L. I. Isaenko

Thermally stimulated luminescence and lattice defects in crystals of alkali metal borate LiB3O5 (LBO)

Abstract The recombination processes and lattice defects in crystals of alkali metal borate LiB 3 O 5 (LBO) were studied by the means of the thermally stimulated luminescence (TL) and electron spin resonance (ESR) techniques. The glow curves, the spectra of the LBO recombination luminescence, and the angular variations of ESR-spectra of the O − center in three different planes were measured in the temperature range from 80 to 400 K . The luminescence bands were assigned to the electron ( E m =4.0 eV ) and hole ( E m =4.2 eV ) recombination processes. The model of the trapped hole center O − was proposed. The processes responsible for the formation of localised electronic excitations in LBO were discussed and compared with those taking place in other wide-gap oxides.

Infrared absorption investigation of the role of octahedral groups upon the phase transition in the Rb 2 KMoO 3 F 3 crystal

The infrared absorption spectra of the oxyfluoride Rb2KMoO3F3 have been measured in the frequency range corresponding to stretching vibrations of the Mo-O anion octahedron with the purpose of clarifying their role in the phase transition. A semi-empirical calculation of two possible configurations of quasioctahedral MoO3F3 groups has been performed. The results of the investigations have demonstrated that some octahedra in the crystal structure change the local symmetry from C3v to C2v due to the phase transition (T = 197 K).

Low-temperature time-resolved spectroscopy of APb2X5 crystals (A ≡ K, Rb; X ≡ Cl, Br)

The paper reports on a low-temperature (8 K) time-resolved spectroscopic study of excitonic states and radiative relaxation of electronic excitations in undoped APb2X5 crystals (A ≡ K, Rb; X ≡ Cl, Br) performed under selective photoexcitation by synchrotron radiation. The study has revealed a variety of channels of radiative relaxation of intrinsic electronic excitations, which should be primarily assigned to specific features of the electronic structure of the crystal.

Transient optical absorption and luminescence of lithium triborate LiB3O5

The transient optical absorption and luminescence of LiB3O5 (LBO) nonlinear crystals in the visible and UV spectral ranges were studied. Measurements made using absorption optical spectroscopy with nsscale time resolution revealed that the transient optical absorption (TOA) in LBO originates from optical transitions in hole centers and that the kinetics of optical density relaxation are rate-limited by interdefect nonradiative tunneling recombination involving these hole centers and the Li0 electronic centers, which represent neutral lithium atoms. At 290 K, the Li0 centers can migrate in a thermally stimulated, one-dimensional manner, a process which is not accompanied by carrier delocalization into the conduction or valence band. It is shown that the pulsed LBO cathodoluminescence kinetics is rate-limited by a recombination process involving two competing valence-band-mediated hole centers and shallow B2+ electronic centers. The radiative recombination accounts for the characteristic σ-polarized LBO luminescence in the 4.0-eV region.

Transient optical absorption and luminescence in Li2B4O7 lithium tetraborate

This paper reports on a study of the transient optical absorption exhibited by Li2B4O7 (LTB) in the visible and UV spectral regions. Using absorption optical spectroscopy with nanosecond time resolution, it is established that the transient optical absorption (TOA) in these crystals originates from optical transitions in hole centers and that the kinetics of the optical-density relaxation is controlled by interdefect tunneling recombination, which involves these hole centers and electronic Li0 centers representing neutral lithium atoms. At 290 K, the Li0 centers migrate in a thermally stimulated, one-dimensional manner, without carrier ejection into the conduction or valence band. The kinetics of the pulsed LTB cathodoluminescence is shown to be controlled by a relaxation process connected with tunneling electron transfer from a deep center to a small hole polaron migrating nearby, a process followed by the formation of a self-trapped exciton (STE) in an excited state. Radiative annihilation of the STE accounts for the characteristic σ-polarized LTB luminescence at 3.6 eV, whose kinetics is rate-limited by the tunneling electron transfer.

Luminescence and electronic excitations in Li6Gd(BO3)3: Ce3+ crystals

This paper reports on a study of the luminescence emitted by Li6Gd(BO3)3: Ce3+ crystals under selective photoexcitation to lower excited states of the host ion Gd3+ and impurity ion Ce3+ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd3+ ions and vibrational energy relaxation between the 6Ij and 6Pj levels. It has been shown that radiative transitions in Ce3+ ions from the lower excited state 5d1 to 2F5/2 and 2F7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.

Raman spectroscopic study of the lattice dynamics in the Rb2KMoO3F3 oxyfluoride

The lattice dynamics of the Rb2KMoO3F3 oxyfluoride has been studied by Raman spectroscopy in the temperature range 7–400 K. A phase transition has been revealed at T ≈ 185 K with decreasing temperature. Anomalies of the frequencies and Raman line half-widths have been analyzed. No condensation of soft lattice modes has been found. The character of changes in the Raman spectra of the Rb2KMoO3F3 oxyfluoride shows that the phase transition is related to variations in the [MoO3F3]3− molecular octahedron.

Thermodynamic properties and structure of oxyfluorides Rb2KMoO3F3 and K2NaMoO3F3

According to the results of calorimetric and structural studies, the Fm{ie1202-1}m phase in K2NaMoO3F3 remains stable at least to 100 K. No ferroelectric transformation assumed earlier has been revealed in a series of Rb2KMoO3F3 samples prepared using various technologies. Only a phase transition of nonferroelectric origin has been observed near 195 K, and its thermodynamic characteristics have been determined. An analysis of the stability of the cubic structure of molybdenum fluorine-oxygen elpasolites-cryolites has been performed in the framework of the hypothesis on strengths of interatomic bonds. The barocaloric effect in Rb2KMoO3F3 has been estimated.

Search for and study of phase transitions in some representatives of the APb2 X 5 family

Single crystals of KPb2Cl5, RbPb2Cl5, and RbPb2Br5 are grown and studied using optical polarization methods. The heat capacity of the crystals is investigated by differential scanning microcalorimetry, and the birefringence and the angle of rotation of the optical indicatrix are measured. The measurements are performed in the temperature range 270–640 K. It is found that KPb2Cl5 undergoes a first-order ferroelastic phase transition at T0↑ = 530 K, T0↓ = 528 K, and ΔH = 1000 ± 200 J/mol. The transition is accompanied by twinning and a change in symmetry mmm ⇄ P21/c. The RbPb2Cl5 crystal remains monoclinic up to the melting temperature. The RbPb2Br5 compound belongs to the I4/mcm tetragonal modification and does not undergo structural transformations.

Ultraviolet Luminescence of Li6Gd(BO3)3:Ce Crystals under Selective Excitation in the Region of 4d → 4f Transitions

The subnanosecond time-resolved ultraviolet luminescence of Li6Gd(BO3)3: Ce crystals under selective excitation by ultrasoft X-rays in the region of the 4d→4f core transitions at temperatures of 7 and 293 K has been investigated for the first time. The performed investigation has revealed the following features: an intense fast component of the luminescence decay kinetics in the subnanosecond range due to the high local density of electronic excitations and the processes of Auger relaxation of the core hole; the modulation of the luminescence excitation spectrum by the “giant resonance” absorption band of the 4d-4f photoionization in the energy range 135–160 eV; and a new broad luminescence band at an energy of 4.44 eV due to the direct radiative recombination between the genetically related electron in the states of the conduction band bottom and hole in the 4f ground state of the Ce3+ ion.