Mikhail G. Brik

A low-temperature co-precipitation approach to synthesize fluoride phosphors K2MF6:Mn4+ (M = Ge, Si) for white LED applications

A new class of Mn4+ activated alkali-metal hexafluoride red phosphors are emerging for white light-emitting diodes because of their sharp red line 2Eg → 4A2g emissions (600–650 nm) excited by irradiation of 4A2g → 4T1g (320–380 nm) and 4A2g → 4T2g (380–500 nm) transitions. However, these phosphors have the drawbacks of difficult control of the Mn valence state during synthesis and lack of underlying mechanisms for structure–photoluminescence relationships. In this study, we explore a novel, highly productive route to the quantifiable synthesis of K2GeF6:Mn4+ by the chemical co-precipitation method at room temperature. The prepared yellowish K2GeF6:Mn4+ powders exhibit a hexagonal shape and high crystallinity without significant defects. The photoluminescence thermal stability and white light-emitting diodes applicability of K2GeF6:Mn4+ suggest that it is a promising commercial red phosphor because of its efficient emission intensity, high color purity and excellent thermal stability. Structural analyses and theoretical calculations reveal that the red shift of the K2GeF6:Mn4+ red phosphor compared with K2SiF6:Mn4+ is due to the longer Ge–F distance and lower effective Mulliken charge of F ions in coordination environments of the MnF62− octahedron. The split feature in K2GeF6:Mn4+ is due to the hexagonal distortion in the host. The structure–photoluminescence mechanism is predicted to be general in hexafluoride red phosphors to tune the optical properties through cationic substitutions and crystal structure adjustments.

Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White Light-Emitting Diodes

Red phosphors AMF6:Mn(4+) (A = Na, K, Cs, Ba, Rb; M = Si, Ti, Ge) have been widely studied due to the narrow red emission bands around 630 nm. The different emission of the zero-phonon line (ZPL) may affect the color rendering index of white light-emitting diodes (WLED). The primary reason behind the emergence and intensity of ZPL, taking KNaSiF6:Mn(4+) as an example, was investigated here. The effects of pressure on crystal structure and luminescence were determined experimentally and theoretically. The increase of band gap, red shift of emission spectrum and blue shift of excitation spectrum were observed with higher applied pressure. The angles of ∠FMnF and ∠FMF(M = Si, Ti, Ge) were found clearly distorted from 180° in MF6(2-) octahedron with strong ZPL intensity. The larger distorted SiF6(2-) octahedron, the stronger ZPL intensity. This research provides a new perspective to address the ZPL intensity problem of the hexafluorosilicate phosphors caused by crystal distortion and pressure-dependence of the luminescence. The efficacy of the device featuring from Y3Al5O12:Ce(3+) (YAG) and KNaSiF6:Mn(4+) phosphor was 118 lm/W with the color temperature of 3455 K. These results reveal that KNaSiF6:Mn(4+) presents good luminescent properties and could be a potential candidate material for application in back-lighting systems.

Crystal field analysis of the energy level structure of Cs2NaAlF6:Cr3+

An analysis of the energy level structure of Cr3+ ions in Cs2NaAlF6 crystal is performed using the exchange charge model (ECM) together with the crystal field analysis/microscopic spin Hamiltonian (CFA/MSH) computer package. Utilizing the crystal structure data, our approach enables modelling of the crystal field parameters (CFPs) and thus the energy level structure for Cr3+ ions at the two crystallographically inequivalent sites in Cs2NaAlF6. Using the ECM initial adjustment procedure, the CFPs are calculated in the crystallographic axis system centred at the Cr3+ ion at each site. Additionally the CFPs are also calculated using the superposition model (SPM). The ECM and SPM predicted CFP values match very well. Consideration of the symmetry aspects for the so-obtained CFP datasets reveals that the latter axis system matches the symmetry-adapted axis system related directly to the six Cr?F bonds well. Using the ECM predicted CFPs as an input for the CFA/MSH package, the complete energy level schemes are calculated for Cr3+ ions at the two sites. Comparison of the theoretical results with the experimental spectroscopic data yields satisfactory agreement. Our results confirm that the actual symmetry at both impurity sites I and II in the Cs2NaAlF6:Cr3+ system is trigonal D3d. The ECM predicted CFPs may be used as the initial (starting) parameters for simulations and fittings of the energy levels for Cr3+ ions in structurally similar hosts.

Optical properties of 3d-ions in crystals : spectroscopy and crystal field analysis

Recent developments in laser crystals doped with 3d-ions.- Spectral properties of cobalt complexes within the ZnSe crystals as new absorbers for the non-linear optical devices.- Exchange charge model of crystal field for 3d ions.- Superposition Model and its Applications.- First-principles calculations of absorption spectra of 3d ions in laser crystals.- Vibronic transitions in spectra of 3d ions in crystals.- Jahn-Teller effect for the 3d3 (3d7) ions in the octahedral (tetrahedral) impurity centers.- Dynamical Jahn-Teller effect in crystals doped with 3d ions.- Spin-Hamiltonian parameters and lattice distortions around 3dn impurities.

Facile synthesis, morphology and photoluminescence of a novel red fluoride nanophosphor K2NaAlF6:Mn4+

A novel red-emitting fluoride phosphor, K2NaAlF6:Mn4+, with nanoscale particle size was synthesized via a cation exchange route. This phosphor absorbs UV and blue light and emits red light at about 630 nm; thus, it has been regarded as an ideal red phosphor for WLEDs. The reaction parameters were systematically investigated and an optimized sample was obtained. K2NaAlF6:Mn4+ showed better photoluminescence properties and its emission wavelength was blue shifted with respect to that of K2LiAlF6:Mn4+ due to differences in the neighbouring alkali metal ions and different distances between the cation and the ligands. These results are in agreement with the trend determined by the recently introduced parameter β1, which describes the energy of the 2Eg → 4A2g transition as a function of the nephelauxetic effect. Finally, these results could further contribute to the optimization of red-emitting phosphors based on Mn4+ ions.

Lattice Parameters and Stability of the Spinel Compounds in Relation to the Ionic Radii and Electronegativities of Constituting Chemical Elements

A thorough consideration of the relation between the lattice parameters of 185 binary and ternary spinel compounds, on one side, and ionic radii and electronegativities of the constituting ions, on the other side, allowed for establishing a simple empirical model and finding its linear equation, which links together the above-mentioned quantities. The derived equation gives good agreement between the experimental and modeled values of the lattice parameters in the considered group of spinels, with an average relative error of about 1% only. The proposed model was improved further by separate consideration of several groups of spinels, depending on the nature of the anion (oxygen, sulfur, selenium/tellurium, nitrogen). The developed approach can be efficiently used for prediction of lattice constants for new isostructural materials. In particular, the lattice constants of new hypothetic spinels ZnRE2O4, CdRE2S4, CdRE2Se4 (RE = rare earth elements) are predicted in the present Article. In addition, the upper and lower limits for the variation of the ionic radii, electronegativities, and their certain combinations were established, which can be considered as stability criteria for the spinel compounds. The findings of the present Article offer a systematic overview of the structural properties of spinels and can serve as helpful guides for synthesis of new spinel compounds.

Photoluminescence of Pr3+-, Dy3+- and Tm3+-doped transparent nanocrystallized KNbGeO5 glasses

In this paper, we present the photoluminescence properties of Pr3+-, Dy3+- and Tm3+-doped potassium?niobium?germanate glasses and glass ceramics. From the x-ray diffraction measurement, the glass structure was established. These glasses have shown strong absorption bands in the near-infrared region. Compared with Pr3+-, Dy3+- and Tm3+-doped glasses, their respective glass ceramics have shown stronger emissions due to the presence of the K3.8Nb5Ge3O20.4 crystalline phase. For Pr3+ -doped glass, two weak emission bands centred at 616?nm(3P0 ? 3H6) and 648?nm(3P0 ? 3F2) and for glass ceramic strong emission bands centred at 533?nm(3P0 ? 3H5), 616?nm(3P0 ? 3H6) and 648?nm(3P0 ? 3F2) along with weak emissions at 688?nm(3P1 ? 3F3), 709?nm(3P0 ? 3F3) and 732?nm(3P0 ? 3F4) have been observed with 450?nm(3H4 ? 3P2) excitation wavelength. Among them, 648?nm(3P0 ? 3F2) has shown bright red emission. For the Pr3+ -doped glass and glass ceramic, upconversion emission spectra have also been measured. Bright blue upconversion luminescence was observed under 1D2 level excitation. With regard to Dy3+?:?glass, a weak emission band centred at 576?nm(4F9/2 ? 6H13/2) and for glass ceramic a blue emission band centred at 485?nm(4F9/2 ? 6H15/2) and a bright fluorescent yellow emission at 576?nm(4F9/2 ? 6H13/2) have been observed, apart from 4F9/2 ? 6H11/2(664?nm) emission transition with an excitation at 389?nm(6H15/2 ? 4I13/2, 4F7/2) wavelength. Emission bands of 1G4 ? 3F4(650?nm), 3F3 ? 3H6(707?nm) and 1G4 ? 3H5(774?nm) transitions have been observed for the Tm3+?:?glass and glass ceramic, with an excitation at 3H6 ? 1G4(473?nm). Among them, the 1G4 ? 3F4 transition (650?nm) shows bright red emission. The stimulated emission cross-sections of all the emission bands of Pr3+, Dy3+ and Tm3+?:?glasses and glass ceramics have been computed based on their measured full width at half maximum (??) and lifetimes (?m).

Optical Spectroscopy and Crystal Field Studies of the Mn4+ Ion (3d3) in the Double Perovskite NaLaMgTeO6

The spectroscopic properties of the Mn4+ ion (3d3) in the double perovskite NaLaMgTeO6 are reported in this work. Evidence is presented for the occupation by the Mn4+ ion of both the six coordinated Mg2+ and Te6+ sites in the host structure. The Mn4+ energy levels are calculated using the exchange charge model of crystal field theory for both occupied sites. The results of our calculations yield the crystal field splitting and Racah parameters of Dq = 2008 cm−1, B = 790 cm−1, C =2881 cm−1, with C/B=3:65 (Mg2+ site) and Dq=2008 cm−1, B=790 cm−1,C =2949 cm−1, with C/B = 3:73 (Te6+ site). A cross-cutting comparative study of the variations in the crystal field splitting and the Racah parameters of the six-coordinated Mn4+ ion in a series of materials with the perovskite structure are presented. Graphical Abstract Optical Spectroscopy and Crystal Field Studies of the Mn4+ Ion (3d3) in the Double Perovskite NaLaMgTeO6

Calculations of complete 4f(n) and 4f(n-1)5d(1) energy level schemes of free trivalent rare-earth ions

Calculations of complete energy level schemes for 4fn and 4fn-1 5d1 electronic configurations of free trivalent rare-earth ions have been performed. Semi-empirical Hamiltonian was used for the calculation of 4fn configuration while a fully relativistic first-principles many-electron method was used for the calculation including 4fn-1 5d1 configuration. As a result, Diekes diagram is extended to about 200000 cm-1 in the case of 4fn electronic configurations and about 370000 cm-1 in the case of 4fn-1 5d1 electronic configurations. Calculated energy levels are plotted as a function of the atomic number.

Mn2+ and Mn4+ red phosphors: synthesis, luminescence and applications in WLEDs. A review

Transition-metal activated phosphors are an important family of luminescent materials that can produce white light with an outstanding color rendering index and correlated color temperature for use in light-emitting diodes. In recent years, work in this quite “hot” research field has focused on the development of Mn2+ and Mn4+ activated red phosphors. In this review article, we provide an overview of recent studies on Mn2+ and Mn4+ doped phosphors, including detailed synthesis routes (solid-state reaction and wet-chemical synthesis) and description of luminescence mechanisms and phosphors’ behaviors; discuss their promising applications in white light-emitting diodes; and present an extensive list of references to representative works in this field.