The optical properties of CuX4(NH3)22- centers in NH4X lattices, where Cu2+ occupied an interstitial position, were predicted by performing calculations with various values of the Cu2+-X- and Cu2+-N distances. The calculations included the effect of the electrostatic potential, which was due to the remainder of the lattice, upon the localized electrons of the center. From the dependence of charge transfer transitions upon the Cu2+-X- and Cu2+-N distances, it was deduced that a 600/cm red-shift which was undergone by the first charge transfer transition of CuCl4(NH3)22- in NH4Cl, just below 243K, involved a 2pm increase in the Cu2+-X- distance. This was in agreement with Raman data. The results emphasized the importance of the electrostatic potential, due to the remainder of the lattice, to an understanding of the properties of impurities at off-center positions.

J.A.Aramburu, M.Moreno: Physical Review B, 1997, 56[2], 604-10