The effect of post-growth isothermal annealing in vacuum and in air on the optical properties of Ca3TaGa3Si2O14 crystal samples of Z and X-cuts has been studied. Spectral dependences of transmission coefficients were measured in the wavelength range (240–700) nm taking into account anisotropy and dichroism. On the Z-cut samples in the initial state an absorption band at λ = 360 nm in the ultraviolet range is observed, in the visible region – two absorption bands at λ = 460 nm and λ = 605 nm. Additionally, a band at λ = 290 nm was observed on the X-cut samples. When the sample was rotated around the direction of the light beam by 90 degrees, a change in the intensity of the absorption bands was observed. Annealing in vacuum leads to a decrease in the intensity of the absorption bands in the near ultraviolet and visible range, except for the absorption band at λ = 605 nm. Annealing in air leads to the opposite effect – an increase in the intensity of the absorption bands, except for the band λ = 605 nm. The value of the anomalous birefringence of the samples was estimated by the Mallard method. The degree of linear dichroism is calculated. It is shown that the degree of dichroism decreases as a result of annealing in vacuum, and increases during annealing in air.

The effect of post-growth isothermal annealing in vacuum and in air on the optical properties of Ca3TaGa3Si2O14 crystal samples of Z- and X-cuts is studied. The spectral dependences of the transmission coefficients are measured in the wavelength range (240-700) nm taking into account anisotropy and dichroism. On the Z-cut samples in the initial state an absorption band at lambda = 360 nm in the ultraviolet range is observed; in the visible region, there are two absorption bands at lambda = 460 nm and lambda = 605 nm. Additionally, a band at lambda = 290 nm is observed on the X-cut samples. When the sample is rotated around the direction of the light beam by 90 degrees, a change in the intensity of the absorption bands is observed. Annealing in vacuum leads to a decrease in the intensity of the absorption bands in the near ultraviolet and visible range, except for the absorption band at lambda = 605 nm. Annealing in air leads to the opposite effect, i.e., an increase in the intensity of the absorption bands, except for the band lambda = 605 nm. The value of the anomalous birefringence of the samples was estimated by the Mallard method. The degree of linear dichroism is calculated. It is shown that the degree of dichroism decreases as a result of annealing in vacuum and increases during annealing in air.

The effect of proton irradiation with a dose of 50 Mrad (Si) on the optical properties and defect formation in crystals of gadolinium-aluminum-gallium garnet is studied during the substitution of aluminum and gallium in the cation sublattice: Gd3Al2Ga3O12 (Al : Ga = 2 : 3) and Gd3Al3Ga2O12 (Al : Ga = 3 : 2). After irradiation with protons, the crystals change color: an additional absorption band appears in the spectrum of each crystal in the wavelength range of 400-500 nm. This is due to the formation of induced structural defects in the form of color centers. The refractive indices n(lambda) are determined by the Brewster spectrophotometric method and barely change for Al : Ga = 2 : 3 crystals, but largely increase for Al : Ga = 3 : 2. In the spectral dependences, there is a noticeable increase in the attenuation of light, which also indicates the formation of additional structural defects.

The influence of proton irradiation with a dose of 50 Mrad (Si) on the optical properties and defect formation in crystals of the gadolinium-aluminum-gallium garnet with the substitution of aluminumand gallium in the cationic sublattice: Gd3Al2Ga3O12 (Al: Ga = 2:3) and Gd3Al3Ga2O12 (Al: Ga = 3:2) was studied. After proton irradiation, color of crystals changes: an additional absorption band appears in the transmittance of each crystal in the wavelength range 400–500 nm. This occures due to the formation of induced structural defects as color centers. The refractive indices n(λ) were determined by the Brewster spectrophotometric method and practically did not change for Al:Ga = 2:3 crystals, but significantly increased for Al:Ga = 3:2. There is a noticeable increase in the attenuation of the light in spectral dependences, which also indicates the formation of additional structural defects.

Gadolinium‒aluminum‒gallium garnet crystals are grown with charge compositions Gd3AlxGa5 – xO12 (x = 1–3) and Gd3Al2Ga3O12:Ce3+. The effect high-temperature annealing in air has on the optical properties of these crystals is established. It is shown that annealing does not affect the state of cerium oxidation. X-ray fluorescence reveals a deficiency of gallium in all the investigated crystals.

The spectral dependences of transmission and absorption in the wavelength range of 200–2500 nm were measured for Ca3TaGa3Si2O14 crystals cut perpendicular to the optical axis in the initial state (without annealing) and after isothermal annealing in vacuum and in air. It was found that annealings in vacuum and air lead, respectively, to decrease and increase in the absorption band intensity. A spectrophotometric method for measuring and calculating the specific rotation angle ρ of the plane of polarization of light in gyrotropic crystals from the transmission coefficient spectra at different angles between the polarizer and analyzer is considered. The transmission spectra were normalized to eliminate the spectral shifts associated with measurement features. Spectral dependences of ρ, approximated by the extended Drude formula, were obtained for all three samples; the influence of the annealing atmosphere on the coefficients of this formula is established.

Представлены результаты комплексных исследований процессов дефектообразования и его влияния на оптические свойства кристаллов гадолиний-алюминий-галлиевых гранатов с частичным замещением галлия на алюминий в катионной подрешетке: Gd 3 AlGa 4 O 12 (Al : Ga = 1 : 4), Gd 3 Al 2 Ga 3 O 12 (Al : Ga = 2 : 3) и Gd 3 Al 3 Ga 2 O 12 (Al : Ga = 3 : 2), а также кристаллов Gd 3 Al 2 Ga 3 O 12 :Сe 3+ (Al : Ga = 2 : 3, легированных церием). Результаты, полученные методами рентгеновской фотоэлектронной спектроскопии и рентгенофлуоресцентного анализа, свидетельствуют о дефиците галлия по отношению к стехиометрическому составу во всех исследованных кристаллах. На основе результатов исследований рассматриваются процессы возникновения ростовых структурных точечных дефектов в кристаллах гадолиний-алюминий-галлиевого граната. Преобладающими точечными дефектами являются F -центры, также показано образование дефектов типа Шоттки и V -центров. Показано, что при легировании гранатов ионами церия также возможно образование дополнительных F ‑центров за счет введения церия. Установлено изменение коэффициентов преломления и показателей ослабления в зависимости от соотношения Al : Ga и легирования церием.

Optical properties of lanthanum-gallium silicate (La3Ga5SiO14), lanthanum-gallium tantalate (La3Ga5.5Ta0.5O14), calcium-tantalum-gallium silicate (Ca3TaGa3Si2O14) crystals in the near ultraviolet (UV) and visible wavelength ranges (200-800 nm) have been studied. For the first time the angles of deviation of the moments of the dipole transition spectral dependences theta(λ) were used to characterize the absorption processes in these crystals. These dependences make it possible to reveal absorption bands that are weakly noticeable on the transmission coefficients spectral dependences, and to estimate the degree of deviation of the dispersion dependence theta(λ) from the value theta=45o corresponding to an ideal ordered crystal. Analysis of theta(λ) dependencies showed that in the studied wavelength range, the lowest deviation from theta =45o is observed in Ca3TaGa3Si2O14, and the highest - in La3Ga5SiO14. The coefficients of the specific rotation angle of the polarization plane rho were obtained by the spectrophotometric interference method in polarized light. The measured values are best approximated by the Vyshin equation. For the first time, dependences of the modified Drude formula 1/rho=f(λ2) were calculated for these crystals. These dependences should be linear in case of an ideal crystal. The largest deviation from the linear dependence is observed for La3Ga5SiO14 crystals, and the lowest for Ca3TaGa3Si2O14. The discrete values of the refractive coefficients No and Ne were measured by the prism method. La3Ga5.5Ta0.5O14 crystals have the highest refractive coefficients, Ca3TaGa3Si2O14 crystals have the lowest. The obtained values are approximated by the Sellmeier equation.

Single crystal calcium molybdate CaMoO 4 is a well-known material. However the interest to CaMoO 4 has recently grown due to a number of its important applications including as a working material in cryogenic scintillation bolometers. CaMoO 4 single crystals acquire blue color during growth due to the presence of color-center type defect centers which are unacceptable for optical applications. Color can be eliminated through annealing in an oxygen containing atmosphere, following which required optical components can be produced from the single crystals by mechanical treatment (cutting, polishing etc.). Therefore assessment of the mechanical properties of these single crystal materials is an important task for the optimal solution of issues occurring in the fabrication of optical components and their further practical application. There are but scarce data on the mechanical properties of CaMoO 4 , and the available ones have been reported without allowance for anisotropy. There is a significant scatter of data on the Mohs hardness of the single crystals, ranging from 3.3 to 6 in different publications. In this work we present data on calcium molybdate single crystals in the initial state and after high-temperature anneals of different durations in an oxygen containing atmosphere. We show that long-term annealing leads to decolorization of the crystals. Calcium molybdate single crystals prove to be quite brittle: the brittleness index Zp of the crystals in the initial state is the highest and equals 5, while annealing reduces the brittleness index to 4. The Palmqvist toughness factors S have been calculated The limit indentation destruction loads F lim have been determined and annealing in an oxygen containing atmosphere has been shown to increase F lim by 2.5 times for the Z cut and by 10 times for the X cut. The microhardness of the crystals has been shown to exhibit a II type anisotropy: the microhardness of all the specimens was higher for the Z cut than for the X cut. The microhardness anisotropy coefficients KH of the specimens have been evaluated. The bond ionicity degree I has been calculated on the basis of the experimentally measured microhardness.

— We present results of a detailed study of defect formation processes and their effect on the optical properties of gadolinium aluminum gallium garnet crystals with partial aluminum substitution for gallium in the cation sublattice—Gd 3 Al 1 Ga 4 O 12 (Al : Ga = 1 : 4), Gd 3 Al 2 Ga 3 O 12 (Al : Ga = 2 : 3), and Gd 3 Al 3 Ga 2 O 12 (Al : Ga = 3 : 2) and cerium-doped crystals: Gd 3 Al 2 Ga 3 O 12 :Сe 3+ (GAGG:Ce). X-ray photoelectron spectroscopy and X-ray fluorescence analysis data demonstrate gallium deficiency relative to the stoichiometric composition in all of the crystals studied. The results obtained are used to analyze processes underlying the formation of structural growth point defects in gadolinium aluminum gallium garnet crystals. F -centers have been shown to be the predominant point defect species. We also have demonstrated the formation of Schottky defects and V -centers. The formation of additional F -centers in cerium-doped garnet crystals is happening. The refractive indices and attenuation coefficients of the crystals have been shown to depend on the Al : Ga ratio and doping with cerium.

We have studied the optical properties and microhardness of gadolinium aluminum gallium garnet crystals with partial substitution of aluminium for gallium, Gd3Al2Ga3O12 (Al : Ga = 2 : 3) and Gd3Al3Ga2O12 (Al : Ga = 3 : 2), and also Gd3Al2Ga3O12:Сe (Al : Ga = 2 : 3) cerium-doped crystals. The Al : Ga ratio and doping with Ce have been shown to influence the light attenuation coefficient and optical band gap of the crystals. Using XANES spectroscopy data, we have evaluated the oxidation state of the cerium in the Gd3Al2Ga3O12:Ce3+ crystal.

Radiation effects in cerium doped Gd3(Al,Ga)5:O12 (or GGAG) single crystals irradiated by swift heavy ions with fluences ranging from 6·1010 to 2·1012 ions/cm2 have been studied. A stable strong induced absorption observed in the spectral range 200–350 nm correlates with the irradiation fluence. It is suggested that several centers are responsible for this induced absorption in GGAG single crystals and their possible origin (F-type centers and V-centers or holes trapped near cation vacancies) is proposed and discussed. The swift heavy ions irradiation strongly modifies the luminescence properties of GGAG, namely, the excitation spectra of the Ce3+ emission, which have been measured over a wide spectral range including vacuum ultraviolet diapason. In particular, it was shown that the formation of the stable radiation defects under swift heavy ions irradiation leads to the effective Ce4+ → Ce3+ transformation in the Mg2+ co-doped GGAG single crystals. The reasons leading to the alteration in the luminescence properties of irradiated GGAG single crystals are elucidated and discussed.

The effect of electron irradiation on the properties of crystals of gadolinium-aluminum-gallium garnets Gd3Al2Ga3O12 (Al : Ga = 2 : 3), Gd3Al3Ga2O12 (Al : Ga = 3 : 2) and crystals doped with cerium Gd3Al2Ga3O12:Ce (Al : Ga = 2 : 3) (GAGG:Ce) is studied. The irradiation of cerium-doped crystals with electron doses from 300 to 2500 Mrad does not affect the optical properties of the crystals, which indicates their resistance to the formation of defects under high-energy irradiation. The optical transmission spectra of undoped Gd3Al2Ga3O12 and Gd3Al3Ga2O12 crystals noticeably decreases after irradiation with electron doses of 300 and 400 Mrad and these initially colorless crystals become brownish. A broad absorption band appears in the wavelength range of 350–500 nm, which is associated with the formation of structural defects such as color centers. The refractive indices of the crystals Gd3Al2Ga3O12 and GAGG:Ce barely change, but Gd3Al3Ga2O12 shows an increase in n(λ) depending on the exposure to high-energy radiation. The characteristic absorption bands of unirradiated GAGG:Ce with maxima at 340 and 440 nm are likely associated with the formation of crystal-lattice defects, and not only with electronic transitions of Ce.

Crystals of gadolinium–gallium–aluminum garnet doped with cerium ions (Gd3Ga3Al2O12:Ce) are analyzed by X-ray diffractometry and X-ray topography. These crystals have a number of unique scintillation characteristics (including a high specific light yield, fast emission kinetics, and high energy and time resolutions), which make them promising for designing detectors of ionizing radiation in a wide energy range. It is shown that the methods used make it possible to carry out the further optimization of preparation processes of bulk crystals by revealing specific features of their growth (e.g., facet inhomogeneity, which is characteristic of Czochralski-grown single crystals).

We performed a study of the behavior of near-electrode processes in the polar cuts of dielectric crystals with no inversion symmetry. We observed and described the temperature behavior of short circuit currents in α-LiIO 3 , NaKC 4 H4O 6 ·4H 2 O, (CH 2 ·NH 2 ·COOH)3·H 2 SO 4 , La 3 GasSiO 14 and La 3 Ga 5.5 Ta 0.5 O 14 crystals with identical current conductive coatings without preliminary affects. The elemental composition and phases on the samples surfaces were studied by X-ray phase analysis and X-ray photoelectronic spectroscopy. On the base of the results of our study we conclude that the formation of the short circuits currents induced by electromotive force is the result of electrochemical processes at the boundary between crystal and current conducting coating and is determined by the anisotropy of the opposite sides of polar cuts. We propose the model of self-decomposition of the polar cuts.

AbstractThe optical and luminescence properties of La_3Ga_5SiO_14 lanthanum–gallium silicate crystals grown in atmospheres of argon and argon with the addition of oxygen are investigated. The results of calculations of the structure of energy bands are presented, obtained using the CASTEP module in the framework of the generalized gradient approximation and the local density approximation. The width of the optical band gap of the crystal is determined to be $$E_{g}^{{{\text{opt}}}}$$ = 5.1 eV. Upon interband excitation, La_3Ga_5SiO_14 crystals grown in argon atmosphere show a luminescence band with a maximum at 430 nm, whereas for a crystal grown in argon with addition of oxygen, two luminescence bands with maxima at 470 and 530 nm dominate in the luminescence spectrum. The nature of the luminescence centers responsible for these bands is discussed with the help of the data for electronic structure calculations. The effect of temperature on the luminescent properties of La_3Ga_5SiO_14 is demonstrated. The presence of traps in La_3Ga_5SiO_14 is shown using thermally stimulated luminescence, and their activation energy is determined.