Instability of some divalent rare earth ions and photochromic effect

TL;DR

This paper investigates the instability of certain divalent rare earth ions in alkaline earth fluorides, linking their electronic structure to the formation of anion vacancies and explaining the photochromic effect observed under irradiation.

Contribution

It reveals the mechanism behind the instability of divalent rare earth ions and their role in photochromic effects through the formation of charged anion vacancies in fluoride crystals.

Findings

Divalent rare earth ions have d1(eg) states in the conduction band, causing instability.

Formation of anion vacancies stabilizes divalent ions during irradiation.

Photochromic effects result from electron transitions between ions and vacancies.

Abstract

It was shown that the divalent rare earth ions (La, Ce, Gd, Tb, Lu, and Y) in cubic sites in alkaline earth fluorides are unstable with respect to electron autodetachment since its d1(eg) ground state is located in the conduction band which is consistent with the general tendency of these ions in various compounds. The localization of doubly degenerate d1(eg) level in the conduction band creates a configuration instability around the divalent rare earth ion that leading to the formation of anion vacancy in the nearest neighborhood, as was reported in the previous paper [Journal of Physics and Chemistry of Solids 74 (2013) 530-534]. Thus, the formation of the stable divalent ions as La, Ce, Gd, Tb, Lu, and Y (PC+ centers) in CaF2 and SrF2 crystals during x-ray irradiation occurs via the formation of charged anion vacancies near divalent ions (Re2+va), which lower the ground state of the divalent ion relative to the conductivity band. Photochromic effect occurs under thermally or optically stimulated electron transition from the divalent rare earth ion to the neighboring anion vacancy and reverse under ultraviolet light irradiation.