_2n+1NH₃)₂CdCl₄ Crystals (n=1;3) with Layer Structure

Abstract

This work investigates the local structure around Cu²⁺ in the two-dimensional perovskite layers (C_nH_2n+1NH₃)₂CdCl₄ (n = 1, 3) doped with Cu²⁺. The interplay between the Jahn-Teller effect and the host lattice compressive strain is analyzed through EPR spectroscopy as a function of the temperature in both powder and single crystals doped with different Cu²⁺ concentrations. The high tendency of Cu²⁺ to form aggregates in these layered systems makes it difficult to study the isolated CuCl₆ ^4- units. Besides, it is shown that Cu²⁺ replaces Cd²⁺ by distorting the compressed D_4h site symmetry to an elongated rhombic CuCl₆ ^4- geometry. The long Cu-Cl axis is randomly distributed between either of the two equivalent Cd-Cl directions within the layer. The temperature dependence of the principal gyromagnetic values and the A_ll hyperfine constant reveal the existence of a dynamic Jahn-Teller effect related, not to three potential wells as it is usually found in octahedral sites, but to two potential wells. This behavior is explained by the energy increase of the potential well associated with the out-of-layer Cu-Cl elongation, due to site anisotropy. The influence of the D_2h symmetry upon the isotropic hyperfine constant is briefly discussed.