Lattice effects on an impurity center: CuX4(NH3)(2)(2-) centers (X=Cl, Br) in NH4X

The optical properties of CuX4(NH3)(2)(2-) centers embedded in NH4X lattices (X=Cl, Br) where Cu2+ occupies an interstitial position have been explored through MS X alpha calculations performed at different values of the Cu2+-X- (R-eq) and Cu2+-N (R-ax) distances. The calculations include the effect of the electrostatic potential due to the rest of the lattice, V-R, upon the localized electrons of the center. It has been shown that V-R decreases significantly the separation between n(L)p(X) and 2p(N) orbitals (n(L)=3 for Cl; n(L)=4 for Br) and thus plays a key role for understanding the existence of four charge transfer (CT) bands in the optical domain. The high splitting found between orbitals mainly built from \2p(z)(N)] and \2p(j)(N)] (j=x,y) is shown to arise from an internal splitting in the NH3 molecule transferred to the complex. From the R-eq and R-ax dependence of CT transitions, it is shown that the 600-cm(-1) redshift undergone by the first CT transition of CuCl4(NH3)(2)(2-) in NH4Cl just below T-c=243 K involves an R-eq increase of similar to 2 pm in agreement with Raman data. The present results stress the importance of V-R for a right understanding of properties due to impurities placed at off-center positions.