Racah Parameters Research Articles

Analysis of crystal-field effects on the energy levels of Mn4+ ions doped in different photoluminescent host lattices, exhibiting lowering of site symmetry: Exchange charge and superposition model calculations, for potential applications

Herein, we presented a comparative analysis of crystal-field (CF) effects on the energy levels of Mn4+ ions doped in the various host lattices exhibiting site symmetry from: triclinic: Y2MgTiO6, La2MgTiO6, Ca2YTaO6, Ca2YSbO6; trigonal: Ba2LaSbO6, LaTiSbO6; tetragonal: Ba2GdTaO6; to octahedral: Li2MgTiO4, Ba2GdSbO6, Ba2YSbO6. For modelling CF parameters (CFPs), we employed exchange charge model (ECM) using one model parameter (G), and superposition model (SPM) using four model parameters. A modified crystallographic axis system (CAS*) was adopted in ECM and SPM calculations for low symmetry hosts. The modelled CFPs served as input for crystal-field analysis/microscopic spin-Hamiltonian program to calculate the Mn4+ energy levels and match them with photoluminescence spectra of Mn4+-doped crystals. The 2nd-rank ECM/CFPs vary with G depending on distortions of the metal (M)-ligand (L) MLn polyhedron. The 4th-rank ECM/CFPs depend predominantly on the exchange charge effects. The ECM yields seemingly a better agreement between the observed and calculated energies than the SPM one. The disparity between the CFP sets calculated using SPM and ECM approaches in CAS*, while yielding same CFP strength, poses dilemmas. To solve these dilemmas, we employed the triclinic standardization of CFP sets, thus uncovering intricate low symmetry aspects inherent in ECM/CFPs, hitherto not realized. This method proved very useful for meaningful analysis and interpretation of CFP sets. Our CFP modelling illustrates importance of well-defined axis systems and adequate treatment of the actual triclinic (and monoclinic) site symmetry of dopant ions, instead of octahedral symmetry (usually adopted by experimentalists), in modelling of CFPs for triclinic symmetry cases. The relations between the Racah parameters, B and C, and 2Eg → 4A2g peak emission energy of Mn4+ ions are extended and suitably modified for triclinic symmetry. These modifications may be significant in engineering the local CF environment by suitably choosing the host crystals and dopant ions to improve the luminescence properties of phosphors for practical applications. The influence of dopant metal ions on the local structure and thus spectroscopic properties of doped crystals was also explored.

Luminescence properties and phase transformation of broadband NIR emitting A2(WO4)3:Cr3+ (A=Al3+, Sc3+) phosphors toward NIR spectroscopy applications

The synthesis, structural, and luminescence properties have been carried out for Cr3+-activated Al2(WO4)3 (AWO) and Sc2(WO4)3 (SWO) phosphors for application in pc-NIR LED. Upon blue excitation, these compounds are capable of exhibiting broadband NIR emission stems primarily from 4T2→4A2 transition in the range of 670–1200 nm (maxima ∼808 nm, FWHM ∼140 nm) for AWO:Cr and of 700–1300 nm (maxima ∼870 nm, FWHM ∼164 nm) for SWO:Cr. The significant shift of NIR emission is attributed to the substitution of AlO6 with larger ScO6 octahedrons. To gain insight into the luminescence the crystal field strength, Racah parameters, nephelauxetic effect, and electron-phonon coupling have been analyzed based on spectroscopic results. The electron-phonon coupling parameter S for SWO:Cr was determined to be 11.5, twice as large as that for AWO:Cr, which is in accordance with its strong thermal quenching. The abrupt changes occurring at 275 K in temperature-dependent luminescence spectra and decay lifetime of AWO:Cr is associated with temperature-driven phase transformation from low-temperature monoclinic to high-temperature orthorhombic phase. Pressure induced amorphization of AWO:Cr at pressures higher than 25 kbar was confirmed by employing high pressure evolution of Raman spectra. A high-power NIR pc-LED, fabricated by coating AWO:0.04Cr on a commercial 470 nm LED chip, shows good performance with an output power of 17.1 mW driven by a current of 320 mA, revealing potential application of studied materials for NIR light source.

The d-d transitions and ligand field parameters for Cr3+/Co2+ doped (Mg, Zn)Al2O4: Multi-reference Ab initio investigations

The normal spinels (Mg, Zn)Al2O4 doped with transition metal (TM) ions Cr3+/Co2+ are versatile materials with important electronic, optical and spectral properties. In addition to being used in many applications, they are excellent systems for testing some models and simulation features. The aim of this paper is to present, in the unified frame, the results on d-d transitions and ligand field parameters (LFPs) for the title systems, based on ab initio calculations, combining periodic density functional theory (DFT) supercell approach with ab initio (AI) multi -reference perturbation theory (MRPT) and multi-reference configuration interaction (MRCI) methods. These AI methods, based on complete active space self-consistent-field (CASSCF) reference, allow to calculate and investigate the energy levels of TM ions and the d-d transitions between them. From the AI results the B and C Racah parameters, the spin-orbit coupling (SOC) constant and the LFPs in the frame of the angular overlap model (AOM) were accurately extracted with ab initio ligand field theory (AILFT) protocol, all with subsequent comparison with the experimental data or existing theoretical results in the literature. The calculation technique presented in this paper serves as a predictive formalism for further studies of larger monomer clusters, for which experimental data is unreliable or unavailable.

Mn5+-activated Ca6Ba(PO4)4O near-infrared phosphor and its application in luminescence thermometry

The near-infrared luminescence of Ca6Ba(PO4)4O:Mn5+ is demonstrated and explained. When excited into the broad and strong absorption band that spans the 500–1000 nm spectral range, this phosphor provides an ultranarrow (FWHM = 5 nm) emission centered at 1140 nm that originates from a spin-forbidden 1E → 3A2 transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 μs. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe–Sugano diagram for this phosphor. We found that 1E emission quenches due to the thermally-assisted cross-over with the 3T2 state and that the relatively high Debye temperature of 783 K of Ca6Ba(PO4)4O facilitates efficient emission. Since Ca6Ba(PO4)4O also provides efficient yellow emission of the Eu2+ dopant, we calculated and explained its electronic band structure, the partial and total density of states, effective Mulliken charges of all ions, elastic constants, Debye temperature, and vibrational spectra. Finally, we demonstrated the application of phosphor in a luminescence intensity ratio thermometry and obtained a relative sensitivity of 1.92%K−1 and a temperature resolution of 0.2 K in the range of physiological temperatures.

Systemic development of Mn4+-activated double perovskite red phosphors for plant growth applications

Developing efficient and stable red phosphor has always been considered as important academic research of LED plant-growth lamps. In this study, Mn4+-activated double perovskite red phosphors, A2BB’O6:Mn4+ (A = Sr, Ba; B = Lu, Gd; B’ = Nb, Ta), were successfully synthesized by solid-phase method. To elucidate the structure-activity relationship between the double perovskite structure and the luminescence of Mn4+, the crystal structure, electronic structure and luminescence properties of Sr2LuNbO6:Mn4+ were first studied in detail using X-ray powder diffraction data, density functional theory calculations and spectroscopy data. The coordination environment of Mn4+ was analyzed by investigating the crystal-field parameters, Racah parameters and nephelauxetic parameter. Then, the effect of structural evolution on the luminescence of Mn4+ was investigated by regulating the local coordination environment of Mn4+ through cationic modification. The results demonstrate that Mn4+ enters the octahedral lattice site with a strong crystal field and covalent environment, which can be excited by a near-ultraviolet LED chip to emit narrow-band far-red light in the double perovskite oxides. Moreover, the Mn–O bond length, octahedral distortion and crystal-system type all affect the luminescence of Mn4+. This systemic development provides some guidance for the exploration of novel Mn4+-activated double perovskite red phosphors.

Zinc borosilicate glasses doped with Co2+ ions: Synthesis and optical properties

New promising optical glasses containing cobalt ions in tetrahedral coordination have been synthesized, their mechanical and optical properties, as well as their characteristic temperatures, have been studied. High-quality glasses of the composition (70-x)ZnO–10SiO2-(20 + x)B2O3 (x = 0; 5; 10 wt%) doped with 0.1, 0.3, and 0.6 wt% CoO have been synthesized for the first time. The selected glass compositions were close to the eutectic one in the ZnO–SiO–B2O3 system. This made it possible to reduce the glass synthesis temperature from the 1350–1500 °C, described in the literature for similar zinc borosilicate glasses, to the 1170 °C. The presence of tetrahedral cobalt ions (CoO4) was identified by the absorption bands at 17000 cm−1 (4A2(4F) → 4T1(4P)) and at 6954 cm−1 (4A2(4F) → 4T1(4F)). Then the intense absorption bands in the visible region have been attributed to the 4A2(4F) → 2A1(2G), 4A2(4F) → 4T1(4P), 4A2(4F) → 2T2(2G), 4A2(4F) → 2T1(2G) and 4A2(4F) → 2E(2G) transitions. The most intense absorption band of octahedral cobalt ions (CoO6) corresponds to the 4T1(F) → 4T1(P) transition and is located in the region of 18000–20000 cm−1. According to analysis of Co2+ absorption spectra, the tetrahedral Co2+ ions concentration increased with increasing the CoO concentration in glasses. To study the nature of the bond between cobalt ion and ligands, the values of the splitting force of the ligand field (10Dq) and Racah parameter B were determined.

Fe2O3 within Na2O–Al2O3– B2O3 glasses to study the structural and optical features changes

This paper aims to study the structural and optical properties of the glass system 75B2O3 + 5Al2O3 + (20-x)Na2O + (x)Fe2O3; (x = 0, 1, 3 and 5 mol. %). The melt quenching approach was utilized to prepare all the glass samples. The Fourier transform infrared (FTIR) results confirmed the back conversion of the BO4 units into BO3 units with a steady increase in nonbridging oxygen content. The absorbance of all the samples was determined in wavelength range (300–650 nm). The overlap between the real absorption edge and an absorption band at about 370 nm produces a pseudo absorption edge accompanied by an error in calculating the optical band gap. To overcome this error, a deconvolution process is performed. The bands from 417 to 469 nm confirm the presence of trivalent iron cations (Fe3+) within the glass matrix. These Fe3+ cations have a distorted octahedral symmetry originating from d-d transitions. The densities (ρemp and ρexp), molar volume (Vm) optical band gap (EOpt), refractive index (n0) and nonlinear refractive index (n2) were determined. The ρemp, ρexp,Vm, n0 and n2 of the prepared glasses show an increasing behavior, with further Fe2O3 doping, while EOpt shows a decreasing one. Further, the addition of Fe2O3 produced an increment in Racah parameters (B and C) values, which in turn reflects the more stability of the bonds between Fe cations and their complexes.

Eu3+ ions doped zinc borotellurite glass system for white light laser application: Structural, physical, optical properties, and Judd-Ofelt theory

The melt-quenching method was used to fabricate zinc borotellurite glasses of composition [{(TeO2)0.7 (B2O3)0.3}0.7 (ZnO)0.3](1-y) (Eu2O3)y, with 0.01 = y ≤ 0.05 M fraction. The calculated density and molar volume values lies between 4.74 and 5.15 g/cm3, and 24.58 and 25.61 cm3/mol. The optical properties were analysed and discussed. The estimated optical band gap (direct and indirect) energies decrease from 3.49 to 3.05 eV and 3.11 to 2.81 eV, whereas the Urbach energy and reflective index increase from 0.262 to 0.334 eV and 2.368 to 2.450 with Eu2O3 content. The evaluated Racah parameters (B and C) ranges from 92.60 to 119.86 cm−1 and from 626.64 to 643.35cm−1, respectively. The calculated Judd-Ofelt Ω2 parameter increases from (2.78 to 3.93)×10−20cm2. Furthermore, the gain bandwidth and optical gain value were found between (14.295 to20.340)×10−28cm2 and (12.576 to 15.693)×10−25cm2, respectively, for the 5D2 → 7F2 transition. In addition, stimulated emission cross-section and branching ratio values of 14.029×10−22cm2 and 66.54% were achieved from TBZEu0.04 prepared glass, indicating a promising potential for white-light lasing application.

Experimental and optical studies of the new organic inorganic bromide: [(C3H7)4N]2CoBr4

The alkylammonium halogenocobalate families are subjected to diverse studies according to their wide field application. In this work, the [(C3H7)4N]2CoBr4 compound was synthesized using a slow evaporation solution growth method at room temperature and characterized by X-ray powder diffraction (XRPD) and UV-VIS-IR absorption spectrometry. The X-ray powder diffraction (XRPD) data confirmed the formation of a single-phase with monoclinic system (C2/c space group). The optical absorption and reflectance spectra of the named compound were measured at the range [240 nm–1900 nm] at room temperature. Using three methods, we have determined the direct optical band gap estimated at 3.8 eV and the Urbach energy. Different models were used to calculate the refractive index, which was found to be in the range of 2.12–2.58. This spectrum consists also of electronic transition d-d of Co2+ (3 d7) ions. This work examines a detailed crystal-field analysis of the UV–Vis–NIR lines associated to the transitions between the Stark levels of Co2+, occupying a Td site symmetry in [CoBr4]2- (organic part). The theoretical study is based on the Racah tensor algebra methods. This analysis permits us to determine the electronic structure of the cobalt ion Co2+ based on the crystal field theory. As a result, Racah and crystal-field parameters have been reliably obtained. A good agreement between the theoretical and the experimental energy levels are obtained. This theoretical study confirms the Td site symmetry of Co2+ in [CoBr4]2- and leads to a satisfactory correlation between experimental and calculated energy levels. In addition to internal transitions, the absorption spectrum shows vibrational peaks witness of anharmonic behavior.

Full understanding of K3GaF6:Cr3+ properties under hydrostatic pressure based on the Tanabe−Sugano energy-level diagram

The aim of this study is to make clear the photoluminescence (PL) properties of Cr3+-activated phosphors under the application of hydrostatic pressure (p). A determination of the excited-state zero-phonon line (ZPL) energies in the 3d3-configuration ions is possible from a set of Racah (B and C) and crystal-field splitting parameters (Dq), and vice versa. Detailed analyses of PL and PL excitation spectra are demonstrated to make possible exact determination of the excited-state ZPL energies, E (2Eg)ZPL and E (4T2g)ZPL, as a function of p. Inputting such pressure-dependent ZPL energies into a newly proposed set of Racah parameter expressions, it is possible to obtain reliable metal−ligand field parameters for 3d3 activator ions in red-emitting phosphors. An example of the analysis method is given on the Cr3+-activated K3GaF6 fluoride phosphor and suggests a change in its phosphor nature from type F–Cr-A [E (2Eg)ZPL > E (4T2g)ZPL] to type F–Cr–B [E (2Eg)ZPL < E (4T2g)ZPL]. The present analysis method can be used to obtain broad perspectives and insight for deeply understanding metal−ligand chemistry and related phenomena in the various 3d3 ion-activated phosphors under the application of external perturbation like hydrostatic pressure or lattice temperature.

New fuel governed combustion synthesis and improved luminescence in nanocrystalline Cr3+ doped ZnAl2O4 particles

This paper describes the solution combustion (SCS) synthesis of nanocrystalline Cr3+ doped ZnAl2O4 (ZAC) particles using a new efficient fuel. A thorough investigation was conducted to comprehend the interaction of dopant (Cr3+) and ZnAl2O4 host matrix, as well as the influence of doping on their structural, optical, and luminescence characteristic properties. The powder samples were thoroughly characterized using XRD, UV–Visible Diffuse Reflectance, and Photoluminescence spectroscopic techniques. The XRD analysis demonstrates the formation of polycrystalline nanoparticles, as well as a decrease in crystallite size with increasing Cr3+ concentration. The band gap energy red-shifted as Cr3+ was incorporated into the host matrix. The diffuse reflectance spectra show two absorption peaks at 395 nm and 540 nm are assigned to the 4A2g →4T1g and 4A2g →4T2g characteristic transitions of Cr3+ ions. The PL emission spectrum, on the other hand, shows four distinct peaks, indicating the manifestation of Cr3+ ions in octahedral symmetry, that is, Cr3+ ions in a strong crystal field. Increases in Cr3+ concentration result in luminescence enhancement up to x = 0.07 and suppression of luminescence observed beyond that concentration. In addition, the dopant ion's Racah parameters, crystal field parameters, and local site symmetry are calculated.

Synthesis and Magnetic Properties of Antimony-Ligated Co(II) Complexes: Stibines versus Phosphines

Herein, we test the hypothesis that neutral, heavy-atom stibine donors can increase the extent of spin-orbit coupling on light, 3d transition metal. To this end, we developed a novel synthetic route toward coordinating a paramagnetic 3d metal ion─cobalt(II)─with neutral stibine ligands. Such complexes have not been reported in the literature due to the weak σ donor strength of stibines and the hard-soft mismatch between a 3d metal and a 5p ligand─which herein has been overcome using alkylated Sb donors. Magnetometry of [(SbiPr2Ph)2Co(I)2] (1) reveals that the stibine complex 1 exhibits a higher magnitude D value (D = |24.96| cm-1) than the spectroscopically derived value for the corresponding phosphine complex 3 (D = -13.13 cm-1), indicative of large zero-field splitting. CASSCF/NEVPT2 calculations corroborate the experimental D values for 1 and 3, predicting D = -31.9 and -8.9 cm-1, respectively. A re-examination of magnetic parameters across the entire series [(ER3)2Co(X)2] (E = P → Sb; X = Cl → I) reveals that (i) increasingly heavy pnictogens lead to an increased X-Co-X bond angle, which is correlated with larger magnitude D values, and (ii) for a given X-Co-X bond angle, the D value is always higher in the presence of a heavy pnictogen as compared with a heavy halide. Ab initio ligand field theory calculations for 1 (stibine complex) and 3 (phosphine complex) reveal no substantial differences in spin-orbit coupling (ζ = 479.2, 480.2 cm-1) or Racah parameter (B = 947.5, 943.9 cm-1), an indicator of covalency. Thus, some "heavy atom effect" on the D value beyond geometric perturbation is operative, but its precise mechanism(s) of action remains obscure.

Luminescence Spectroscopy of 3d3 (Mn4+, Cr3+) Ions in Multiple Octahedral-Site Phosphors

Investigating spectroscopic properties of luminescent ions in multiple sites of host materials is not only of scientific interest, but also technological importance. The present investigation focuses on this problem and reports the analysis results of the optical properties of 3d 3 ions in the multiple octahedron sites. The phosphor materials considered are double-perovskite La4Ti3O12:Mn4+ and Ca3−x Lu x Hf2Al2+x Si1−x O12:Cr3+ (x = 0−1.0) phosphors. An analysis method is demonstrated for obtaining reliable and site-dependent crystal-field (Dq) and Racah parameters (B and C) based on a ligand field theory with paying an attention to difficulty in the exact estimation of such important crystal-field-related parameters. For example, the difference in the crystal-field splitting energies between the slightly and highly-distorted octahedron sites determined are ΔDq ∼ 1615 and 405 cm−1 (x = 0) for the [MnO6]8− and [CrO6]9− octahedrons, respectively. All results are in satisfactory agreement with the Tanabe−Sugano energy-level diagram plots, promising good understanding of such 3d 3-ion-activated phosphors with multiple octahedron sites.

Mn4+-activated double perovskite red phosphor for multifunctional applications

Recently, Mn4+ activated red double perovskite red phosphors have gained great attention due to their particular luminescent properties. In this study, a novel far-red phosphor Ba2LaTaO6: Mn4+ was successfully prepared by solid-state reaction. The crystal structure and bandgap structure were analyzed using X-Ray Diffraction (XRD) Rietveld refinement and density functional theory (DFT) calculations. The photoluminescent performance of Ba2LaTaO6: Mn4+ was investigated to evaluate its availability in plant growth LED and temperature sensing. The results indicate that Ba2LaTaO6: Mn4+ can match ultraviolet chips and emit a narrow-band far-red light centering at 690 nm. The emission band of the fabricated LED matches well with the absorption bands of chlorophyll-a and b. In addition, Ba2LaTaO6: Mn4+ has a high temperature sensitive (3.2% K−1 @ 450 K), which is of great value in thermal sensing. The luminescent mechanism was dissected in-depth based on configurational-coordinate model, crystal-field and Racah parameters. It is expected that the research of Ba2LaTaO6: Mn4+ offers insight into developing new functional materials.

Impact of Cr3+ substitution on the nephelauxetic ratio and Racah parameter of Cr-Mn-Zn nanoferrites

For the first time, we present the ligand field splitting (10Dq) nephelauxetic ratio (β), Racah parameter (B) and stability of Cr3+ within Mn–Zn nanoferrite which synthesized using the citrate route. These parameters were determined via optical absorption spectra. The obtained bands in the significance range of optical absorption spectra are wide as well as asymmetric; therefore the deconvolution process becomes a necessary task. The values of 10Dq and B have been estimated from the produced bands. The values of 10Dq are observed to increase from 16366.61 to 16447.37 cm−1 whereas, B values are observed to decrease from 829.81 to 760.80 cm−1 with additional Cr3+ substitution. The increasing 10Dq and decreasing B values, evidence for bonds between Cr3+ ions with their environments became a further covalent (less ionic). Moreover, the values of β are observed to decrease for further Cr3+ substitution. This decrease of β produces less stability between Cr3+ and its ligand. Furthermore, Dq/B values declare that Cr3+ centers are in the low-field sites by a slight tendency towards the high-field sites with further Cr/Fe substitution process. Therefore, it was suggested that Cr3+ substituted Mn–Zn nanoferrites are excellent candidates for diverse optical applications such as a tunable laser.

Characterization of Cuban and Brazilian natural zeolites by photoacoustic spectroscopy and electron paramagnetic resonance.

This report describes the photoacoustic and electron paramagnetic resonance investigations of Brazilian and Cuban zeolites. Photoacoustic optical absorption measurements indicate the presence of iron (Fe3+) ions with their respective transition bands for both zeolites. Two species of manganese (Mn2+ and Mn3+) were identified in the Cuban sample and the electronic transitions assigned. Iron and manganese ions were confirmed through nonradiative relaxation (τ) and characteristic diffusion (τβ) times evaluation, whose values were found to be τBRA = 5.40 ms, τCUB = 4.60 ms, τβBRA = 387 μs and τβCUB = 305 μs. Crystal field (Dq-BRA/Dq-CUB = 1048 cm-1/945 cm-1) plus Racah (B-BRA/B-CUB = 457 cm-1/813 cm-1 and C-BRA/C-CUB = 3655 cm-1/2496 cm-1) parameters were assessed as well. Paramagnetic resonance corroborated Fe3+ ions present in the Brazilian zeolite occupying sites showing axial and/or rhombic symmetry distortions. For the Cuban sample, results reveal the characteristic hyperfine sextet lines of Mn2+ overlapping the Fe3+ line. Values of Landé factor and isotropic hyperfine splitting constant were found to be 2.0 and 9.7 mT, respectively. This tells us that the Mn2+ lies in octahedral symmetry probably replacing calcium ions and point towards an ionic bonding character of the Mn2+ and its surroundings.

Dual role of oxygen-related defects in the luminescence kinetics of AlN:Mn2.

This study presents the impact of temperature and pressure on AlN:Mn2+ luminescence kinetics. Unusual behavior of Mn2+ optical properties during UV excitation is observed, where a strong afterglow luminescence of Mn2+ occurs even at low temperatures. When the temperature increases, the contribution of the afterglow luminescence is further enhanced, causing a significant increase in the luminescence intensity. The observed phenomena may be explained by an energy diagram in which the ON-VAl complex in AlN:Mn2+ plays a key role. Hence the ON-VAl complex defect in AlN:Mn2+ plays a double role. When the ON-VAl defect is located close to Mn2+ ions, it is responsible for transferring excitation energy directly to Mn2+ ions. However, when the ON-VAl defect complex is located far from Mn2+ ions, its excited state level acts as an electron trap responsible for afterglow luminescence. Additionally, three models have been tested to explain the structure of the emission spectrum and the strong asymmetry between the excitation and emission spectra. From the most straightforward configuration coordinate diagram through the configuration coordinate diagram model assuming different elastic constants in the excited and ground-states ending by a model based on the Jahn-Teller effect. We proved that only the Jahn-Teller effect in the excited 4T1 electronic state with spin-orbit coupling could fully explain the observed phenomena. Finally, high-pressure spectroscopic results complemented by the calculations of Racah parameters and the Tanabe-Sugano diagram are presented.

High-Pressure Photoluminescence Properties of Cr3+-Doped LaGaO3 Perovskites Modulated by Pressure-Induced Phase Transition.

The photoluminescence properties of Cr3+-doped LaGaO3 perovskites are investigated by high-pressure spectroscopy. The pressure-induced phase transition from orthorhombic (Pbnm) to rhombohedral (R3̅c) at around 2 GPa is confirmed by Raman spectroscopy. Cr3+-doped LaGaO3 shows deep-red emission peaks around 730 nm due to the zero-phonon line (R-line) and the phonon sidebands, which correspond to Cr3+: 2Eg → 4A2g transitions in the ideal octahedral site and the Cr-Cr pair luminescence (N-line) under ambient condition. Under a high pressure, the R-line shifts to a lower energy at a rate of -13 cm-1/GPa. From the pressure dependence of photoluminescence excitation (PLE) spectra, it is suggested that the redshift of the R-line is caused by the decrease of Racah parameters B and C. Moreover, the N-line luminescence becomes stronger relative to the R-line with increasing pressure and the N-line/R-line can be used to monitor the phase transition pressure. Under a high pressure, the tilt angle of the GaO6 octahedral unit becomes smaller. It implies that the enhanced N-line luminescence is caused by the stronger superexchange interaction between Cr3+ ions due to the increased Cr-O-Cr bond angle closer to 180°.

Deep‐red‐emitting Ca2ScSbO6:Mn4+ phosphors with a double perovskite structure: Synthesis, characterization and potential in plant growth lighting

AbstractMn4+‐activated double‐perovskite type Ca2ScSbO6 (CSS) phosphors were synthesized via a high‐temperature solid‐state reaction. The phase purity and crystal structure of obtained samples were investigated by powder X‐ray diffraction (XRD). The successful incorporation of Mn4+ ions into CSS lattice was confirmed by the combination of energy‐dispersive X‐ray spectra (EDX) and X‐ray photoelectron spectra (XPS) results. The luminescent properties of the CSS phosphors, including the photoluminescence (PL) and PL excitation (PLE) spectra, commission international de l'clairage (CIE) chromaticity coordinates, fluorescence decay curves, quantum yields and temperature‐dependent PL spectra, were investigated in detail. Under 310‐nm excitation, the optimized CSS:0.3%Mn4+ phosphor exhibited bright deep‐red emission covering a narrow band from 640 nm to 720 nm, which overlaps with the absorbance of phytochromes. The Racah parameters B, C, and local crystal strength Dq were calculated to be 870, 2703, and 1984 cm–1, respectively. Particularly, the emission intensity of CSS:0.3%Mn4+ still remained 61.4% at 423 K compared with that at room temperature. Therefore, all these outstanding luminescent properties provided the as‐synthesized phosphors a great potential in plant growth lighting.

Intense Blue Chromophores in Cobalt Doped Phenacite‐Type Zinc Germanate System through Jahn‐Teller Distortion of Co Tetrahedron

AbstractIn the quest for new blue chromophores with less usage of cobalt alternatives to the carcinogenic and expensive zinc‐cobalt aluminate system, we developed cobalt doped phenacite type zinc germanate system. Rietveld analysis reveals that the CoO4 tetrahedron intensely experienced Jahn‐Teller distortions leading to irregular geometry on substitution. The UV‐vis‐IR absorption spectra exhibit intense Co2+ d‐d transitions with three fold splitting associated with the CoO4 tetrahedron in the visible and IR region, indicating low symmetry. The crystal field strength 10Dq, Racah parameter and Nephelauxetic ratio variations reflect on the improved covalency of the Co2+ environment. Thus the synergetic effect of Jahn‐Teller distortions and covalency of the CoO4 tetrahedron lead to strong absorptions in the green to red region resulting in intense blue colors with low cobalt content (2.2 to 8.9 mol %). The typical composition Zn1.6Co0.4GeO4 exhibits intense blue hue b*=−58.64, much higher than that of the commercial CoAl2O4 system.