Recharging processes of Cr ions in Mg2SiO4 and Y3Al5O12 crystals under influence of annealing and γ - irradiation (original) (raw)
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International Conference on Solid State Crystals 2000: Growth, Characterization, and Applications of Single Crystals, 2001
The influence of reducing and oxidizing annealing on optical absorption spectra of the yttrium aluminium garnet (YAG) single crystal co-doped with Mg and Cr is investigated using step-by-step isothermal and isochronous thermal treatment in H 2 or air flow. The changes in the spectra were analyzed using decomposition on elementary absorption bands of Gaussian shape. The separeted absorption bands attributed to the Cr 4 ÷ ions occupying octahedral and tetrahedral sites in garnet lattice demonstrate different behaviour. The analysis allowes to distinguish the electronic recharging process Cr 4 +Oct] _ &+[Oc] taking place in the octahedral sites of garnet lattice and having lower activation energy from the chromium migration process Cr 4 +[,it<-• Cr 4 ÷[oti happening at higher temperatures or longer exposure. Estimations show that approximately 0.2% of total number of chromium ions occupied tetrahedral sites forming phototropic centers in the YAG: Mg, Cr crystal.
physica status solidi (c), 2005
Peculiarities of absorption spectra and recombination luminescence of Y 3 Al 5 O 12 (YAG) and Cd 3 Ga 5 O 12 (GGG) single crystalline films co-doped with chromium and magnesium have been studied. The change of impurities concentration and annealing of samples in the reducing atmosphere have an influence on absorption in the visible and UV range. Using the results on absorption coefficient measurements in the band of 480 nm and in the absorption bands of Cr 3+ ions, it was determined, that near 50% of the chromium ions located in octahedral sites are in the Cr 4+ state in films with high chromium concentration. The investigation of thermostimulated luminescence confirms the existence of Cr 2+ trap centres in the irradiated films caused by the magnesium dopant.
Spectral and Generation Properties of a New Laser Crystal (Cr[sup 3+],Li) : Mg[sub 2]SiO[sub 4]
Physics of the Solid State, 2005
It is well known that, in the forsterite structure, Cr 3+ ions substitute for Mg 2+ ions in two nonequivalent crystallographic positions, M 1 and M 2, and form Cr 3+ ( M 1) and Cr 3+ ( M 2) isolated centers with different spectral luminescent properties . It is generally agreed that, both in Cr 3+ : Mg 2 SiO 4 and in (Cr 3+ , Li) : Mg 2 SiO 4 crystals, the wide-band luminescence of Cr 3+ ions in the spectral range 850-1000 nm is caused by isolated chromium ions located in the mirror-symmetry magnesium positions M 2 [2, 3].
Excited state absorption in materials containing Cr 3+ and Cr 4+ ions
Journal of Alloys and Compounds, 2002
The ground state absorption (GSA), the excited state absorption (ESA) and fluorescence properties (low-and room temperature 31 41 emission spectra) of Cr , Cr -doped single crystals: Y Al O (YAG) and SrGdGa O (SGG) have been studied at various 3 5 12 3 7 temperatures in a broad spectral range (300-1700 nm). In the measured characteristics of those crystals we have observed features which 31 41
Effect of thermal history on high-valence chromium ion dissolution in merwinite (3CaO·MgO·2SiO2 )
Journal of the American Ceramic Society
Solubility and local structure of transmission elements in calcium silicate compounds has not been well understood. We investigate the local structure of chromium ions dissolved in merwinite (3CaOÁMgOÁ2SiO 2 ) of a monoclinic crystal structure. The acceptance of doping elements into merwinite has not been reported before. We found that chromium ions are soluble in merwinite in air and that chemical valence of the dissolved Cr ions varies with annealing temperature. The absorption edge in the x-ray absorption near edge structure (XANES) of Cr-doped merwinite indicated that octahedrally coordinated Cr 3+ ions were mainly formed when annealed at 1673 K in air. A pre-edge peak was also detected, indicating the existence of tetrahedrally coordinated high-valence Cr ions. Conversely, through annealing of merwinite at 1123 K in air, tetrahedrally coordinated Cr 6+ ions were found to be the main form of chromium. XANES spectra simulated by first-principle calculations were used to explain the structural features in the observed spectra. We propose the coexistence of Cr 3+ ions in octahedral Mg 2+ sites and high-valence Cr ions in tetrahedral Si 4+ sites. In addition, a change in the chromium ion oxidation state in tetrahedral coordination sites was suggested by XANES spectroscopy of Cr-doped merwinite synthesized at 1673 K and reannealed at 1123 K.
Polarizing Spectroscopy of Y3Al5O12, SrAl2O4, CaAl2O4 Crystals Containing Cr4+ Ions
The s p e c t r o s c o p i c model of a doping center -Cr4+ i o n , occupying t e t r a h e d r a l s i t e of YAG-lattice (Cr:+) is developed by t h e p o l a r i z i n g spectroscopy i n v e s t i g a t i o n s of C r , Mez+-doped YAG s i n g l e c r y s t a l s and t h e c a l c u l a t i o n of energy l e v e l s f o r an e l e c t r o n c o n f i g u r a t i o n dZ i n a c r y s t a l f i e l d of Dzd p o i n t group symmetry. New c r y s t a l s : SrAlZ0, and CaAlZ0,, c o n t a i n i n g t e t r a h e d r a l l y coordinated Cr4+ i o n s , a r e introduced.
Optical Materials, 2003
Thermal treatment of YAG crystals co-doped with Cr 4þ and Ca 2þ exhibits reversible oxidation-reduction reactions of the chromium ions, which depend on both temperature and ambient atmosphere. Dynamics of these reactions were studied by the spectral absorption and fluorescence characteristics of Cr,Ca:YAG crystals following different thermal treatment procedures. The absorption spectra were resolved into different peaks, of modified Lorentzian line-shape. Full reduction of Cr 4þ into Cr 3þ is obtained by prolonged heating under vacuum, or in an ambient atmosphere of Ar:5 vol.% H 2 mixture at $1000°C. Saturated oxidation is obtained by prolonged heating at elevated temperatures in free air. The oxygen diffusion coefficient is estimated as (1.85 AE 0.2) Â 10 À6 cm 2 /s at 850°C, with an activation-energy of (1.7 AE 0.07) eV. Only a small fraction of Cr 3þ ion concentration transforms into Cr 4þ. The latter may reside in either octahedral or tetrahedral sites, each exhibiting a different absorption spectrum. Transition from octahedral sites to tetrahedral ones is thermally activated, although no unique thermal activation energy may be assigned to the process. Knowing the process parameters now allows one to optimize oxidization procedures administered to Cr,Ca:YAG crystal devices that are used as laser gain media or as saturable absorbers.
The Journal of Chemical Physics, 2019
This paper focuses on the kinetics of Cr 4+ formation in Cr,Ca:YAG ceramics prepared by solid-state reaction sintering. The kinetics of Cr 4+ formation was studied by annealing of Cr,Ca:YAG ceramics in ambient air under different temperatures at different times, resulting in the transformation of Cr 3+ to Cr 4+. The activation energy (Ea) of Cr 3+ oxidation determined by the Jander model was 2.7 ± 0.2 eV, which is in good correlation with the activation energy of innergrain oxygen diffusion in the YAG lattice. It is concluded that Cr 3+ to Cr 4+ transformation in YAG ceramics is limited by oxygen diffusion through the grain body. It was established that in Cr,Ca:YAG ceramics, the intralattice cation exchange, in which the Cr 4+ ions exchange positions with the Al 3+ ions, switching from "A" to "D" sites, is faster than Cr 3+ to Cr 4+ oxidation. In the temperature range of 900-1300 ○ C, the reaction enthalpy of Al 3+ /Cr 4+ ion exchange between octahedral "A" and tetrahedral "D" lattice sites is close to zero, and this exchange ratio is thermodynamically driven by entropy.
Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy
Physics of the Solid State, 2009
Divalent and trivalent chromium ions Cr 2+ and Cr 3+ replacing magnesium ions at octahedral positions in Mg 2 SiO 4 : Cr and Mg 2 SiO 4 : Cr : Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65-230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is = 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr 2+ ions are distributed over the M1 and M2 positions in a ratio of approximately 2 : 1. A change in the oxygen partial pressure and the chromium concentration, as well as an additional doping with lithium, does not lead to substantial changes in the distribution of divalent chromium ions over the positions. It is shown that an increase in the oxygen partial pressure from 0.01 to 2.00 kPa results in a decrease in the coefficient of divalent chromium distribution between the crystal and the melt. Doping with lithium also decreases the concentration of Cr 2+ centers. In crystals grown without lithium, approximately half of the trivalent chromium ions are associated with magnesium vacancies. The addition of lithium leads to the destruction of these associates, an increase in the concentration of individual Cr 3+ centers, and the for mation of lithium associates with trivalent chromium ions. The conditions for the formation of associates of trivalent chromium ions with lithium ions are optimum when the crystal contains approximately identical amounts of Cr 3+ and Li + ions. Doping with lithium increases the concentration of Cr 3+ ions and, thus, decreases the fraction of Cr 2+ and Cr 4+ ions in the total content of chromium centers.