Configurational instability at the excited impurity ions in alkaline earth fluorites

Abstract

The formation of intrinsic defects by ionizing radiation in some ionic crystals in the anion sublattice is only associated with the instability of the self-trapped exciton up to now. In this paper we propose a new mechanism for the formation of the defects in the anion sublattice associated with the Janh-Teller instability occurring near the cation impurities in the excited state. The instability occurs when the degenerate excited state of the impurity ion is localized in the conduction band. We believe that the configuration interaction between the discrete impurity level and host continuum (effect Fano) plays an important role in this process.

Instability of some divalent rare earth ions and photochromic effect

“…According to the mechanism proposed in our previous paper [18], there are two necessary conditions for the formation of PC + centers (Re 2+ v a ) by x-ray radiation at Table 2. High-frequency (ε ∞ ) and static (ε S ) dielectric constant [81,82] and Gd 2+ with adjacent anion vacancy.…”

“…Efficient creation of self-trapped holes indicates that the electron is captured by the trivalent rare earth ions and an unstable configuration of the divalent rare earth ion is created. Further the unstable configuration transforms into PC + centers by mechanism proposed by us in the previous paper [18]. Since the initial state of the PC + has been the unstable divalent rare earth ion, we want to describe the transitions of PC + center as d→d and d→f transitions of the divalent rare earth ion.…”

“…(d(e g ) level is the ground state for divalent La, Ce, Gd, Tb, Lu, and Y ions and is the excited state for Eu 2+ and Yb 2+ ) ( Figure 9). In our previous article [18], we thought that quantum interference phenomenon between the discrete d(e g ) level and a continuum of conduction band (Fano effect) as well as valence fluctuation creates the resonant electron-phonon interaction that leads to the enhancement the Jahn-Teller distortion. Perhaps the differences between the formation the photochromic centers and the "anomalous" luminescence are in the time interval of both processes.…”

Перенос заряда и центры F типа в щелочноземельных фторидах, активированных ионами Cd-=SUP=-2+-=/SUP=- или Zn-=SUP=-2+-=/SUP=-

“…Однако легирование кристаллов некоторыми ионами двухвалентных переходных металлов, такими как Cd 2+ , Zn 2+ [2][3][4], или некоторыми трехвалентными ионами редкоземельных катонов, такими как Y, La, Ce, Gd, Tb, Lu [5][6][7][8], приводит к образованию примесного дефекта, который содержит анионную вакансию. То есть легирование некоторыми катионами увеличивает эффективность образования радиационных дефектов в анионной подрешетке, и механизмы образования таких дефектов могут быть различными.…”

Radiation defects in BaF2-Cd

“…(ii) The centers, including the impurity ion and anion vacancy are created at 77 K and their formation is not related to the heat‐activation process of the motion of anion vacancies. It seems that the formation is linked to the configuration instability at the impurity ion trapping an electron (maybe a hole) . The photochromic centers are produced either by X‐irradiation or by additive coloration (by heating the crystals in a calcium atmosphere) of CaF crystals doped with certain rare‐earths ions (La, Ce, Gd, Tb, and Lu) or Y .…”