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 Cr 3+ centers I and II at room temperature in KCl single crystals are theoretically investigated from the perturbation calculations for a rhombically distorted octahedral 3d 3 cluster. The impurity systems are attributed to the doped Cr(CN) 6 3− groups into KCl replacing the host KCl 6 5 − ones, associated with two nearest neighbor potassium vacancies V K in [011] and [ 0 1 ¯ 1 ¯ ] axes in center I and one nearest neighbor V K along [ 0 1 ¯ 1 ] and another next-nearest neighbor V K along [100] axis in center II, respectively. In center I, the four coplanar and two axial ligands CN − undergo the shifts ∆ R 1 (≈0.0044 nm) away from the V K and ∆ R 2 ′ (≈0.0144 nm) away from the central ion along Z axis, respectively, because of the electrostatic interactions. In center II, the impurity Cr 3+ is found to undergo the shift ∆ R C (≈0.0063 nm) towards the nearest neighbor V K along [ 0 1 ¯ 1 ] axis, while the two ligands in [001] and [ 0 1 ¯ 0 ] axes closest to the V K undergo the shifts ∆ R 1 (≈0.0081 nm) away from the respective V K , and the ligand intervening in the V K and the central ion experiences the shift ∆ R 2 (≈0.0238 nm) away from the V K 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 |Δg CT /Δg CF |>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. [ABSTRACT FROM AUTHOR]