The luminescence spectrum of ReCl6 2- doped in various host lattices with antifluorite-type structure

The vibrational structure derived from the Γ₇(² T _2g ) → Γ₈(⁴ A _2g ) phosphorescence spectrum at liquid nitrogen temperature of a number of cubic host crystals A₂BX₆ (A = K, Rb, Cs; B = Pt, Sn, Pb, Te; X = Cl, Br) doped with ReCl₆ ^2- has been investigated. The best resolved spectra were obtained for those host crystals absorbing appreciably more of the exciting light than ReCl₆ ^2-. This suggests an energy transfer mechanism from host molecules to the chromophore. The spectral bands are assigned to normal frequencies of the octahedral chromophore, to lattice vibrations and to various combinations. For a complete assignment, vibrational states must be considered which belong to special points in the Brillouin zone other than the zone centre. The zero phonon transitions vary in a definite pattern for host crystals which have closed electron shells. The shifts, which depend more on the cation A⁺ than on the host complex BX₆ ^2-, are discussed by means of ligand field theory.     

Influence of chemical bond length changes on the crystal field strength and “ligand-metal” charge transfer transitions in Cs2GeF6 doped with Mn and Os ions

Detailed study of dependence of the crystal field strength 10Dq and lowest charge transfer (CT) energies for different interionic distances in Cs₂GeF₆:Mn⁴⁺ and Cs₂GeF₆:Os⁴⁺crystals is presented. The calculations were performed using the first-principles discrete-variational Dirac-Slater (DV-DS) method. As a result, the functional dependencies of 10Dq and lowest CT energy on the metal-ligand distance R were obtained without any fitting or semiempirical parameters. It was shown that 10Dq depends on R as 1/Rⁿ, with n=4.0612 and 4.3874 for Cs₂GeF₆:Mn⁴⁺ and Cs₂GeF₆:Os⁴⁺, respectively. Two approximations (linear and quadratic) are obtained for the dependence of the lowest CT energy on R; CT energy decreases when R increases with dE(CT)/dR=−638 and −1080 cm⁻¹/pm for Cs₂GeF₆:Mn⁴⁺ and Cs₂GeF₆:Os⁴⁺, respectively, if the linear approximation is used. These values can be used for estimations of the lowest CT energies for Mn⁴⁺ and Os⁴⁺ ions in other hosts with fluorine ligands. Estimations of the electron-vibrational interaction (EVI) constants, Huang-Rhys parameters, and Stokes shifts for all the above-mentioned crystals were performed using the obtained 10Dq and E(CT) functions.     

The absorption and magnetic circular dichroism of spectra of Cs2NaPrCl6

The room temperature absorption and magnetic circular dichroism spectra and the absorption spectrum at liquid helium (liquid He) temperature have been measured for Cs₂NaPrCl₆. At room temperature the crystal is cubic and the Pr³⁺ sites have O _h symmetry. All terms above 15 000 cm^-1, except ¹S₀, have been assigned and a previous assignment in PrCl₆ ^3- has been shown to be incorrect. The transition at 20 631 cm^-1 is assigned to ³H₄(A _1g ) →³P₁(T _1g ), in contradiction to previous assignments of Pr³⁺ spectra in other systems. A rich vibrational structure was observed in every transition. Vibrations have been assigned using the site group approximation and there is substantial agreement with the vibrational assignments in Cs₂NaEuCl₆. A crystal phase transition takes place between room temperature and liquid nitrogen temperature and the O _h forbidden transitions, A _1g →E_g and T _2g , are observed. At lower temperatures many additional lines are observed but it is unclear presently whether they are due to lower symmetry or a breakdown of the site group approximation.     

Temperature Dependence of Integrated Intensities of Vibrational Bands in M2GeF6 IR-Absorption Spectra

The paper presents experimental results of studies on the temperature dependence of integrated intensities of vibrational bands corresponding to ₃(GeF ₆) in M₂GeF₆ IR-spectra. It is shown that unlike Na₂GeF₆, Rb₂GaF₆, and Cs₂GeF₆, where the dependence of intensity on temperature is monotonous, in the case of K₂GeF₆ there exist anomalous temperature ranges of 220-240 and 150-170 K, where the intensity of a band, corresponding to ₃(GeF^2- ₆) changes abruptly. The study of heat evaluation, heat capacity, unit cell parameters, and intensities of selective reflexes in K₂GeF₆ X-ray diffraction patterns indicates the presence in this compound, as distinguished from others in the seris considerod, of phase transitions of the first order. On the basis of IB, ¹⁹F NNR, X-rat and calorimetric data, a possible mechanism of polymorphic transformation is discussed.     

Pressure dependence study on the electron-phonon coupling of thulium hexachloroelpasolite

Raman spectra of Cs₂NaTmCl₆ have been recorded using a diamond anvil cell at ambient temperature. The vibrational energy of each of the Raman-active TmCl₆⁻³ moiety modes increases linearly with pressure. The integrated band areas of the ν₁(a_1g) and ν₂(e_g) modes are independent of applied pressure. However, the band area of the ν₅(t_2g) mode shows an anomalous behaviour, which has been qualitatively interpreted as due to electron-phonon coupling of the aΓ₅ electronic state with the Γ₁+ν₅(t_2g) vibronic state. This interaction between the coupled states is strongest between ca. 10 and 13 GPa at ambient temperature. The results serve to emphasize the specificity of the occurrence of strong electron-phonon coupling for particular transitions of a given rare earth ion.     

Cs2XF6 (X=Si,Ge) compounds: Common and different features as uncovered by the first-principles calculations

Results of ab initio calculations of the electronic, optical and elastic constants for Cs₂GeF₆ and Cs₂SiF₆ are reported for the first time. Both compounds are the direct band gap dielectrics, with the calculated band gaps 6.926 eV (Cs₂SiF₆) and 6.417 eV (Cs₂GeF₆). Analysis of the calculated elastic constants and Cauchy condition shows both crystals as slightly covalent compounds. However, with increased pressure chemical bonds in Cs₂GeF₆ turn to be more ionic, whereas in Cs₂SiF₆ the covalent character of chemical bonds is enhanced. Pressure dependence of the chemical bond lengths and lattice constants was determined and represented as the second power functions of external pressure. Different trends in the values of the Mulliken charges for both compounds were found. The obtained results are applicable to the analysis of the luminescence properties of impurity ions at varying pressure or to the microscopic studies of crystal field effects in these crystals.     

Ab initio and crystal field studies of the Mn4+-doped Ba2LaNbO6 double-perovskite

A detailed study using the ab initio DFT-based calculations of the electronic and optical properties of pure and Mn⁴⁺ doped Ba₂LaNbO₆ is presented in this paper. Comparison of the calculated electronic bands structure, density of states diagrams, and dielectric functions for the pure and doped crystal reveals the changes induced by the Mn⁴⁺ impurity ions. In addition, the energy levels of the Mn⁴⁺ ion in the ordered perovskite Ba₂LaNbO₆ are calculated by the exchange charge model (ECM) of crystal field theory and compared with the experimental data that has been presented in the literature. The calculated Mn⁴⁺ energy levels are in good agreement with the experimental spectra. Additionally, the excitation band shapes of the ⁴A_2g(⁴F)–⁴T_2g(⁴F) and ⁴A_2g(⁴F)–⁴T_1g(⁴F) transitions are modeled to estimate the zero-phonon lines (ZPL) positions and the effective number of phonons, which are involved in the corresponding excitation transition. The results of our calculations yield the crystal field parameter of Dq=1780 cm^?1 and Racah parameters B=670 and C=3290 cm^?1, with C/B=4.9 for the Mn⁴⁺ ion in the double-perovskite Ba₂LaNbO₆.     

Infrared laser magnetic resonance detection of SD at 1885·76 cm-1

The visible and near infrared luminescence spectra of MoCl₆ ^3- in Cs₂NaMCl₆ (M=Sc, Y, In) and MoBr₆ ^3- in Cs₂NaYBr₆ have been measured between 15 000 cm^-1 and 3000 cm^-1 at liquid helium temperatures. Comparison with new electronic absorption, infrared and Raman spectra have enabled five luminescence transitions between the states derived from the t _2g ³ configuration to be assigned unambiguously. Each electronic transition shows extensive vibronic structure which can be analysed to yield the vibrational frequencies of the MoX ₆ ^3- ion in each state. The spectra are strongly influenced by resonant interactions between the MoX ₆ ^3- ion and the internal and lattice modes of the host lattices and there is an enormous variation in the intensities of the vibronic origins.     

Optical absorption spectrum of Cr3+ ion doped in zinc cesium sulphate hexahydrate

Optical absorption spectrum of Cr³⁺ ion doped in zinc cesium sulphate hexahydrate single crystal has been studied both at room and liquid nitrogen temperatures. From the nature and position of the bands a successful interpretation of all the bands could be made assuming octahedral symmetry for the Cr³⁺ ion in the crystal. The observed bands are assigned to the transitions from the ground ⁴A_2g(F) state to the excited ²E_g(G), ²T_1g(G), ⁴T_2g(F) and ⁴T_1g(F) states.The crystal field parameters Dq = 1735 cm^?1, B = 635 cm^?1 and C = 4.75 B are found to give a good fit to the observed band positions.     

Radiative properties of α-ZnAl2S4:V spinel type single crystals

The absorption and luminescent properties of α-ZnAI₂S₄:V spinel type crystals in the temperature range 10-300 K are investigated. The spectra are assigned to the electronic transitions of trivalent vanadium ions located in octahedral sites. It is shown that at low temperatures the three main components of the revealed IR luminescence spectra are caused by the ¹A_1g(¹G)→¹E_g(¹D), ¹T_2g(¹D), ³T_2g(³F)→³T_1g(³F), and ¹E_g(¹D)→³T_1g(³F) transitions. The observed dependencies of the emission components intensities on temperature are explained assuming that there is a phonon assisted tunnelling between ³T_2g(³F) and ¹E_g(¹D) states. On temperature rise, the ³T_2g(³F)→³T_1g(³F) vibronic transitions suppress other emission channels, which leads to the enhancement of the integral luminescence intensity and to the broadening of the spectrum centred at λ=1.4 μm.     

Electronic and vibrational absorption spectra in falcondoite

Optical absorption spectrum of Ni²⁺ ion in Falcondoite, a new mineral has been studied at 300 K. From the nature and positions of the observed bands a succesful interpretation of all the bands could be made assuming octahedral symmetry for the Ni²⁺ ion in the crystal. The bands at 9255, 15 380 and 27 390cm⁻¹ are assigned to ³T_2g(F), ³T_1g and ³T_1g(P) and the other band at 24385 cm⁻¹ assigned to ¹T_2g(D). The crystal field and the Racah parameters are evaluated to be Dq = 925 cm⁻¹, B = 1000 cm⁻¹ and C = 4095 cm⁻¹. NIR and IR spectra of the sample are also studied. The fundamental vibrational modes of H₂O are identified in the IR spectrum. The bands observed in the NIR spectrum are due to overtones and combination tones of water molecule.     

Indirect measurements of the Jahn-Teller quenched spin-orbit splitting in the 5T2g-state of KMgF3:Fe2+

The energy separation of the first excited spin-orbit States Γ_3g, Γ_4g from the Γ_5g ground state, in the orbital ⁵T_2g triplet state of Fe²⁺ in KMgF₃, has been estimated from temperature dependence measurements on the 7860 cm^-1 zero-phonon-line (ZPL) transition from Γ_5g to the orbital ⁵E_g doublet state. Using a simplified crystal field energy level model, we find the Γ_5g Γ_3g, Γ_4g separation to be ～30 cm^-1, indicating that the vibronic Jahn-Teller coupling is considerably stronger in KMgF₃:Fe²⁺ than in MgO:Fe²⁺. Far infrared absorption data on KMgF₃:Fe²⁺ in magnetic fields up to 6T, are found to be consistent with this interpretation.     

Optical properties of Mn-activated Na2SnF6 and Cs2SnF6 red phosphors

Alkaline hexafluorostantanate red phosphors Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺ are synthesized by chemical reaction in HF/NaMnO₄ (CsMnO₄)/H₂O₂/H₂O mixed solutions immersed with tin metal. X-ray diffraction patterns suggest that the synthesized phosphors have a tetragonal symmetry with the space group D_4h¹⁴ (Na₂SnF₆:Mn⁴⁺) and a trigonal symmetry with the space group D_3d³ (Cs₂SnF₆:Mn⁴⁺). Photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and the Raman scattering techniques are used to investigate the optical properties of the phosphors. The Franck-Condon analysis of the PLE data yields the Mn⁴⁺-related optical transitions to occur at ∼2.39 and ∼2.38 eV (⁴A_2g→⁴T_2g) and at ∼2.83 and ∼2.76 eV (⁴A_2g→⁴T_1g) for Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺, respectively. The crystal field parameters (Dq) of the Mn⁴⁺ ions in the Na₂SnF₆ and Cs₂SnF₆ hosts are determined to be ∼1930 and ∼1920 cm⁻¹, respectively. Temperature-dependent PL measurements are performed from 20 to 440 K in steps of 10 K, and the obtained results are interpreted by taking into account the Bose-Einstein occupation factor. Comprehensive discussion is given on the phosphorescent properties of a family of Mn⁴⁺-activated alkaline hexafluoride salts.     

The excitonic absorption spectrum of thin Ag2ZnI4 films

The electron absorption spectrum of thin films of the Ag₂ZnI₄ complex compound is studied at photon energies of 3–6 eV. It is established that the interband absorption edge corresponds to an allowed direct transitions across the energy gap E _g=3.7 eV. A strong exciton band is adjacent to the absorption edge at E _ex=3.625 eV (80 K); in the 80–390 K range, the temperature behavior of the half-width Γ of this band is determined by the exciton-phonon interaction typical of quasi-one-dimensional excitons. At T≤390 K, a discontinuity in the slope of the E _ex(T) and Γ(T) dependences is observed. This discontinuity is associated with the generation of Frenkel defects and is accompanied by the transfer of Ag ions to the interstitial sites and vacancies of the crystal lattice of the compound.     

Electron-vibration effects in Cs2SiF6 : Mn4+

We report a study of the ⁴ A _2g →² T _1g absorption band of Mn⁴⁺ in Cs₂SiF₆. The band shows several lines or groups of lines associated with transitions from the ⁴ A _2g ground state to the spin-orbit components (² T _1g )Γ₈ and (² T _1g )Γ₆ coupled to the three odd-parity vibrations v ₆(t _2u ), v ₄(t _1u ) and v ₃(t _1u ). The absorptions associated with the (² T _1g )Γ₈ electronic state have structure whereas those associated with the (² T _1g )Γ₆ do not. It is shown that the structure is a consequence of splitting of the Γ₈ × v vibronic multiplets by electron-vibration interaction. The intensity of the ⁴ A _2g →(² T _1g )Γ _i + v_j vibronic transitions are expressed in terms of a small number of parameters; two parameters for v(t _1u ) modes and three for v(t _2u ) modes. Plausible but not good fits to the low temperature Zeeman data and vibronic splitting patterns are obtained. The excitation spectrum of the Cs₂SiF₆ : Mn⁴⁺ in the region of the ⁴ A _2g →² E_g and ⁴ A _2g →² T _1g is recorded using a c.w. dye laser. This reveals numerous weaker lines involving combinational modes and even-parity modes v₅ (t _2g ), v ₂(e_g ) and v ₁(a _1g ). Several interesting electron-vibrational effects are observed. These are illustrated and discussed qualitatively.     

Luminescence and crystal field analysis of Eu in Eu[Co(CN)6]·4H2O

The vibrational spectra of Eu[Co(CN)₆]·4H₂O and luminescence spectra of Eu³⁺ in this compound, using 355 nm excitation at temperatures down to 10 K, have been assigned. A clear distinction is made between the n=5 and 4 members of the Ln[M(CN)₆]·nH₂O series from the vibrational spectra. The electronic spectra show prominent vibronic structures, particularly for the ⁵D₀→⁷F₂ sideband. A resonance occurs between the transitions ⁵D₀→⁷F₁(III) and ⁵D₀→⁷F₀+ν(Eu−N). A crystal field analysis of the derived energy data set is presented for Eu³⁺ in eight coordination geometry.     

Energy levels of terbium(III) in the elpasolite Cs2NaTbBr6. II. A correlation crystal field analysis

A set of more than 100 electronic energy levels in Cs₂NaTbBr₆, extending from the ground state to ⁵H₄, is used to test different models of the correlation crystal field (CCF). These are based on Judd's orthogonal g_iQ ^(k) two-electron operators, and more specifically on contributions due to spin-correlation, or ligand polarization. Similar data from Cs₂NaTbCl₆ and Cs₂NaTbF₆ has also been analysed. Only fourth-rank operators make clear improvements to the quality of the fit in deviant multiplets. Empirically the g₇ ⁽⁴⁾ and g₉ ⁽⁴⁾ operators are found to be the most effective. Although fourth-rank operators achieve modest success in correcting the calculated spread of the multiplets, no single operator has a significant impact on the shortcomings of the one-body crystal field. This result is discussed in terms of the limitations of an effective-operator Hamiltonian.     

Crystal-field hamiltonian and spin-orbit interaction for d and d ions at low C3 symmetry sites: V:Al2O3 and Ni:LiNbO3

The experimental studies have provided evidence of the occurrence of transitions from the ³T_1g(³F) ground state to the crystal-field levels ³T_2g(³F), ³T_1g(³P) and ³A_2g(³F) for the V³⁺ centres in Al₂O₃ crystal; and from the ³A_2g(³F) ground state to the crystal-field levels ³T_2g(³F), ³T_1g(³F) and ³T_1g(³P) for the Ni²⁺ centres in LiNbO₃ crystal (levels are assigned to irreps of the O_h point symmetry group). Using the experimental spectroscopic data, theoretical calculations of the crystal-field levels of V³⁺:Al₂O₃ and Ni²⁺:LiNbO₃ are carried out based on the Racah theory. The observed crystalline-field splittings of the V³⁺ and Ni²⁺ terms were accounted for using a C₃ symmetry Hamiltonian. The spin-orbit interaction was taken into account in this work. The Racah, crystal-field and spin-orbit parameters, which fit experimental and theoretical energy levels, have been reliably obtained. A good agreement between the theoretical and experimental results for the energy levels of V³⁺:Al₂O₃ and Ni²⁺:LiNbO₃ has been obtained.     

Electronic Absorption Spectrum of Fe3+ Doped in Ammonium Perchlorate Single Crystal

Abstract

Single crystals of Iron - doped ammonium perchlorate were grown at room temperature. The electronic absorption bands observed at room and liquid air temperatures have been assigned transitions from the ground ⁶A_1g state to the excited ⁴A_1g, ⁴E_g, ⁴T_1g and ⁴T_2g states. The crystal field parameters Dq = 870 cm^?1, B = 615 cm^?1 and C = 4.2 B are found to give a good fit to the observed band positions.     

Electronic absorption spectrum of Ni2+ in lithium potassium sulphate single crystal

Single crystals of nickel-doped lithium potassium sulphate were grown by slow evaporation method at room temperature. From the nature and position of the bands observed, a successful interpretation of all the bands could be made assumingO _h symmetry for the Ni²⁺ ion in the crystal. The bands have been assigned transitions from the ground³A_2g(F) state to the excited³T_2g(F),¹E_g(D),³T_1g(F),¹T_2g(D) and³T_1g(P) states. The crystal field parameters derived areDq=910cm^–1,B=890cm^–1 andC=3560cm^–1.The authors wish to express their thanks to Prof. K. Sreerama Murthy for his constant encouragement throughout this investigation. The authors are also thankful to Prof. Mihir Chowdhury, Indian Association for the cultivation of Science, Calcutta for giving permission to take the spectra.