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The results of high-pressure spectroscopic studies of near-stoichiometric ${\mathrm{LiNbO}}_{3}:\mathrm{C}\mathrm{r}(0.1%)$ and ${\mathrm{LiNbO}}_{3}:\mathrm{M}\mathrm{g}\mathrm{O},\mathrm{C}\mathrm{r}(0.1%)$ crystals are reported. Much lower inhomogeneous broadening of electronic transitions in near-stoichiometric crystals than in congruent ones allows us to obtain better resolution of the spectroscopic measurements. The results of measurements show that ${\mathrm{Cr}}^{3+}$ ions occupy mostly the same centers in near-stoichiometric crystals and in the congruent ones. In near-stoichiometric crystals doped only with chromium, the ${\mathrm{Cr}}^{3+}$ ions substitute the ${\mathrm{Li}}^{+}$ sites in the ${\mathrm{LiNbO}}_{3}$ host forming one major center (denoted as center \ensuremath{\gamma}) and two lower concentration centers, denoted by \ensuremath{\alpha} and \ensuremath{\beta}. In near-stoichiometric crystals doped additionally with a proper amount of MgO, the additional low-strength crystal-field center \ensuremath{\delta} dominates the spectrum. This center corresponds to the ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Nb}}^{5+}$ sites. The hydrostatic pressure above 75 kbar transforms this center into the high-strength crystal-field center. Due to better resolution of measurements, it was possible to determine the splitting of the ${}^{2}E$ level for the \ensuremath{\delta} center, which is equal to about 17 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Estimated values of the energy difference between the position of the ${}^{4}{T}_{2}$ and the ${}^{2}E$ levels for centers \ensuremath{\beta}, \ensuremath{\gamma}, and \ensuremath{\delta} in these near-stoichiometric ${\mathrm{LiNbO}}_{3}$ crystals are equal to -20, -242, and more than -1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively.

The results of high-pressure studies of ${\mathrm{LiNbO}}_{3}:\mathrm{C}\mathrm{r}(0.2%),$ Mg crystals doped with two concentrations of magnesium (2% and 5.5%) are reported. The results reveal information about the electronic structure of different ${\mathrm{Cr}}^{3+}$ centers in lithium niobate crystals. There are three major ${\mathrm{Cr}}^{3+}$ centers (denoted by \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma}) in the crystal with 2% of magnesium. These centers correspond to ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Li}}^{+}$ sites with different crystal field. At ambient pressure the \ensuremath{\alpha} center experiences strong crystal field and the \ensuremath{\beta} and \ensuremath{\gamma} centers are the intermediate crystal-field centers. The energy differences between the ${}^{4}{T}_{2}$ and ${}^{2}E$ levels are positive and negative for the \ensuremath{\beta} and the \ensuremath{\gamma} centers, respectively. Additional broadband luminescence observed in the sample with 5.5% of magnesium even at pressure of almost 100 kbar testifies that another very weak crystal-field center exists in this crystal. This center (denoted by \ensuremath{\delta}) correponds to ${\mathrm{Cr}}^{3+}$ ions located in ${\mathrm{Nb}}^{5+}$ sites. The R lines of the ${\mathrm{Cr}}^{3+}$ centers exhibit very large redshift with pressure of about 3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/kbar.

Optical absorption and electron paramagnetic resonance (EPR) have been applied in parallel to investigate ${\mathrm{LiNbO}}_{3}$:Cr[0.1%] and ${\mathrm{LiNbO}}_{3}$:Cr[0.1%], Mg[X%] crystals (X=2, 4, 5, 5.5, and 6). From the data obtained, it has been concluded that ${\mathrm{Cr}}^{3+}$ ions occupy both ${\mathrm{Li}}^{+}$ and ${\mathrm{Nb}}^{5+}$ sites. In Mg undoped crystals the optical absorption and EPR spectra are attributed to ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Li}}^{+}$ site. In crystals with Mg doping levels greater than 4.6 mol % both ${\mathrm{Li}}^{+}$ and ${\mathrm{Nb}}^{5+}$ sites are occupied, their relative concentration being governed by the ${\mathrm{Mg}}^{2+}$ content. The double occupancy affects the overall absorption spectrum, giving rise to reproducible color changes in the crystal. Finally, the threshold found for the occupation of ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Nb}}^{5+}$ sites is discussed considering the different models related to the nonstoichiometry of ${\mathrm{LiNbO}}_{3}$. \textcopyright{} 1996 The American Physical Society.

Ces centres (F 2 + ) A sont crees, entre 77 et 300 K, en deux etapes: le processus a basse temperature est associe a une reorientation a courte distance, tandis que le processus de diffusion des lacunes anioniques opere dans l'etape a haute tenperature. Caracterisation des dommages crees sous eclairement et analyse des phenomenes de blanchiment

Selective R-line emission of the Cr31 ions in near stoichiometric LiNbO3 crystals codoped with MgO has been studied at low temperature under excitation into the 4A2!4T1 transition. The emission intensity of R lines associated with Cr31 ions located in Nb51 sites, @Cr#Nb centers, was observed to decrease with the illumination time, whereas those ascribed to other Cr31 ions, such as those located in the Li site, @Cr#Li centers, are optically stable. The selective bleaching is explained by considering the photoinization of Cr31 ions in Nb sites and the formation of Nb41 complexes ~small polaron!.

In this work, experimental results describing Cr 3+ sites in LiNbO 3 :Cr and LiNbO 3 :MgO,Cr previously determined with electron-paramagnetic-resonance (EPR), electron-nuclear-double-resonance, and optical techniques, are correlated in terms of the formation of three chromium sites: Cr 3+ ions in Li + and Nb 5+ positions, and a Cr 3+ (Nb 5+ )-Mg 2+ center that only appears in the double-doped system

The optical spectroscopy (luminescence and decay-time measurements) of ${\mathrm{Pb}}^{2+}$ in doubly doped NaCl:${M}^{2+}$,${\mathrm{Pb}}^{2+}$ (M=Mg, Ca, Sr, Pb) has been studied. Two emission bands have been observed, one of them appearing at 310 nm in all systems whereas the position of the other band (at about 380 nm) is affected by the additional ion ${M}^{2+}$. These results have been interpreted in terms of a Jahn-Teller model which considers two kinds of minima in the adiabatic potential-energy surface (APES). The role of the ${M}^{2+}$ ion in the ${\mathrm{Pb}}^{2+}$APES is discussed.

The real and imaginary parts of the dielectric constant of LiTa${\mathrm{O}}_{3}$ have been measured from 20 Hz to 20 kHz at $Tg{T}_{c}$ under moderate oxygen partial pressures to prevent dielectric losses from becoming too high. A very large frequency-dependent contribution to the dielectric constant is observed, much higher than the normal ferroelectric contribution, which may be attributed to mobile charged point defects, presumably connected with the nonstoichiometry of the crystal. The data can be interpreted satisfactorily within the framework of a modified Debye model allowing for collective defect migration under the low-frequency ac field.

It is shown in this work that the room-temperature excitation spectra of as-grown crystals of LiF, NaF, NaCl, KCl, and KBr doped with ${\mathrm{Mn}}^{2+}$ reveal the existence of double excitation peaks. These peaks are related to the formation of precipitated phases containing ${\mathrm{Mn}}^{2+}$ in the alkali halide lattice. In the case of LiF:${\mathrm{Mn}}^{2+}$ the position of the [$^{4}A_{1}(G)$, $^{4}E(G)$] and $^{4}T_{1}(G)$ peaks suggests an anomalously small ${\mathrm{Mn}}^{2+}$- ${\mathrm{F}}^{\ensuremath{-}}$ distance for the precipitated phase.

The thermally activated processes occurring in the dark after room-temperature $F$-light bleaching of x- or $\ensuremath{\gamma}$-irradiated NaCl have been investigated. It has been shown that, near room temperature, the concentration of free-anion vacancies decays exponentially with time. The kinetics of the process has been studied by putting the crystal in the dark for a variable time after bleaching and then subjecting it to a short illumination or irradiation pulse which detects the disappearance of the vacancies. It has been observed that the decay in vacancy concentration is closely associated with a decrease in the ${F}^{\ensuremath{'}}$-band intensity and with increases in the $F$- and $M$-band intensities. The experimental information has led to the conclusion that vacancies become trapped at ${F}^{\ensuremath{'}}$ centers according to a first-order reaction, giving rise to $F$ and $M$ centers through the mechanisms ${F}^{\ensuremath{'}}+\mathrm{anion}\mathrm{vacancy}\ensuremath{\rightarrow}2F$, and ${F}^{\ensuremath{'}}+\mathrm{anion}\mathrm{vacancy}\ensuremath{\rightarrow}M$, respectively. The activation energy for the trapping process has been found to be $E=0.48\ifmmode\pm\else\textpm\fi{}0.06$ eV. These phenomena also occur after an x or $\ensuremath{\gamma}$ irradiation, and are shown to provide a coherent picture of the previously reported anomalous coloring behavior appearing at the start of a new irradiation period.