4T2g State Research Articles

Insights into luminescence thermal quenching of Mn4+-doped BaLa(Na/Li)(W/Te)O6 double perovskite red phosphors

A series of Mn4+ doped BaLa(Na/Li)WO6 and BaLaLi(W/Te)O6 phosphors were synthesized and studied. Substitution reactions of Na+ by Li+ and W6+ by Te6+ allows the management of the energy gap between Mn4+ 2Eg and 4T2g states and improves the thermal stability of the phosphors.

Upconversion luminescence in Cr3+:YAG single crystal under infrared excitation

The present work concerns the upconversion luminescence in Cr3+:YAG single crystal under infrared laser irradiation. During the experiment, 404 nm, 808 nm, 980 nm, and 1064 nm laser excitation sources in range of −150 °C–600 °C were employed. A strong red light emission was detected in the bulk of the samples under the infrared excitation. The red emission originated from 4T2g→4A2g transition of Cr3+ ions and are only observed at temperatures higher than 150 °C; the profile of the emission curve is Gaussian-type with maxima at 500 °C, 600 °C, and 620 °C for λexc 808 nm, 980 nm, and 1064 nm respectively. This is attributable to the high-temperature nature of the upconversion luminescence, where the energy gap between the energy of a laser photon and the 4T2g state is compensated by simultaneous absorption of two photons.

Temperature dependence (13–600 K) of Mn4+ lifetime in commercial Mg28Ge7.55O32F15.04 and K2SiF6 phosphors

The fluorescence lifetime of the Mn4+2Eg→4A2g transition in commercial Mg28Ge7.55O32F15.04 (MFG) and K2SiF6 (KSF) phosphors are measured in the temperature region of 13–600 K. In MFG, the observed decrease in the lifetime with increasing temperature is correlated with increasing probability of phonon induced transitions (vibronic emission) without any contribution of nonradiative transitions. In KSF, the decrease in lifetime in the temperature region of 13–500 K is associated with an increase in the vibronic transition probability. Above 500 K, the rapid decrease in the lifetime is correlated with the onset of non-radiative relaxation. The differences between the thermal quenching behavior of Mn4+ luminescence in the two phosphors in explained with respect to the energy position of the 4T2g state.

Dynamic Jahn–Teller effect for V2+ in MgO single crystal

In this paper we studied the dynamic Jahn–Teller effect in the excited state 4T2g of the V2+ ions doped in MgO crystal, in octahedral coordination. Calculation of the fine structure of the energy levels in the frame of crystal field theory, estimation of the vibronic coupling constants and the Jahn–Teller stabilization energy are presented. The dynamic Jahn–Teller effect in the lowest excited 4T2g state of the V2+ ions is considered using the Ham effect and analysis of the displacements of ligands, due to the combined effect of the a1g and eg normal modes of the obtained octahedral cluster.

Theoretical study on ultrafast intersystem crossing of chromium(III) acetylacetonate

In the relaxation process from the 4T2g state of chromium(III) acetylacetonate, CrIII(acac)3, ultrafast intersystem crossing (ISC) competes with vibrational relaxation (VR). This contradicts the conventional cascade model, where ISC rates are slower than VR ones. We hence investigate the relaxation process with quantum chemical calculations and excited-state wavepacket simulations to obtain clues about the origins of the ultrafast ISC. It is found that a potential energy curve of the 4T2g state crosses those of the 2T1g states near the Franck–Condon region and their spin–orbit couplings are strong. Consequently, ultrafast ISC between these states is observed in the wavepacket simulation.

Interionic Energy Transfer in Y3Al5O12 : Ce3+, Pr3+, Cr3+ Phosphor

We present an investigation of dynamical processes of nonradiative energy transfer (ET) in Y3Al5O12:Ce3+, Pr3+, Cr3+ phosphor. The analysis based on Inokuti-Hirayama model indicates that the ET from the lowest 5d state of Ce3+ to the 4T2g state of Cr3+, and from the state 1D2 of Pr3+ to 4T2g state of Cr3+ are both governed by electric dipole-dipole interaction. For Pr3+ concentration of 0.005 in YAG: Pr3+ Cr3+ and for Ce3+ concentration of 0.01 in YAG: Ce3+ Cr3+ the rate constant and critical distance are evaluated to be 7.3×10−37 cm6s−1 and 0.22 nm for Pr3+ - Cr3+ ET and 1.03×10−36 cm6s−1 and 0.63 nm for Ce3+ - Cr3+ ET, respectively.

Optical investigation of 6A 1g( 6S)→ 4T 2g ( 4D) absorption band in magnetic MnF 2 and RbMnF 3 compounds

Spectroscopic measurements of the spectral lines in MnF 2 and RbMnF 3 have been carried out in the temperature range 10–300 K. Observations of exciton–magnon sidebands in the fine structures of the 6A 1g → 4T 2g ( 4D) state of Mn 2+ ions are reported at low temperatures. The temperature dependence of peak positions, oscillator strength and half line widths have been investigated for selected bands. The analysis of the temperature dependence of these parameters confirms that the fine structure of the D band is mainly attributed to spin–orbit interaction combined with spin multiplicity in the ordered state. The ratios for the separation energies between these lines are fitted with the ratios expected from Landé interval rule.

Optical Properties of the (CrF6)3−Complex in A2BMF6:Cr3+Elpasolite Crystals: Variation with M−F Bond Distance and Hydrostatic Pressure

This work investigates the photoluminescence (PL) properties of the Cr (3+)-doped and Cr (3+)-pure fluoroelpasolites along the A 2BMF 6 series and as a function of pressure. In particular, we focus on the variation of the crystal-field spectrum and the associated PL. The results are explained on the basis of the octahedral (CrF 6) (3-) complex subjected either to external pressure or the internal pressure exerted by different crystal hosts. We have established structural correlations between the crystal-field parameter 10Dq and the Cr-F distance, R Cr-F, from which we have determined the local structure around the Cr (3+) impurity, allowing the host material effect on the Cr-F bonds to be studied. As salient features, we show, first, a weak dependence of the first excitation energy, E 1, usually identified as 10Dq, with R Cr-F as E 1 = KR Cr-F (-3.3), and, second, an increase of the Stokes shift upon R Cr-F reduction or with increasing pressure. We associate this unusual behavior with the existence of state mixing among (4)T 2g(F), (2)E g(G), and (2)T 1g(G) states in the first excitation band of Cr (3+). Finally, high-pressure experiments performed on Rb 2KCrF 6 indicate that the excited-state spin crossover, (2)E g(G) <--> (4)T 2g(F), takes place around 7 GPa. The results indicate the suitability of the selected A 2BMF 6:Cr (3+) elpasolites to establish structural correlations between PL and R Cr-F.

Jahn–Teller effect in the 4T 2g excited state of Cr 3+ ion in Cs 2NaYF 6 crystal

Calculations of the fine structure of Cr 3+ energy levels in Cs 2NaYF 6 accompanied by estimations of the Jahn–Teller (JT) stabilization energy in the first excited 4T 2g state of Cr 3+ ion are presented. Two independent approaches—effective second-order spin–orbit Hamiltonian and analysis of the potential energy surfaces—are used. The JT energy was estimated to be 216 and 257 cm –1 in the first and the second models, respectively. It is shown that the octahedral [CrF 6] 3− complex undergoes an equatorial expansion by 0.09 Å and an axial elongation by 0.02 Å due to the combined effect of the a 1g and e g normal modes.

Fluorescence and phosphorescence of Cr 3+ in cubic hosts

The low-temperature well-resolved 4T 2g → 4A 2g fluorescence spectrum of Cs 2NaYF 6:Cr 3+ is presented and interpreted. Strong vibrational progressions in the α 1g symmetric stretching and ε g stretching modes are based upon the zero phonon line and odd-parity vibronic origins. The Jahn–Teller stabilization energy in the 4T 2g state is 255 cm −1. The Cr 3+ emission has previously been assigned to Tm 3+ upconversion. The 4T 2g barycentre is strongly linearly correlated with metal–halogen distance for Cr 3+ diluted into elpasolite hosts. More generally, for Cr 3+ in other hosts, the metal–ligand distance divided by ligand formal charge exhibits a fairly linear relation with 4T 2g– 4A 2g energy separation, by contrast with the independence upon 2E g– 4A 2g energy separation.

Manifestation of vibronic interaction in the fine structure of Cr3+ energy levels in laser crystal LiCaAlF6:Cr3+

Theoretical study of the fine structure of the 4T2g level of Cr3+ ions in LiCaAlF6 is carried out. The spin–orbit splitting of the 4T2g term in the intermediate field and the vibronic coupling with the eg local mode of crystal lattice is found. The energies of the spin–orbit components of 4T2g are found by means of diagonalization of the Eisenstein matrices using the parameters (Dq,B,C,ζ) corresponding to zero phonon line of 4T2g→4A2g emission. Dynamical Jahn–Teller effect in 4T2g state, which quenches the total splitting of 4T2g state by Ham effect, is taken into account. Using the Ham reduction factor, the Jahn–Teller stabilization energy has been calculated.

Influence of hydrostatic pressure on the Jahn–Teller effect in the T2g4 excited state of CrCl63− doped Cs2NaScCl6

The T2g4→4A2g luminescence of a 4.1% Cr3+ doped Cs2NaScCl6 crystal is studied as a function of hydrostatic pressure at room temperature and 15 K. The vibrational fine structure observed in the low-temperature variable pressure emission spectra is analyzed with a two configurational coordinate approach, involving the totally symmetric a1g and the eg Jahn–Teller normal coordinate. Increasing hydrostatic pressure is found to reduce the tetragonal distortion of the CrCl63− unit in the electronic T2g4 state resulting from the Jahn–Teller effect. Additionally, pressure impedes expansion along the a1g coordinate of the CrCl63− complex upon T2g4→4T2g photo-excitation, and thus has a greater influence on the T2g4 excited state than on the A2g4 ground state. The absolute Cr3+–Cl− average distance reduction upon increasing pressure is estimated using a simple point charge model.

Ab Initio calculations of [CoY6−nXn]2+ complexes

The complete active space self-consistent field (CASSCF) and multireference second order perturbation theory (CASPT2) calculations of [CoF6]4−, [Co(H2O)6]2+, [Co(NH3)6]2+, [Co(H2O)5X]2+, and [Co(H2O)4X2]2+ complexes (X=CH3OH, CH3SH, CH3NH2) are reported. The potential energy surfaces of 10 lowest quartet states of [Co(H2O)5X]2+ complexes near the equilibrium geometry are calculated and splitting of triple-degenerate T1g4(F), T2g4(F), and T1g4(P) electronic states of [Co(H2O)6]2+ complex induced by the substitution of one or two water ligands is characterized and quantified. It is shown that the energy differences between originally degenerate states are almost invariant to the changes of metal–ligand distances, and despite their proximity, the crossing does not occur. The coefficients of leading configuration of multireference wave functions of [Co(H2O)5X]2+ and [Co(H2O)4X2]2+ complexes are shown to approach unity and the usage of single-reference methods is justified. As a consequence, interaction energies of the studied functional groups with Co2+ are computed also at the HF (Hartree–Fock), DFT (density functional theory), and MP2 (second-order Moller–Plesset) levels. They are compared to CASSCF calculations and to the equivalent calculations done for Zn2+ and Ni2+ ions. The computational methodology for the accurate calculations of various cobalt (II) ionic complexes is described and the implications for the theoretical investigation of interactions of chemically and biologically important functional groups with Co2+ are discussed.

Optical properties of α-ZnAl2S4:Cr single crystals

The optical properties of the chromium doped spinel type semiconductor α-ZnAl2S4 have been examined over the temperature range 2–540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state A2g4 and the excited levels Eg2, T1g2, T2g2, T2g4, and T1g4 of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the Eg2, T1g2, and T2g4 levels and the energy gaps between the Eg2–T1g2 and Eg2–T2g4 states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a α-ZnAl2S4:Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm−1 at 198 K were obtained.

Luminescence of the Cr 3+ ions in spinel-type α-ZnAl 2S 4:Cr single crystals

Photoluminescence (PL) of chromium-doped spinel-type semiconductor α-ZnAl 2S 4 has been studied in the temperature range 2–540 K. The transitions between ground state 4A 2g and 2E g, 2T 1g, 2T 2g, 4T 2g, 4T 1g excited levels of Cr 3+ ions in absorption and emission spectra have been observed. A fit of the measured temperature dependence of the PL decay, using a four-level model, gave the lifetimes of the 2E g, 2T 1g and 4T 2g levels. The energy gaps between 2E g– 2T 1g and 2E g– 4T 2g states have been determined and they are very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr 3+ ions in the α-ZnAl 2S 4:Cr spinel host has been constructed.

Photoluminescence of the Cr 3+ ions in α-ZnAl 2S 4:Cr single crystals

Photoluminescence measurements have been performed on chromium doped α-ZnAl 2S 4 spinel-type crystals in the temperature range 2–540 K. a single configuration coordinate diagram for the Cr 3+ ions in the α-ZnAl 2S 4 host is constructed to interpret the absorption and emission spectra and to allow a determination of the energy gaps between the 2E g − 2T 1g and 2E g − 4T 2g states. A fit of the measured temperature dependence of the luminescence decay, using a simplified four level model gave the lifetimes of the 2 E g , 2 T 1 g and 4 T 2 g excited levels and confirmed the gap values obtained from steady state measurements.

Optical properties of tsavorite Ca3Al2(SiO4)3:Cr3+, V3+ from Kenya

Previously, a relatively large splitting of the 2Eg state of Cr3+ in grossular Ca3Al2[SiO4]3 has been reported [M. Czaja and Z. Mazurak, Opt. Mater 3 (1994) 95]. The absorption and luminescence spectra of crystals containing Cr3+ and V3+in grossular Ca3Al2(SiO4)3 garnets (variety tsavorite) have been studied for the first time. From these spectra measured at room and liquid helium temperatures we determined energies of R2, R1, and 4T2g levels of Cr3+. The lifetimes and splitting of the 2Eg state have been measured by tuning the excitation energy within the inhomogeneous profile of the R1 and R2 lines of the split 2Eg → 4A2g transition. To account for the thermal quenching of the R-lines, it is necessary to recognize that the two components of the doublet may have remarkably different radiative lifetimes (7500 and 2500 μs). These lifetimes were obtained from low temperature decay and from the temperature dependence of the intensity ratio of the R2, R1 components. These separate lifetimes as well as the lifetime of the 4T2g state were then set to fit the temperature dependence of the joint effective lifetime. The optical fluorescence of a V3+ dopant is characterized by the broad band infrared fluorescence spectrum associated with short decay time < 1μs.

Coupling with the Jahn‐Teller mode for triplet states of MF6 (M = Mn2+, Cr3+) complexes: Dependence on the M‐F distance and influence on the stokes shift

AbstractThe microscopic origin of the VE coupling constant with the Jahn‐Teller mode Eg for orbital triplet states corresponding to several excited states of MnF4+6 and the 4T2g state of CrF3−6 is explored by molecular orbital (MO) calculations. VE is shown to be determined by the splittings δe and δt induced in the antibonding e and t levels by a Qθ(∼3z2− r2) distortion and so VE &lt; 0 for the 4T2g state of CrF36 leading to a compressed octahedron as equilibrium geometry. VE values have been derived from self‐consistent charge extended Hückel and MSXα calculations performed at different Qθ values. The results for the 4T1g(G), 4T2g(G), 4T2g(D), and 4T1g(P) states of MnF46 are reasonably close to the experimental figures. For the first excited state 4T1g(G) the value VE ∼ 60 cm−1/pm found for a metal‐ligand distance R equal to 213 pm leads to a Huang‐Rhys factor SE = 1.5 also close to experimental findings. On passing from this case to the 4T2g state of CrF36−[VE] increases by a factor of about two but SE =S 1.2 in agreement with experimental data for Rb2KGaF6: Cr3+. The latter figure implies a Stokes shift E0S(E) due to the Eg mode equal to 1200 cm−1 which is about 50% of the total Stokes shift. As a salient feature it is shown that for both MnF4−6 and CrF3−6 complexes VE strongly depends upon R, as it also happens to the VA coupling constant with the symmetric A1g mode. This reasonably explains the increase of the Stokes shift upon increasing R recently observed for Mn2+−doped fluoroperovskites. Although pure crystalfield (CF) theory gives rise to VE values much smaller than the experimental ones it is shown that, the relation VA/VE = \documentclass{article}\pagestyle{empty}\begin{document}$\documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt 2 $\end{document}$\end{document} derivedforthe first excited state of MnF4−6 and CrF3−6 is no far from the results obtained through MO calculations.

Excited-state absorption spectra of V2+-doped fluoroperovskites. An ab initio model potential embedded-cluster study

In this paper we present an ab initio model potential embedded-cluster study of the electronic structure of the local excited states of V2+-doped KMgF3, KZnF3, and CsCaF3 fluoroperovskites, which are more directly involved in their potential laser activity: the 4T2g, 4Ta1g, and 4Tb1g states of the (V F6)4− embedded-cluster. The ab initio model potential embedded-cluster model used allows for the consideration of intracluster electron correlation and Jahn–Teller coupling in the excited states, as well as of lattice effects which include quantum mechanical interactions resulting from a relaxed, dipole polarized crystal lattice. The embedding potentials enable the geometry optimization of the embedded-cluster electronic ground-state but also of the excited states; as a consequence, the ab initio calculation of the vertical ground-state absorption, excited-state absorption, and emission spectra is possible and it is done in this work, without resorting to the use of experimental structural parameters of the local defect, which, in any case, are not available for the ground state and not attainable for the excited states. The agreement of the calculated ground-state absorption, excited-state absorption, and emission transition energies with the available experimental data is satisfactory and systematic. Our results allow for the discussion of (a) the competition between the infrared 4T2g→4Ta1g excited-state absorption and the 4T2g→4A2g spontaneous emission, (b) the overlap between the excited-state absorption bands and the pumping bands, and (c) the 4A2g→4T2g reabsorption, for all three V2+-doped fluoroperovskites, as well as their host dependency. The overall conclusion is that the laser efficiency is expected to deteriorate in the investigated series, going from KMgF3:V2+ to CsCaF3:V2+.

Optical properties of tsavorite Ca 3Al 2(SiO 4) 3:Cr 3+,V 3+ from Kenya

Previously, a relatively large splitting of the 2E g state of Cr 3+ in grossular Ca 3Al 2[SiO 4] 3 has been reported [M. Czaja and Z. Mazurak, Opt. Mater 3 (1994) 95]. The absorption and luminescence spectra of crystals containing Cr 3+ and V 3+in grossular Ca 3Al 2(SiO 4) 3 garnets (variety tsavorite) have been studied for the first time. From these spectra measured at room and liquid helium temperatures we determined energies of R 2, R 1, and 4T 2g levels of Cr 3+. The lifetimes and splitting of the 2E g state have been measured by tuning the excitation energy within the inhomogeneous profile of the R 1 and R 2 lines of the split 2E g → 4A 2g transition. To account for the thermal quenching of the R-lines, it is necessary to recognize that the two components of the doublet may have remarkably different radiative lifetimes (7500 and 2500 μs). These lifetimes were obtained from low temperature decay and from the temperature dependence of the intensity ratio of the R 2, R 1 components. These separate lifetimes as well as the lifetime of the 4T 2g state were then set to fit the temperature dependence of the joint effective lifetime. The optical fluorescence of a V 3+ dopant is characterized by the broad band infrared fluorescence spectrum associated with short decay time < 1μs.