Neutral atoms in ionic lattices: Stability and ground-state properties of KCl : Ag-0

The equilibrium geometry of Ag-0 centers formed at cation sites in KCl has been investigated by means of total-energy calculations carried out on clus ters of different sizes. Two distinct methods have been employed: First, an ab initio wave-function based method on embedded clusters and second, dens ity-functional theory (DFT) methods on clusters in vacuo involving up to 11 7 atoms. In the ab initio calculations the obtained equilibrium (AgCl-)-Cl- 0 distance R-e is 3.70 Angstrom, implying a large outward relaxation of 18% , along with 7% relaxation for the distance between Ag-0 and the first K+ i ons in [100] directions. A very similar result is reached through DFT with a 39-atom cluster. Both approaches lead to a rather shallow minimum of the total-energy surface, the associated force constant of the A(1g) mode is se veral times smaller than that found for other impurities in halides. These conclusions are shown to be compatible with available experimental results. The shallow minimum is not clearly seen in DFT calculations with larger cl usters. The unpaired electron density on silver and Cl ligands has been cal culated as function of the metal-ligand distance and has been compared with values derived from electron-paramagnetic resonance data. The DFT calculat ions for all cluster sizes indicate that the experimental hyperfine and sup erhyperfine constants are compatible when R-e is close to 3.70 Angstrom. Th e important relation between the electronic stability of a neutral atom ins ide an ionic lattice and the local relaxation is established through a simp le electrostatic model. As most remarkable features it is shown that (i) th e cationic Ag-0 center is not likely to be formed inside AgCl, (ii) in the Ag-0 center encountered in SrCl2, the silver atom is probably located at an anion site, and (iii) the properties of a center-like KCl:Ag-0 would exper ience significant changes under hydrostatic pressures of the order of 6 Gpa .