Theoretical studies of the defect structures for the two Cr3+ centers in KCl

The spin Hamiltonian (SH) parameters (i.e. the zero-field splitting parameters (ZFSPs) and g factors) and local structures of the two Cr3+ centers I and II at room temperature in KCl single crystals are theoretically investigated from the perturbation calculations for a rhombically distorted octahedral 3d3 cluster. The impurity systems are attributed to the doped Cr(CN)63− groups into KCl replacing the host KCl65− ones, associated with two nearest neighbor potassium vacancies VK in [011] and [01¯1¯] axes in center I and one nearest neighbor VK along [01¯1] and another next-nearest neighbor VK along [100] axis in center II, respectively. In center I, the four coplanar and two axial ligands CN− undergo the shifts ∆R1 (≈0.0044 nm) away from the VK and ∆R2′ (≈0.0144 nm) away from the central ion along Z axis, respectively, because of the electrostatic interactions. In center II, the impurity Cr3+ is found to undergo the shift ∆RC (≈0.0063 nm) towards the nearest neighbor VK along [01¯1] axis, while the two ligands in [001] and [01¯0] axes closest to the VK undergo the shifts ∆R1 (≈0.0081 nm) away from the respective VK, and the ligand intervening in the VK and the central ion experiences the shift ∆R2 (≈0.0238 nm) away from the VK along [100] axis. The charge-transfer (CT) contributions to g-shifts are found to be opposite in sign and more than half (characterized by the ratios |ΔgCT/ΔgCF|>50%) in magnitude compared with the CF ones for both centers. The local structures and the microscopic mechanisms of the relevant impurity and ligand shifts are discussed for the two centers.