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The results of the study of the temperature dependence of attenuation and velocity of ultrasonic waves are presented for CaF2:Cr and CaF2:Ni crystals in which Cr2+ and Ni2+ ions substitute calcium ions, forming Jahn–Teller (JT) complexes [CrF8]6– and [NiF8]6–. The Cr2+ and Ni2+ ions in the fluorite structure have threefold orbital degeneracy in the ground state, are described by the T $$ \otimes $$ (e + t2) Jahn–Teller effect (JTE) problem, and have an adiabatic potential energy surface (APES) defined in the five-dimensional space of trigonal and tetragonal symmetrized coordinates whose lower sheet is a simply connected surface with orthorhombic minima separated by potential energy barriers. The temperature dependence of the ultrasonic attenuation and velocity exhibits anomalies typical of the relaxation contributions of systems of JT complexes in the low-temperature region. The effect of tunneling relaxation mechanisms (direct and two-phonon transitions) on the JT contribution to the complex elastic moduli and to the error in determining the relaxation time is analyzed. Based on the approach that takes into account both tunneling and activation mechanisms of relaxation, the activation energies and the constants characterizing these relaxation mechanisms are determined in CaF2:Cr2+ and CaF2:Ni2+ crystals.

The temperature dependences of attenuation and the velocity of ultrasonic waves at frequencies of 26–158 MHz in a CaF2 fluorite crystal at the substitution of Jahn–Teller Cr2+ centers for calcium ions have been studied. An abnormally high relaxation rate, which is two orders of magnitude higher than the relaxation rate in other previously studied CaF2:Ni2+ and SrF2:Cr2+ fluorites, has been found in the system of Jahn–Teller complexes in the low-temperature region. It has been shown that the global minima of the adiabatic potential energy surface of the Cr2+ $${\text{F}}_{8}^{ - }$$ complexes in the CaF2:Cr2+ crystal also have orthorhombic symmetry but are separated by significantly lower potential energy barriers than in CaF2:Ni2+ and SrF2:Cr2+ crystals. It has been found that tunneling relaxation mechanisms (direct and two-phonon transitions) are dominant, rather than thermal activation, in CaF2:Cr2+ in the temperature range where the Jahn–Teller effect is manifested in an ultrasonic experiment. The parameters characterizing these relaxation mechanisms have been determined.

Photoluminescence spectra of impurity centers containing anion vacancy Cd+ $${{{v}}_{a}}$$ or Zn+ $${{{v}}_{a}}$$ in alkali-earth fluorides have been studied. A comparison with luminescence spectra of photochromic centers (PCs), which can be considered as perturbed F centers, shows that luminescence of centers Cd+ $${{{v}}_{a}}$$ or Zn+ $${{{v}}_{a}}$$ is similar to that of F centers; therefore, it is caused by charge transfer from an impurity ion to an anion vacancy.

A new approach is introduced to obtain information about the adiabatic potential energy surface of the Jahn-Teller (JT) complexes subject to theT⊗e+t2problem in doped crystals. It employs simulation of the contribution of the JT sub-system to ultrasonic attenuation with account of three mechanisms of relaxation and subsequent comparison with the experimental data on temperature dependence of the attenuation obtained with the use of doped crystal and nominally pure one. The approach has been applied to the CaF

The absorption spectra of X-ray irradiated alkaline earth fluoride (CaF2, SrF2, BaF2) crystals doped with Sm, Нo, Er, Tm ions are investigated by thermal bleaching of the sample in the 300–700 K temperature range. X-ray irradiation creates absorption bands of inter-configuration 4fn-4fn−1-5 d1 transitions of divalent impurity ions and F3− and (F2−)ii hole centers. The thermal oxidation of the divalent RE ions have been observed. Two possible mechanism of thermal oxidation of divalent lanthanide ions in alkaline-earth fluorides are discussed.

The biological activity of laser radiation (wavelength, 632.8 nm) when it interacts with human serum albumin is demonstrated. The most noticeable effect of exposure to laser radiation is observed when the energy density is 28.8 J cm−2. It is assumed that three-photon absorption is the main mechanism of biological stimulation.

A new approach is introduced to obtain information about the adiabatic potential energy surface of the Jahn–Teller (JT) complexes subject to the T ⊗ e + t 2 problem in doped crystals. It employs simulation of the contribution of the JT sub-system to ultrasonic attenuation with account of three mechanisms of relaxation and subsequent comparison with the experimental data on temperature dependence of the attenuation obtained with the use of doped crystal and nominally pure one. The approach has been applied to the CaF2 crystal doped with chromium. The experiment has been carried out at the frequencies of (13–158) × 106 Hz at low temperatures (4–170 K). Peak for all the ultrasonic normal modes propagating along the [110] axis has been observed at about 8 K. It has been considered as the JT effect manifestation by the sub-system of Cr2+ F 8 − complexes. Symmetry properties of the global minima and saddle points have been identified, the JT stabilization energies and coordinates of the extrema points in the five-dimensional space of symmetrized displacements have been evaluated. Besides, a broad shoulder has been found at 15–40 K and attributed to contribution of the Cr3+ F 8 − sub-system.

The absorption spectra of radiation-colored CaF2, SrF2, and BaF2 crystals activated by trivalent Pr, Sm, Нo, Er, Tm, and Yb (rare-earth, RE) ions are studied. It is shown that ionizing radiation reduces the impurity ions to the divalent state. The temperature resistance of divalent RE ions of radiation-colored CaF2 crystals correlates with the chemical stability of the compounds with divalent RE ions. The photochromic centers are produced in CaF2-Pr crystal colored by radiation at room temperature and heated to 200°C.

The ground state of monovalent ions of transitional metals in alkaline-earth fluorides is localized in the conduction band. It is shown by the example of monovalent Co and Ni ions in CaF2 and SrF2 crystals that this is responsible for the unstable state of these ions in cubic environments. A stable state is achieved by the monovalent ions shifting to off-center positions.

The mechanism of coloration of alkaline-earth fluoride crystals CaF2, SrF2, and BaF2 in calcium vapors in an autoclave with a cold zone is studied. It was found that the pressure in the autoclave upon constant evacuation by a vacuum pump within the temperature range of 500–800°C increases due to evaporation of metal calcium. In addition to the optical-absorption bands of color centers in the additively colored undoped crystals or to the bands of divalent ions in the crystals doped with rare-earth Sm, Yb, and Tm elements, there appear intense bands in the vacuum ultraviolet region at 7.7, 7.0, and 6.025 eV in CaF2, SrF2, and BaF2, respectively. These bands belong to the Ha - hydrogen centers. The formation of hydrogen centers is also confirmed by the appearance of the EPR signal of interstitial hydrogen atoms after X-ray irradiation of the additively colored crystals. Grinding of the outer edges of the colored crystals leads to a decrease in the hydrogen absorption-band intensity with depth to complete disappearance. The rate of hydrogen penetration inside the crystal is lower than the corresponding rate of color centers (anion vacancies) by a factor of tens. The visible color density of the outer regions of the hydrogen-containing crystals is several times lower than that of the inner region due to the competition between the color centers and hydrogen centers.

A new model of photochromic centers formed by ionizing radiation or the additive coloration of alkaline earth fluorides doped with rare-earth ions (La, Ce, Gd, Tb, Lu, and Y) is proposed, based on original experimental results and the literature data.

The absorption spectra of x-irradiated alkaline-earth fluoride (CaF 2 , SrF 2 , BaF 2 ) crystals doped with Nd 3+ ions have been investigated. X-irradiation results in creating the absorption bands of inter-configuration 4f n –4f n−1 –5 d 1 transitions of Nd 2+ . The charge reduction of the neodymium by irradiation is not temperature-stable and the ions reoxidation (Nd 2+ → Nd 3+ ) occurs under heating to 570 K in CaF 2 , 520 K in SrF 2 and 470 K in BaF 2 .

We have studied the absorption spectra of radiation-induced divalent cerium and praseodymium ions in crystals of alkaline-earth fluorides. Using ab initio quantum-mechanical methods, we have calculated absorption spectra of divalent praseodymium ions in CaF2 crystals for the first time. The theoretical spectrum agrees rather well with the experimentally registered spectra.

Ionization radiation is shown to reduce impurity ions to the univalent state in strontium and calcium fluoride crystals doped by divalent cadmium and zinc ions. In this case, a univalent ion is surrounded by eight equivalent fluorine ions and exhibits cubic symmetry Oh. At room temperature, the symmetry of the center is revealed to be sequentially lowered to C3v and then to C2v owing to the addition to the nearest environment of the impurity univalent ion of one or two anion vacancies, respectively, which are intrinsic defects not forming in undoped strontium and calcium fluoride crystals. Stable intrinsic defects are assumed to form through the separation of anion vacancy-interstitial fluorine ion pairs in the electric field induced by the reduced impurity ions. This electric field lowers the energy barrier to thermal separation of charged intrinsic defects.

This paper reports on a study of cross-luminescence in barium fluoride crystals doped by a variety of impurities (K+, Cd2+, Y3+, Yb3+, La3+). It is shown that doping of the crystal with a trivalent impurity gives rise to the formation of an additional cross-luminescence band peaking at 7.5 eV, the intensity of this band increasing with increasing impurity concentration.

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., Comment: 22 pages, 9 figures

Optical absorption excitation and emission of alkaline-earth fluoride crystals doped with impurities, which did not introduce a new absorption band up to 8 eV, were investigated. Comparison of emission of BaF 2 –LaF 3 , BaF 2 –KF and doubly doped BaF 2 –LaF 3 , KF shows that interstitial fluorines are not the main exciton suppressors as thought before.The two stages of exciton suppression on the plot of exciton intensity against impurity concentration were observed. The first stage has the interaction length near 130 A, while the interaction lengths of second stage are 10–30 A, depending on impurity.

Strong absorption in the vacuum ultraviolet region near the exciton edge was observed in Cd doped CaF 2 , SrF 2 and BaF 2 crystals. The Cd + + absorption band at 8.6 eV was observed in low doped BaF 2 crystals. In SrF 2 and CaF 2 the maximum of Cd + + band cannot be resolved due to stronger overlapping with exciton bands. The calculations of geometry of Cd + + and Cd + centres and Cd- related electron transitions were done using ab - initio Hartree-Fock and density functional methods. The experimental and calculated results lead us to the conclusion that strong Cd + + absorption bands belong to allowed transitions from the nearest fluorides to the Cd + + ion. Lattice relaxation around Cd + centres, created by x-irradiation, and optical transitions of these centres were also calculated. The calculated energies are in good agreement with experimental ones.

Defect formation in BaF2 single crystals doped with cadmium is investigated by optical absorption, electron paramagnetic resonance (EPR) and thermoluminescence. An optical absorption band observed in X-irradiated at 295 K crystals CaF2 (at 3.8 eV), SrF2 (at 3.95 eV) and BaF2 (at 4.08 eV) is assigned to absorption of Cd+-ions. In addition, two type hole centers: pure Vk and perturbed Vk-centers, are observed in BaF2–1%CdF2 crystals X-irradiated at 77 K.

Exciton luminescence was investigated in La-doped BaF2 crystals. Two luminescence bands at 4.26 and 3.82 eV related to excitons have been observed. The excitation spectrum of the 4.26 eV band of BaF2–LaF3 was similar to that of the exciton emission band of pure BaF2 crystals. The excitation spectrum of the 3.82 eV band shows a new band within the 8–9 eV region. The band becomes most prominent for BaF2–3 wt% LaF3 and has a maximum at 8.7 eV at 290 K. The decay time for the 4.26 eV emission band of BaF2–3% LaF3 considerably increased with decreasing temperature like the exciton decay time for pure BaF2 ,w here the decay time of unperturbed excitons grows to 121 µ sa t 17 K. In contrast to this, the decay time of the 3.82 eV band increases from 4.1 µ sa t 290 K to 6.2 µ sa t1 5K .T he significant reduction of the triplet–singlet decay time can be associated with strong interaction of excited electrons with lanthanum ions. It was concluded that the 4.26 eV emission band is associated with weakly perturbed excitons, while the 3.82 eV band is associated with strongly perturbed excitons. The possible spatial configurations of excitons are discussed.

In this work, we carry out investigation of the suppression of the self-trapped exciton emission in BaF2:La crystals with various La concentrations (up to 5%), including measurements of thermo-luminescence, optical absorption of hole centers in the temperature range from 77 K to room temperature. Exciton luminescence was also investigated. Two luminescence bands at 4.0 and 3.67 eV were observed. It was concluded that the 4.0 eV emission band belongs to weakly perturbed excitons, while the 3.67 eV band belongs to strongly perturbed excitons.

Nominally pure BaF 2 single crystals were investigated at 77 v K with optical absorption and electron paramagnetic resonance to understand the mechanism of radiation damage. We find that X-irradiation at 77 v K of undoped BaF 2 produces V k - and F -centres having absorption bands at 3.4 and 2.3 v eV respectively.

The luminescence properties of Ba1−xLaxF2+x single crystals have been investigated for La concentrations up to 5 mol % . The luminescence spectra of the crystals have been recorded during X-ray excitation. The intensity of the exciton emission in BaF2–5% LaF3 is reduced by a factor of 10, whereas the intensity of the cross-luminescence remains almost constant. Besides the main emission bands of the cross-luminescence at 6.3 and 5.6 eV , an additional weak emission band at about 7.2 eV has also been observed. We suggested that the newly observed emission arises from a radiation transition between the interstitial Fi−-ion and the outermost Ba2+ 5p core band. We found that with increasing La-doping, the edge of fundamental absorption of Ba1−xLaxF2+x crystals shifts to lower energies.

Anion hydrogen atom centres are studied by optical and ESR techniques in LiF-H − and NaF-H − crystals. The ESR spectra of these centres are observed after X-ray irradiation of the crystals at 77 K. The optical absorption spectra of H s,a 0 centres consist of two bands at 2.4 and 3.5 eV for LiF-H − and at 2.95 and 3.8 eV for NaF-H − , which are due to the electron charge transfer from a fluorine ion to the 1 s state of the hydrogen atom

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., 10 pages, 2 Figures, article

The emission of self-trapped excitons produced by vacuum ultraviolet excitation as well as X-ray excitation or photostimulation in BaFCl and BaFBr crystals was studied. The exciton emission bands at 4.2 and 5.15 eV in BaFBr and at 3.3 eV in BaFCl were observed upon irradiation into the Cl or Br exciton absorption bands. The 5.15 eV emission band in BaFBr thermally transforms to the 4.2 eV band with increasing temperature within the range 50-80 K. The 3.3 eV emission in BaFCl increases in the range 20-50 K in BaFCl, while the lifetime of the emission decreases. These facts indicate rather clearly the potential barrier between two different exciton configurations. The low-temperature exciton configuration was tentatively assigned to the Vk+e type and the other to F+H type.

The formation of radiation defects in calcium and strontium fluoride single crystals doped with cadmium or zinc has been investigated by luminescence and absorption spectroscopy, as well as by electron spin resonance spectroscopy. It was found that x-irradiation could convert divalent impurity ions located at essentially cubic sites into the univalent state. Three types of Cd(+) or Zn(+) centers differing by local environment with point symmetries O(h), C(3v) and C(2v) are identified in the crystals. The formation of the last two results from the interaction between reduced impurities in the cubic environment and anion vacancies. The latter are intrinsic radiation defects and are not created by x-irradiation in undoped crystals. We also discuss the possible implications of the electric field of the charge impurity defects on separation of the intrinsic radiation defects in these crystals.

The ESR spectrum of an unperturbed hydrogen atom on a cation site is observed after vacuumultraviolet irradiation of LiF-H−, Mg2+ crystals at room temperature. A production mechanism involving optical destruction of magnesium-vacancy dipoles and U-centres, release of the cation vacancies from the radiation-induced Mg-related centres, and subsequent capture of hydrogen atoms by the free cation vacancies, is proposed. [Russian Text Ignored].

The H2- centre in NaF is studied by electron spin resonance (ESR). The ESR spectrum shows a triplet due to the hyperfine interaction of an unpaired electron with the two equivalent protons. Each line of the triplet is further split owing to the superhyperfine interactions with the neighbouring nuclei of the first two shells (as in the case of the F centre) and the centre may also be described as being a neutral H2 molecule embedded in an F centre.

Atomic hydrogen centres in LiF–H− and LiF–H−, Mg2+ are studied by optical and ESR techniques. X-irradiation of these crystals at 80 K leads to the formation of the H-centres which are thermally stable up to 110 K. After thermal annealing of the H-centres in LiF–H−, Mg2+ crystals a new atomic hydrogen centre is observed which is interpreted as an interstitial hydrogen atom stabilized by a complex of the Mg2+ ion and a positive ion vacancy (H(Mg)-centre). Thermal decay of the (H(Mg)-centres occurs at 430 K and leads to the formation of a hydrogen atom in the cation vacancy (H(Mg)-centre). [Russian Text Ignored.]

Two types of tetragonal centre have been discovered in LiF:Co. One centre produced by X-irradiation at 77 K is converted by heating into another centre. The latter is stable up to 180 degrees C. A large splitting with two equivalent fluorine atoms and a small splitting with four equivalent fluorine atoms are observed in both centres. HFS caused by the Co nucleus is absent. These defects are molecular ions F65- bonded with an impurity. The broad line of Co+ is observed too.

On mesure les intensites relatives des bandes en reduisant l'imprecision due a la non uniformite de la distribution des impuretes a travers le cristal

The absorption, emission, excitation spectra, and polarized luminescence of magnesium centres in sodium fluoride are studied. Three types of magnesium centres are observed. The first one, which is produced by X-irradiation at 300 K and absorbs at 3.32 eV, is attributed to the Mgcentre. The two other centres have the electronic structure of the Mg0 atom perturbed by lattice defects. Optical excitation of the Mg0 type centres leads to two emission bands, which are attributed to the 3P1 1S0 and 1P1 1S0 transitions. However, the structure of both of these centres are not still completly established. [Russian Text Ignored.]

The H-centre in LiF is studied by optical and ESR techniques. The centre is produced by reaction between H and H-centres. The H-centre ESR spectrum shows three broad lines which are due to the hyperfine interaction of an unpaired electron with the two protons. The resolved superhyperfine structure (as in case of F-centre) arises from the interaction with the lithium nuclei in a first shell and the fluoride nuclei in a second shell. Optical absorption band of the H-centre at 5.05 eV is similar to that of the F-band, only slightly shifted to higher energies. The centre has the character of molecule-ion as well as of F-centre. [Russian Text Ignored].

Processes of defect creation by VUV-excitation are studied by the thermoluminescence method. NaCl crystals doped with Mg or Mn are irradiated at 80 K by monochromatic VUV-light in the region of 7 to 8 eV. Excitation spectra of the glow peaks at 195 K (NaCl-Mg) and 220 K (NaCl-Mn) have maxima at 7.5 and 7.4 eV, respectively. It is concluded that the TL excitation spectra correspond to the formation of an exciton near the divalent metal (Mg2+ or Mn2+) of the impurityvacancy complex. The decay of these excitons leads to formation α-Iz pairs at 80 K. [Russian Text Ignored.]