Neutral atoms in ionic lattices: Stability and ground-state properties ofKCl:Ag0

Abstract

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 clusters of different sizes. Two distinct methods have been employed: First, an ab initio wave-function based method on embedded clusters and second, density-functional theory ͑DFT͒ methods on clusters in vacuo involving up to 117 atoms. In the ab initio calculations the obtained equilibrium Ag 0 -Cl Ϫ distance R e is 3.70 Å, implying a large outward relaxation of 18%, along with 7% relaxation for the distance between Ag 0 and the first K ϩ ions in ͗100͘ 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 several 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 clusters. The unpaired electron density on silver and Cl ligands has been calculated as function of the metal-ligand distance and has been compared with values derived from electron-paramagnetic resonance data. The DFT calculations for all cluster sizes indicate that the experimental hyperfine and superhyperfine constants are compatible when R e is close to 3.70 Å. The important relation between the electronic stability of a neutral atom inside an ionic lattice and the local relaxation is established through a simple electrostatic model. As most remarkable features it is shown that ͑i͒ the cationic Ag 0 center is not likely to be formed inside AgCl, ͑ii͒ in the Ag 0 center encountered in SrCl 2 , the silver atom is probably located at an anion site, and ͑iii͒ the properties of a center-like KCl:Ag 0 would experience significant changes under hydrostatic pressures of the order of 6 GPa.

Neutral atoms in ionic lattices: Excited states ofKCl:Ag0

“…From the dependence of the CT excitation energy with the distance, we have given additional support to the outward displacement of 20% of the Cl ions surrounding the Ag 0 impurity, as proposed in previous studies [13][14][15] and in the preceding paper. 16 A model has been proposed to explain the surprisingly low energy of the CT transition. In spite of the very simple nature, the model serves to understand trends in excitation energies in a variety of impurities in ionic insulators.…”

“…Further details concerning the geometry relaxation can be found in Ref. 16. The rest of the atoms of the clusters, the AIEMP's, and the point charges are fixed at the lattice positions of the KCl crystal.…”

“…Results are reported for the 39-atom cluster, AgCl 6 K 12 Cl 8 K 6 Cl 6 2Ϫ described in the previous paper. 16 The details of the , are also given there.…”

Quantum Mechanical Study of the Lattice Distortions Induced by Aliovalent Pb2+ Impurities in Alkali Halides

“…The electronic spectra of doped ionic crystals are known to be strongly coupled with the crystal field, and thus are quite sensitive to the lattice distortions around the impurity. Employment of extended X-ray-absorption fine structure (EXAFS) technique would allow in principle the experimental measurement of these distortions, but in practice this is difficult in many cases: for example, when the impurity concentration is low [2][3][4], or when two or more impurity charge states coexist [5]. This is the main reason why reliable ab initio calculations of the local structure around defects are both timely and desirable.…”

Lattice Distortions Induced by As3+, Sb3+, and Bi3+ Substitutional Impurities in KCl: An Embedded Cluster Study

“…Much less is known, however, about the detailed local distortion induced by the introduction of the dopant. Employment of an extended X-ray absorption fine structure (EXAFS) technique would allow, in principle, the experimental measurement of these distortions, but in practice, this is difficult in many cases, for example, when the impurity concentration is low − or when two or more impurity charge states coexist . This is the main reason that reliable ab initio calculations of the local structure around defects are both timely and desirable.…”