Jahn-teller effects in (3d)2 ions in Al2O3

APR experiments have been carried out on V³⁺ and Cr⁴⁺ ions in Al₂O₃ partly to remove discrepancies in earlier measurements, and partly to provide additional information necessary for the theoretical analysis which follows. We find |G_-| ～ 45± 10 cm^?1 and ～ 160±30 cm^?1 for V³⁺ and Cr⁴⁺ respectively. It is shown that all the data for both ions can only be explained satisfactorily if a moderately strong Jahn-Teller effect operates in the cubic ³T₁, ground state of the ion. Full details are given, γ is found to be 0.14 and 0.36 for V³⁺ and Cr⁴⁺ respectively and important contributions from second-order Jahn-Teller effects are also present. The results also show that it is necessary to use a multimode full-lattice model of the Jahn-Teller effect as the constraints of the cluster model are broken. The implications of the proposed model on thermal conductivity, phonon spectroscopy and optical measurements are also briefly discussed. It is also pointed out that some important errors exist in the literature concerning the interpretation of experimental data for these systems.     

Some detailed investigations of optically detected Ē(2 E) →4 A 2 cross relaxation in ruby

Cross relaxation betweenē(² E) and⁴ A ₂ states of Cr³⁺ in ruby at an applied external magnetic field ofH=5336 Oe was measured by monitoring the ¦? 1/2〉_ex→ ¦? 3/2〉_g optical transition in the temperature region of 1.6 to 4.2 °K. The chromium concentration varied from 2.9· 10^?4 to 4 · 10^?6 Cr³⁺/Al³⁺. With a concentration greater than 2 · 10^?5, the light intensity of the observed transition increases when cross relaxation takes place, while below this value it decreases. By measurement of the fluorescent intensity of one transition and simultaneously inducing EPR ground state transitions, we monitored the effect of trapping. Taking the value for trapping from fluorescence decay time measurements, we have used rate equations for calculating the actual change of excited state population when cross relaxation occurs. With this phenomenological model we are able to explain our experimental data. Finally some calculations for the effective spin temperature in theē(² E) state as a function of Cr³⁺ concentration as well as for various applied magnetic fields have been done.     

Spectrum and energy levels of Cr3+ ions and exchange-coupled Cr3+ pairs in lanthanum aluminate

From the optical spectra the energy levels of the Cr³⁺ ion in LaAlO₃ were determined. In the cubic field approximation we obtained with the parametersDq=1750cm^-1,B=661 cm^-1, andC=2908 cm^-1 a good agreement between the calculated and measured energies. For higher Cr³⁺ concentrations the fluorescence spectrum of the exchange-coupled first nearest (1N) Cr³⁺ pairs was separated from the vibronic fluorescence spectrum of the single ions by selective excitation. From the experimental data the energy level scheme of the 1N pairs is constructed for the states ¦⁴ A ₂·⁴ A ₂〉 and ¦⁴ A ₂·² E〉. In the ground state the pairs are ordered antiferromagnetically due to an isotropic exchange interaction with an exchange integralJ=(?68 ± 1) cm^-1. From the fluorescence decay the intrinsic lifetimes of the fluorescent states of the single ions, the 1N and the 2N pairs were determined to be equal to (86± 4) msec, (1.75± 0.25) msec, and (31 ± 5) msec respectively. The fluorescence decay shows an energy transfer between the single ions and the pairs.     

Supertransferred hyperfine structure in EPR of V4+ ion in Ca3In2Ge3O12 garnet

Interaction of an unpaired electron of the tetrahedral V⁴⁺ ion in Ca₃In₂Ge₃O₁₂ garnet with the four nearest In nuclei gives rise to clearly resolved structure of EPR spectra. We report the observation and analysis of these spectra at three frequencies and at temperature 77 K. The temperature dependence between liquid helium and the room temperatures was also studied. Parameters of the spin Hamiltonian are g_∥=1·8735; g_⊥=1·9825; ∣ ¦A∥|=150·4×10^?4cm^?1; ¦ A_⊥¦ = 36·7 × 10^?4 cm^?1. The supertransferred hyperfine interaction was found to be isotropic, absolute value of the corresponding parameter a is 22·1 × 10^?4 cm^?1.     

The determination of Li+ mobility in solid electrolyte Li1.3Al0.1Zn0.1Ti1.8P3O12 in view of ionic diffusivity and conductivity

Nasicon-type solid electrolyte Li_1.3Al_0.1Zn_0.1Ti_1.8P₃O₁₂ was prepared by citric acid-assisted acrylamide polymerisation gel method. X-ray diffraction pattern showed that the introduction of Zn²⁺ in the parent matrix Li_1+x Al _x Ti_2?x P₃O₁₂ made it easier to get high-purity rhombohedral structure (space group $ R\overline 3 C $ ) Li_1.3Al_0.1Zn_0.1Ti_1.8P₃O₁₂ without the evidence of impurity secondary phase. The Li⁺ kinetics were investigated by complex impedance in bulk pellet and ionic conductivity in battery-type composite cathode, respectively. Grain-interior resistance measured by galvanostatic intermittent titration technique, potential step chronoamperometry, and AC impedance spectroscopy at 20 °C varies in the range 1.2–1.95?×?10^?4?S?cm^?1, which is in good agreement with that obtained by complex impedance method 1.5?×?10^?4?S?cm^?1.     

Condensation Product of 1-Naphthaldehyde and 3-Aminophenol: Fluorescent “on” Probe for Ce3+and “off” Probe for Dichromate (Cr2O72−)

A new probe (Z)-3-((naphthalen-1-ylmethylene)amino)phenol has been synthesized by condensation reaction between 1-naphthaldehyde and 3-aminophenol for the fluorescent sensing of Ce³⁺ by “on” mode and dichromate (Cr₂O₇^2?) by “off” mode. Metal ions—Ag⁺, Al³⁺, As³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Ce⁴⁺, Co²⁺, Cr³⁺, Cr⁶⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, La⁺, Li⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, Zn²⁺and anions Br^?, C₂O₄^2?, CH₃COO^?, Cl^?, CO₃^2?, F^?, H₂PO₄^?, HCO₃^?, HF₂^?, HPO₄^2?, I^?, MnO₄^?, NO₃^?, OH^?, S^2?, S₂O₃^2?, SCN^?, SO₄^2? do not interfere. The limit of detection (LOD) for sensing Ce³⁺ and Cr₂O₇^2? ions are 1.286?×?10^–7 M and 6.425?×?10^–6 M, respectively.

     

Austauschwechselwirkungen,Intrabandschwingungen und Ti3+-Kristallfeldaufspaltung in dotiertem Al2O3

The absorption spectra of ruby with various Cr concentrations have been studied in the wavelength region between 50 μ and 1 mm at liquid helium and nitrogen temperatures. At liquid helium temperatures two strong absorptions have been found at 37.7 and 106 cm^?1 and weak absorptions at 18.8, 30.7, 33.2 and 43.0 cm^?1. These lines depend on the Cr concentration and vanish at N₂ temperatures. The two strong absorptions have also been observed in Al₂O₃ crystals doped with Ti or V. Sapphire did not show any of these absorptions. The origin of the two strong absorptions in Al₂O₃ crystals doped with Ti, V or Cr are discussed by consideration of magnetic dipole transitions of exchange coupled Cr³⁺ - pairs, the crystal field splitting of Ti³⁺ impurities and impurity — induced lattice modes. The weak absorption lines, on the other hand, may be assigned to transitions between the exchange levels of the second-nearest and fourth-nearest Cr³⁺ pairs if one makes use of the data obtained from the EPR and fluorescence spectra. For the second-nearest Cr³⁺ pairs group theory of the exchange interaction predicts transitions withΔS=2, whereas for the fourth-nearest neighbours no restriction forΔS exists.     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

Resonant cross-relaxation between Cr3+ and V3+ in Al2O3

Cross-relaxation, which occurs when a V³⁺ transition is resonant with a monitored Cr³⁺ transition, leads to a marked reduction of the spin-lattice relaxation time of Cr³⁺ in Al₂O₃. Measurements and an analysis of the temperature dependence of this effect give a value of 8.34 ± 0.49 cm^?1 for the zero-field splitting of V³⁺, an ion which is strongly coupled to the lattice.     

Structural studies of Cr-doped mullite derived from single-phase precursors

Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr₂O₃ using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr₂O₃). A second phase, namely θ-(Al,Cr)₂O₃, was revealed by XRD in the sample containing 9.60 wt% Cr₂O₃. The structure of mullites was refined by the Rietveld method, location of Cr³⁺ was performed by the EPR spectroscopy. At low chromium doping level (Cr₂O₃ content less than ∼5 wt%) Cr³⁺ ions were substituted for Al³⁺ in the AlO₆ octahedra of the mullite structure (M1 site). For higher doping level, Cr³⁺ ions were additionally substituted for Al³⁺ in the AlO₆ octahedra of the second phase [θ-(Al,Cr)₂O₃ at 1400 °C, or α-(Al,Cr)₂O₃ at 1600 °C] which segregated in the system. Substitution of Cr³⁺ for Al³⁺ on M1 site in the mullite structure resulted in increase of average distances in (M1)O₆ octahedron and decrease of average distances in T*O₄ tetrahedron, while average distances in TO₄ tetrahedron stayed almost constant.     

N-lines in the luminescence spectra of Cr3+-doped spinels: IV. Excitation spectra

Excitation spectra (T = 75–300 K; λ_exc = 450–630 nm) which were measured for the R-lines of Cr³⁺-doped oxides (α-Al₂O₃, β-Ga₂O₃) and for different luminescence lines (R-lines, N-lines) of Cr³⁺-doped spinels (MgAl₂O₄, ZnAl₂O₄, ZnGa₂O₄) are reported. The excitation maxima observed for different luminescence lines of a given compound exhibit considerable differences: 530 nm ? λ^max_exc ? 565 nm for MgAl₂O₄; 530 nm ? λ^max_exc ? 580 nm for ZnAl₂O₄; 545 nm ? λ^max_exc 555 nm for ZnGa₂O₄. According to the interpretation of N-lines to arise from different classes of Cr³⁺ ion swith different short range orderd, the excitation maximum of one distinct line should entirely correspond to the transition Δ : ⁴T₂ ← ⁴A₂ of that Cr³⁺ class from which the line arises. By this method spectroscopic data about the different kinds of Cr³⁺ ions present in a given sample can, therefore, be obtained which are not available from absorption measurements. The experimetal data were found to be in agreement with the results of model calculations. Restrictions which limit the accuracy and relevance of the data are discussed.     

Luminescence of Cr3+ in mullite transparent glass ceramics

Silicate glasses containing more than 10 mole% Al₂O₃ phase separate upon quenching, and when heat treated form transparent glass ceramics with scattering loss less than 0.01 cm⁻¹. Cr³⁺ used as a dopant is shown to reside in the mullite crystalline phase. The luminescence quantum efficiency is increased from less than 1% to about 30% by the crystallization process.     

EPR and Optical Absorption Study of Cr3+-Doped Dipotassium Tetrachloropalladate

X-band electron paramagnetic resonance (EPR) study of Cr³⁺-doped dipotassium tetrachloropalladate single crystal is done at liquid nitrogen temperature. EPR spectrum shows two sites. The spin-Hamiltonian parameters have been evaluated by employing hyperfine resonance lines observed in EPR spectra for different orientations of crystal in externally applied magnetic field. The values of spin-Hamiltonian and zero-field splitting (ZFS) parameters of Cr³⁺ ion-doped DTP for site I are: g _x  = 2.096 ± 0.002, g _y  = 2.167 ± 0.002, g _z  = 2.220 ± 0.002, D = (89 ± 2) × 10^?4 cm^?1, E = (16 ± 2) × 10^?4 cm^?1. EPR study indicates that Cr³⁺ ion enters the host lattice substitutionally replacing K⁺ ion and local site symmetry reduces to orthorhombic. Optical absorption spectra are recorded at room temperature. From the optical absorption study, the Racah parameters (B = 521 cm^?1, C = 2,861 cm^?1), cubic crystal field splitting parameter (Dq = 1,851 cm^?1) and nephelauxetic parameters (h = 2.06, k = 0.21) are determined. These parameters together with EPR data are used to discuss the nature of bonding in the crystal.     

The structure of a propagating MgAl2O4/MgO interface: linked atomic- and μm-scale mechanisms of interface motion

To understand how a new phase forms between two reactant layers, MgAl₂O₄ (spinel) has been grown between MgO (periclase) and Al₂O₃ (corundum) single crystals under defined temperature and load. Electron backscatter diffraction data show a topotaxial relationship between the MgO reactant and the MgAl₂O₄ reaction product. These MgAl₂O₄ grains are misoriented from perfect alignment with the MgO substrate by ~2–4°, with misorientation axes concentrated in the interface plane. Further study using atomic resolution scanning transmission electron microscopy shows that in 2D the MgAl₂O₄/MgO interface has a periodic configuration consisting of curved segments (convex towards MgO) joined by regularly spaced misfit dislocations occurring every ~4.5 nm (~23 atomic planes). This configuration is observed along the two equivalent [1 0 0] directions parallel to the MgAl₂O₄/MgO interface, indicating that the 3D geometry of the interface is a grid of convex protrusions of MgAl₂O₄ into MgO. At each minimum between the protrusions is a misfit dislocation. This geometry results from the coupling between long-range diffusion, which supplies Al³⁺ to and removes Mg²⁺ from the reaction interface, and interface reaction, in which climb of the misfit dislocations is the rate-limiting process. The extra oxygen atoms required for dislocation climb were likely derived from the reactant MgO, leaving behind oxygen vacancies that eventually form pores at the interface. The pores are dragged along by the propagating reaction interface, providing additional resistance to interface motion. The pinning effect of the pores leads to doming of the interface on the scale of individual grains.     

Relaxation phenomena in Cr3+ doped Al2O3−Fe 2 57 O3 systems

Mössbauer absorption spectra have been measured for Cr³⁺ doped Al₂O₃?Fe ₂ ⁵⁷ O₃ systems for different values of the Cr³⁺ concentration at room temperature. The cross relaxation between Fe³⁺ and Cr³⁺ ions, which destroys the paramagnetic hyperfine structure of Fe⁵⁷ observed in undoped Al₂O₃?Fe ₂ ⁵⁷ O₃, is thoroughly studied. The experimental results suggest a new kind of cross-relaxation process involving three spins, i.e. two Fe³⁺.ions and one Cr³⁺. The process, though it is a higher-order one, is highly effective because it is energy-conserving.     

Optical properties of transparent cobalt-containing magnesium aluminosilicate glass-ceramics doped with gallium oxide for saturable absorbers

Transparent glass-ceramic materials based on glasses of the MgO–Al₂O₃–SiO₂–TiO₂ system doped with CoO and Ga₂O₃ are synthesized. The secondary heat treatment of the initial glasses at temperatures of 800–950°C leads to precipitation of nanosized (6–7 nm) crystals of magnesium aluminogallium spinel doped with cobalt ions and magnesium aluminotitanate solid solutions. The optical absorption spectra of the initial glass and glass-ceramic materials are studied. It is shown that the absorption band caused by the ⁴A₂(⁴F)→ ⁴T₁(⁴F) transitions of tetrahedrally coordinated Co²⁺ ions in glass-ceramics with nanosized Co:Mg(Al,Ga)₂O₄ crystals is shifted to longer wavelengths (up to ~1.67 µm) compared to the position of this band in materials with Co:MgAl₂O₄ crystals. The synthesized glass-ceramics are characterized by a relatively low saturation fluence FS ~ 0.5 ± 0.1 J/cm² at a wavelength of 1.54 µm, as well as by a high radiation resistance to nanosecond laser pulses, which is no lower than ~15 ± 2 J/cm². This explains their attractiveness as materials for saturable absorbers for erbium lasers emitting in the spectral range 1.5–1.7 µm.     

Charge transfer reaction of multi-charged oxygen ions with O2

The rate of transfer of electrons from O₂ to O²⁺ and O³⁺ has been measured at energies ? 2 eV using a stored ion technique. The rate for O²⁺ is k = 1.0(0.3) × 10^?9 cm³/s and for O³⁺, k = 2.5(0.3) × 10^?9 cm³/s, compared to calculated Langevin rates of 1.8 × 10^?9 cm³/s and 2.7 × 10^?9 cm³/s respectively.     

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

Magnesium aluminate (MgAl₂O₄) doped with trivalent chromium (Cr³⁺) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr³⁺ ions. The luminescence of Cr³⁺-activated MgAl₂O₄ exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper ²E_g→⁴A_2g ground state of Cr³⁺ ions. By correlating EPR and optical data the crystal field splitting parameter (D_q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr³⁺ ions with the ligands and the Cr³⁺ ions are in distorted octrahedral environment.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

Light induced charge transfer processes in Cr doped Bi12GeO20 and Bi12SiO20 single crystals

Optical absorption and ESR spectra of Bi₁₂GeO₂₀ doped with Cr were measured before and after illumination with visible light. It was found that Cr⁴⁺ ions in tetrahedral position are responsible for light induced ESR and optical spectra. The g-factor of the center is 1.945 ± 0.002. Crystal field parameters for the Cr⁴⁺ center are D_q = 820 cm^?1 and B = 429cm^?1. The photochromic effect is explained in terms of a Cr⁵⁺?Cr⁴⁺ charge transfer process.