Experiments carried out with the use of ultrasonic technique in doped crystals have established three mechanisms of relaxation of the Jahn-Teller subsystem at low temperatures. All of them have a quantum nature. The Debye relaxation function f(D) = 1/(1 + i omega tau) is changed due to the temperature-dependent relaxation time tau(T) provided the frequency omega is fixed. The dependence of tau(T) derived in an experiment on dissipation or/and dispersion of an ultrasonic wave makes it possible to obtain the temperature dependence of f(D)(T). As a consequence of quantum nature of the relaxation mechanisms, the dependence of tau(T) at low temperatures is an immanent characteristic of the system of Jahn-Teller complexes in a given matrix. Therefore graphical representations of -Im f(D)(T) and -Im f(D)(T)/T can serve as fingerprints of a certain Jahn-Teller complex.

We present the results of our ultrasonic studies in a fluorite (CaF 2 ) single crystal with isovalent substitution of Ca 2+ ions by copper ions performed at frequencies 23–163 MHz in the temperature range 4–240 K. Anomalies typical for the relaxational contributions of systems of Jahn–Teller complexes have been found in the temperature dependences of the attenuation and velocity of the longitudinal and transverse normal modes associated with the elastic moduli c L = c 44 + ( c 11 + c 12 )/2 and c E = ( c 11 – c 12 )/2 at temperatures below 80 K. The Cu 2+ ions in a cubic environment in the ground state possess threefold orbital degeneracy, are described by the T ⊗ ( e + t 2 ) Jahn–Teller effect problem, and have an adiabatic potential energy surface specified in the five-dimensional space of trigonal and tetragonal symmetrized coordinates. An analysis of the experimental data has allowed us to determine the symmetry properties of the extrema (global minima and saddle points) of the lowest adiabatic potential energy sheet and to find the linear (tetragonal and trigonal) and quadratic vibronic coupling constants of the Jahn–Teller Cu 2+ F_8^ - complexes in the CaF 2 matrix appearing in the vibronic Hamiltonian, the Jahn–Teller stabilization energies, and the coordinates of these extrema in the five-dimensional space of normal coordinates.

A study is performed of the optical spectra of alkaline earth fluorides and CdF 2 crystals activated by chromium ions in the trivalent state. The absorption spectra of chromium ions are obtained for the divalent state, along with absorption and emission spectra for a bound Cr 2+ –Cr 3+ pair of CaF 2 crystals.

Photoluminescence spectra of impurity centers containing anion vacancy Cd-Va(+) or Zn-Va(+) 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-Va(+) or Zn-Va(+) is similar to that of F centers; therefore, it is caused by charge transfer from an impurity ion to an anion vacancy.

The photoluminescence spectra of impurity centers containing the anionic vacancy Cd+va or Zn+va in alkaline-earth fluorides are studied. A comparison with the luminescence spectra of photochromic centers (РC), which can be considered as perturbed F centers, shows that the luminescence of the Cd+va or Zn+va centers is similar to the luminescence of F centers and, therefore, is associated with charge transfer from an impurity ion to an anionic vacancy.

In CaF2 crystal doped with Ni2+ ions, attenuation of all the normal ultrasonic modes with the wave vector k parallel to < 110 > were investigated at 14-161 MHz in the temperature region of 5-120 K. The observed peaks of relaxation origin were interpreted as manifestation of the Jahn-Teller effect subject to the T-1g circle times (e(g) + t(2g)) problem. The experimental data made it possible to estimate the parameters of adiabatic potential energy surface: the Jahn-Teller stabilization energies, the linear and quadratic vibronic coupling constants, location of the saddle points and the global minima defined in the 5-dimensional space of symmetrized (tetragonal and trigonal) coordinates. (C) 2020 Elsevier B.V. All rights reserved.

It is shown the photochromic PC centers that form during the radiative coloration of alkaline-earth fluoride crystals doped with a number of rare-earth ions (La, Ce, Gd, Tb, Lu, and Y) can be attributed to DX-type centers.

Показано, что фотохромные РС-центры, образующиеся при радиационном окрашивании кристаллов щелочноземельных фторидов, легированных рядом редкоземельных ионов (La, Ce, Gd, Tb, Lu и Y), можно отнести к центрам DX-типа.

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.

The absorption spectra of X-ray irradiated alkaline earth fluoride (CaF2, SrF2, BaF2) crystals doped with Sm, Ho, 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 4f(n)-4f(n-1).5d(1) transitions of divalent impurity ions and F-3(-) and (F-2(-))(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 mechanism of coloration of alkaline-earth fluoride crystals CaF 2 , SrF 2 , and BaF 2 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 CaF 2 , SrF 2 , and BaF 2 , respectively. These bands belong to the H a - 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.

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.