Tanabe-Sugano Diagram Research Articles

Synthesis, structure and properties of compounds derived from Kotoite-related structures: A2MB2O6 (A = Zn, Ba, Pb; B = Pb, Ba, Zn, Cu).

A series of compounds with the general formula A2MB2O6 (A = Zn, Ba, Pb; B = Pb, Ba, Zn, Cu) related to the kotoite structure has been prepared employing high-temperature solid state methods. The substitution of transition elements in place of Zn2+ ions resulted in colored compounds. The optical absorption spectra could be explained based on Tanabe-Sugano diagram and allowed d-d transitions. Dielectric studies on Ba2ZnB2O6, (Ba1.5Pb0.5)ZnB2O6, PbZn2B2O6, Pb1.5Zn1.5B2O6, BaZn2B2O6 and Pb2CuB2O6 at room temperature indicate reasonable values at low frequencies, which decrease on increasing frequencies. Magnetic study of synthesized single-phase compounds BaZnCoB2O6 and Pb2CuB2O6 have been performed. The substitution of Eu3+, Tb3+ and Tm3+ in place of Ba2+ in Ba2ZnB2O6, gives rise to the expected emission of red, green and blue colors. Suitable modifications of the different phosphors in Ba2ZnB2O6, resulted in white-light emission in Ba2ZnB2O6. The Pb2CuB2O6 compound was found to be a good catalyst in the ipso-hydroxylation of arylboronic acids.

Selecting ns2 electron (Sb3+) and d3 electron (Cr3+) co-doped in lead-free halide double perovskite to achieve blue and near-infrared luminescence

The double-perovskite Cs2NaInCl6 has emerged as a highly favored candidate due to its remarkable stability and eco-friendly properties. However, it is noteworthy that this material does not demonstrate the desired optical performance in the crucial visible and near-infrared (NIR) spectrum, which may limit its potential applications in certain domains. We develop such characteristics in Cs2NaInCl6 by co-doping Sb3+ (s-electron) and Cr3+ (d-electron) ions. Doping with Sb3+ allows for 1S0→3P1 transition absorption, which then emits blue photoluminescence of self-trapped exciton (STE). By co-doping Sb3+ and Cr3+ ions, it causes the transfer of excitation energy from STE to Cr3+, yielding strong NIR emission at 950 nm. Temperature-dependent Photoluminescence (PL), PL excitation, PL decay and Tanabe-Sugano diagram provide insight into photophysical processes. Furthermore, a novel dual-modal optical information encryption based on the integration of blue and NIR luminescence printing patterns using Cs2NaInCl6: Sb3+ and Cs2NaInCl6: Sb3+/Cr3+, respectively, is successfully demonstrated.

Color tuning by Si/Ge substitution in Cr4+ doped melilite inorganic pigments

Rockery constitutes one of the significant elements of Chinese royal garden design, and it becomes an emerging interdisciplinary topic covering both science and art. In this work, we report an attempt to restore the color of the ancient rockeries by finding the appropriate inorganic pigments. Chromium is doped as activators into strontium melilites with varying Si/Ge ratio, and the synthesized powders give a series of color changing from ice blue to dark-sea green. With the structure and spectroscopic characterizations, we discuss the substitutions of Cr in Mg/Si/Ge sites of the melilite host. The Tanabe-Sugano diagram of 3d2 configuration is employed to explain the change of diffuse reflectance spectra caused by Si/Ge substitution. With the work, we hope to inspire the interdisciplinary research and promote the rockery heritage restoration and protection.

Intense NIR-II luminescence through stabilization of tetrahedral (Mn5+O4) lumophore in apatites with high external quantum efficiency

The discovery of highly efficient near-infrared (NIR) emitting phosphors is essential for multiple spectroscopic applications. Pentavalent-manganese (Mn5+) is a rarely-reported NIR-II activator due to the its instability in inorganic solids. In this study, Mn5+-activated apatite phosphors A5(B1-xMnxO4)3X (A = Sr, Ba; B = P, V; X = OH, F, Cl, Br) were reported. Benefiting from the discrete P/VO4 tetrahedron and stabilization effect of alkaline-earth cation, Mn5+ existed as MnO43− anion can be effectively incorporated into the A5(P/VO4)3X. A selection of 16 different apatites is conducted to test the structural and compositional effect on Mn5+ stabilization and luminescence. The structure-property relationship was discussed with a detailed analysis on Mn5+ Tanabe-Sugano diagram and Dq/B calculation. Particularly, the optimized Ba5(VO4)3Cl: 0.01Mn5+ (λex = 650 nm, λem = 1181 nm) reveals an exceptional external quantum efficiency of 61.1 % with vivid turquoise body color. The multiparametric thermometry by luminescence intensity ratio (LIR) and 1E lifetime were demonstrated with excellent linear regressions (R2 > 99.9 %) and high relative sensitivities of > 1.0% K−1 in the range of physiological temperatures. A NIR-II phosphor-converted light-emitting diode (pc-LED) is fabricated with a NIR output power of 30.18 mW at 60 mA driven current and photoelectric conversion efficiency of 6.63 %.

Host Dependency of Boundary between Strong and Weak Crystal Field Strength of Cr3+ Luminescence.

Cr3+ doped near-infrared phosphors hold significant applications and generate considerable research interest. The critical parameter for assessing the strength of the crystal field for Cr3+ in the Tanabe-Sugano diagram is the boundary value of Dq/B, representing the ratio of crystal field splitting to the Racah parameter B. Nevertheless, there are conflicting values for this parameter, as reported in various studies, such as 2.1, 2.2, and 2.3 for C/B = 4.5-4.8. Moreover, some Cr3+ doped phosphors with wide-band emissions exhibit a Dq/B value that falls within the region of a contradictory strong field. In this study, we numerically determine the boundary value of Dq/B, which distinguishes between strong and weak fields. The results then demonstrate a dependence on the host material and are correlated with the values of Racah parameters B and C. This work resolves the inconsistency between the boundary values of Dq/B and the emission profile of Cr3+, providing researchers with a more profound comprehension of Cr3+ luminescence.

Unveiling Local Electronic Structures of Manganese‐Doped Double Perovskites Red Emitters

AbstractOver the past few years, light‐emitting diodes (LEDs) have emerged as an efficient replacement for standard incandescent and fluorescent lamps, due to their superior efficiency, energy conservation, service longevity, reliability, and affordability. However, identifying a red emitter for white LEDs has presented significant challenges. This study seeks to synthesize and analyze a novel red emitter, investigating its potential use in white LEDs and indoor plant growth lighting. The Sr2Lu(Nb,Mn)O6 is manufactured employing the traditional high‐temperature solid‐phase method. The internal and external quantum efficiencies (IQE and EQE) of double perovskites reached 32.39% and 21.67%. The g value calculates to 2.146, which corroborates that the magnetic dipolar transitions of Mn4+ ions inhabit symmetric octahedral sites. First‐principles investigations are utilized to unveil local electronic structures. Emission spectra reveal a substantial broad red emission peak at 688 nm. The Tanabe–Sugano diagram is applied to understand the luminescence mechanism. Additionally, the evaluation of parameters such as the internal UV emission ratio, crystal field strength, and Racah parameters is conducted. The luminescent intensity of the Sr2Lu(Nb,Mn)O6 can still be maintained at 60% upon reaching a temperature of 423 K. The findings suggest that Sr2Lu(Nb,Mn)O6 offers a novel alternative for white LEDs and indoor plant growth lighting.

Structural and optical properties of Zn2.95Ga2−xSnO8:xCr3+: An excellent X-ray charging-based persistent phosphor

This study investigates structural and luminescence properties of Cr3+ doped Zn2.95Ga2SnO8 phosphor. XRD analysis determines the phase purity and lattice parameters, while SEM and TEM analysis were utilized to study the particle size and surface morphology of the sample. UV–visible absorption and PL excitation spectra provided information on the excitation bands and the band-gap of both the undoped and Cr3+ doped Zn2.95Ga2SnO8. The optical band-gap of the material decreases upon doping with Cr3+ ions. Analysis of the Racah parameter and Tanabe Sugano diagram highlights a robust crystal field within Cr3+: Zn2.95Ga2SnO8, causing 2Eg level to become the lowest excited state with sharp emission at 699 nm. Concentration quenching at higher Cr3+ concentrations (> 0.6 mol%) is primarily attributed to the quadrupole-quadrupole interactions. Two distinct traps with trap depths at 0.51 eV and 0.73 eV were identified by using thermoluminescence measurement. The material gives persistent luminescence with a decay time of more than 30 min when charged with blue light (420 nm) and an excellent decay time of more than 16 h when charged with X-rays. Notably, this is the first report on X-ray charging-based persistent luminescence in Zn2.95Ga2SnO8. The outstanding persistent luminescence makes this material highly promising for various applications.

Unveiling the luminescence property of Li2MgGeO4:Mn4+ featuring the tetrahedral crystallographic-site occupancy of Mn4.

Owing to the occupying tendency of Mn4+ at octahedral sites, doping Mn4+ activators in tetrahedral structures poses challenges and hence is seldom reported. In this work, tetrahedrally sited Mn4+ phosphors were studied. By combining X-ray diffraction (XRD) data with Rietveld refinement analysis, the location of Mn4+ was determined. It was found that by adding excessive raw MgO, the phosphor synthesis temperature can be improved, enhancing the crystallinity of the crystal and thus improving the emission performance of the phosphor. In addition, excessive raw MgO forms a second phase in an LMGO matrix, which does not change the doping site for Mn4+. The Tanabe-Sugano diagram of Mn4+ in the tetrahedral field and the energy-level diagram of these phosphors were constructed for the first time, and the excitation and emission mechanisms are discussed in detail. With 1.2-fold excess of raw MgO, the prepared sample (LMGO-Mn-1.2) shows the best luminescence, demonstrating red emissions peaked at 656 nm and affording an emission intensity enhancement of over 50 times compared to a stoichiometric LMGO:Mn4+ system. At 150 °C, LMGO-Mn-1.2 keeps 90% emission intensity compared to that at room temperature. Finally, a high-efficiency warm white light-emitting diode was built. This work provides new insights into the study of Mn4+-activated phosphors in a tetrahedron crystal field.

Boltzmann thermometer with broadband emission in Mn4+-Doped β-Ga2O3 crystals

The luminescence properties of Mn4+-doped β-Ga2O3 are strongly affected by the surrounding crystal field within the [MnO6] octahedral site, which is substitute for GaⅡ site, as determined through the first-principles calculations. The crystal field strength parameters Dq, B, and C are computed, and the Tanabe-Sugano diagram is redrawn for C/B = 6.7. The temperature dependence of the optical spectra is thoroughly examined, revealing the thermal quenching mechanisms involving the thermal activation energy of 2Eg and 4T2g, which are 378 ± 26 cm−1 and 3527 ± 25 cm−1, respectively. In order to further investigate the temperature sensing characteristics, the absolute (Sa) and relative (Sr) sensitivities are obtained. At 303 K, the Sr is determined to be 0.72 %K−1, while Sa is estimated to be in the range of 10−2 (below 0.05 K-1) for the Mn4+ doped β-Ga2O3 thermometers. The thermal resolution δT is demonstrated to be below 0.1 K, surpassing that of traditional Nd3+ and Er3+-doped thermometers. Moreover, a linear relationship between the energy separation ΔE of the thermally coupled 2Eg-4T2g energy levels and the crystal field strength Dq is observed in numerous materials. With this consideration, the nature of the thermometer, along with its Sa and Sr, can be roughly predicted and designed for various operational conditions.

A high-efficiency double perovskite phosphor SrLaNaTeO6:Mn4+: Potential applications in w-LEDs and indoor plant growth lighting

A novel deep-red phosphor with Mn4+-doped double perovskites SrLaNaTeO6 (SLNTO) was synthesized by the conventional high-temperature solid-state method. SLNTO contains both [NaO6] and [TeO6] octahedra. The octahedral structure of double perovskites facilitates the doping of Mn4+. The band gap (Eg) of the direct semiconductor SLNTO is 2.88 eV. With an excitation wavelength of 322 nm, SLNTO:Mn4+ has a 700 nm deep-red wavelength emission assigned to the energy level transition from 2E1g to 4A2g of Mn4+ ions. Photoluminescence (PL) of the sample and phytochrome far-red light (PFR) absorption are satisfactory matches. According to the Tanabe-Sugano diagram, Mn4+ is located in the strong crystalline field for 2E1g. A 2.0 mol% concentration of Mn4+ is optimal; further high doping results in concentration quenching, ascribed to the neighbour ion interaction and dipole‒dipole (d-d) interactions. The internal quantum efficiency (IQE) is 87.84 %. Additionally, the electroluminescent (EL) spectrum of the encapsulated plant growth lamp overlaps with the chlorophyll a and b absorption spectra. Its Commission Internationale de l'Eclairage (CIE) coordinates of SLNTO:2.0 mol%Mn4+ phosphor with a near-ultraviolet (n-UV) chip are located in (0.720, 0.280). The phosphor-converted white light-emitting diode (pc-WLED) has a high colour rendering index (Ra = 96) with a suitable correlated colour temperature (CCT = 6240 K). The results are satisfactory for white light-emitting diodes (w-LEDs) and plant growth lamps.

Synthesis, structural and optical properties of a novel double perovskite for LED applications

Amidst growing concerns for environmental sustainability and green ecology, the utilization of Light Emitting Diodes (LEDs) for indoor plant cultivation has emerged as an influential area of research. Nonetheless, the growth of indoor plants is often constrained by inadequate lighting. Consequently, designing novel LEDs to create an appropriate light environment has emerged as a focal point in research. As a new type of photocatalytic material with excellent optical properties and light stability, double perovskite has broad application prospects in LEDs manufacturing. In this context, this article aims to synthesize a new Ca2Lu(Nb,Mn)O6 double perovskite and study its optical properties in LEDs lighting for indoor plant growth. A new Ca2Lu(Nb,Mn)O6 double perovskite were synthesized using the traditional high-temperature solid-phase method. The crystal structure and luminescence properties of the double perovskite were characterized in detail using powder X-ray diffraction (XRD), Rietveld refinement, excitation and emission spectra, decay lifetimes, quantum efficiency (QE), electron paramagnetic resonance (EPR) spectra, temperature-dependent emission spectra, and the Commission International de l’Eclairage (CIE) color coordinates. The photoelectric properties of Ca2Lu(Nb,Mn)O6 and Ca2LuNbO6 double perovskite have been explored through first-principles investigations. The absorption spectra from 270 to 600 nm indicates that it can be effectively excited by ultraviolet (UV) light. The emission spectra show a strong broad red emission peak at 684 nm. The luminescence mechanism has been studied using the Tanabe-Sugano diagram. In addition, the internal UV emission ratio β1, crystal field strength Dq, and Racah parameters B and C were evaluated. The thermal stability of the sample was studied through emission spectra at heating temperatures from 303 to 573 K. The results indicate that Ca2Lu(Nb,Mn)O6 double perovskite provides a new solution for indoor plant growth.

Broad Dual Emission by Codoping Cr3+ (d→d) and Bi3+ (s→p) in Cs2 Ag0.6 Na0.4 InCl6 Double Perovskite.

A phosphor emitting both white light and broad near-infrared (NIR) radiation can simultaneously provide visual inspection and early signs of rotting of food products. The broad NIR emission is absorbed by the vibrational overtones of water molecules present in food items, providing the non-invasive image contrast to assess the food freshness. Here we design a phosphor, namely, Cr3+ -Bi3+ -codoped Cs2 Ag0.6 Na0.4 InCl6 , that simultaneously emit warm white light and broad NIR (1000 nm) radiation with quantum yield 27 %. This dual emitter is designed by combining the features of s2 -electron (Bi3+ ) and d3 -electron (Cr3+ ) doping in a weak crystal field of the halide perovskite host. excitation of Bi3+ , using a commercial 370 nm ultraviolet light-emitting-diodes (UV-LED), yields both the emissions. A fraction of the excited Bi3+ dopants emit the warm white light, and the other fraction transfers its energy non-radiatively to Cr3+ . Then the Cr3+ de-excites emitting broad NIR emission. Temperature dependent (6.4-300 K) photoluminescence in combination with Tanabe-Sugano diagram show that the Cr3+ experiences a weak crystal field ( =2.2), yielding the NIR emission. As a proof of concept, we fabricated a panel containing 122 phosphor-converted LEDs, demonstrating its capability to inspect food products.

Crystal Field Splitting Energy (Δo) and Racah Parameters (B) of Some Metal-Saccharine and Metal-Saccharine-Ethylenediamine Complexes

[M(sac)2(H2O)4].2H2O [sac = saccharinato anion, M = Fe(II), Co(II), Ni(II), and Cu(II)], and [M(sac)2(en)2].2H2O [sac = saccharinato anion, en = ethylenediamine, M = Fe(II), Co(II), Ni(II) and Cu(II)] were prepared in an aqueous medium. The compounds are crystalline of different colors and are ambiently stable. By examining their physico-chemical properties and relevant literature, the metal(II) ions in both [M(sac)2(H2O)4].2H2O and [M(sac)2(en)2].2H2O complexes are octahedrally coordinated. In the former by four neutral H2O molecules and two monoanionic sac ligands while in later, the octahedral sites fulfilled by two neutral bidentate ethylenediamine molecules and two monoanionic sac ligands. Splitting of the crystal field (Δo) and Racah parameter (B) of two sets of complexes [M(sac)2(H2O)4].2H2O and [M(sac)2(en)2].2H2O are estimated from their electronic spectra using Tanabe-Sugano diagram of Co(II) and Ni(II) complexes while Δo values of Fe(II) and Cu(II) derivatives are calculated directly from their absorption maxima. Our experimental results show that for the studied ligands and the divalent transition metal ions, the Δo values vary according to the following order: Cu(II) > Fe(II) > Co(II) > Ni(II). Dhaka Univ. J. Sci. 71(1): 6-12, 2023 (Jan)

Optical spectroscopy of the PbI2 single crystals doped with Fe, Ni, and Mn

The optical absorption and photoluminescence spectra of the PbI2:Fe, PbI2:Ni, and PbI2:Mn single crystals grown from the vapour phase are studied. The investigated crystals reveal a fundamental absorption edge at 510–530 nm. The PbI2:Fe and PbI2:Mn crystals also show ligand-to-metal charge transfer bands in the UV–violet range. Charge transfer transitions are also observed in the luminescence spectra of the PbI2:Fe crystal. The PbI2:Ni and PbI2:Mn crystals show an intrinsic luminescence band at 530–540 nm. The PbI2:Mn crystal also exhibits a weakly-resolved emission band near 620 nm assigned to Mn2+ ions. The corresponding excitation spectrum shows several bands ascribed to the 3d – 3d transitions of Mn2+ ions. Analysis of the crystal field splitting for Mn2+ ions in the PbI2:Mn crystal is carried out as well as the Racah parameters (B, C) and crystal field strength (Dq) are calculated. The Tanabe–Sugano diagram for Mn2+ ions in the PbI2:Mn crystal is built and discussed.

An UV/Vis/NIR optical absorption spectroscopic and color investigation of transition-metal-doped gahnite (ZnAl2O4 spinel) crystals grown by the flux method

Abstract Synthetic flux-grown end-member gahnite, ZnAl2O4, and several different colored crystals doped with one or more transition metals including Mn, Ni, Cr, Co, and Fe were studied by electron microprobe methods and UV/Vis/NIR single-crystal optical absorption spectroscopy. The first major objective was to measure and assign the various electronic absorption features. The second was to analyze quantitatively the crystal colors using the experimental spectra and the CIE 1931 color-space-chromaticity diagram. The microprobe results show that the doped gahnites have transition metal concentrations between about 0.001 and 0.1 cations per formula unit. The spectrum of colorless, nominally pure ZnAl2O4 displays no absorption in the visible region. Microprobe analysis of a light-blue gahnite crystal reveals small amounts of Ni and Mn. The UV/Vis/NIR spectrum does not indicate any dd-electronic transitions relating to Mn. All absorption features also cannot be fully interpreted using Tanabe-Sugano diagrams for Ni2+ in either octahedral or tetrahedral coordination. A series of seven slightly different colored gahnites with differing concentrations of Cr3+ and most also containing smaller amounts of Ni was investigated. The spectrum of a one pink crystal shows two intense absorption features in the visible region. They are assigned to spin-allowed 4A2g → 4T2g (4F) and 4A2g → 4T1g (4F) transitions of VICr3+. Other spectra display additional weak bands and lines that are most probably spin-forbidden dd-transitions of Ni2+. These gahnites with Ni and Cr show varying purple colorations depending on the concentrations of both metals. Two more deeply blue gahnites contain Co2+ as demonstrated by their UV/Vis spectra but not by microprobe analysis. Two intense absorption features at ~7440 and ~16 850 cm–1 are observed and assigned to the spin-allowed transitions 4A2 → 4T1 (4F) and 4A2 → 4T1 (4P) of Co2+, respectively. Complex absorption fine structure, caused by spin-orbit and/or vibronic interactions, is also observed. Three different gahnites with yellow to orange colorations contain measurable Mn. Their spectra are similar in appearance and display several weak IVMn2+ spin-forbidden transitions located above 20 000 cm–1. The spectra of two green gahnites show several Fe spin-forbidden electronic transitions arising from single, isolated IVFe2+ and VIFe3+ cations between 10 000 and 25 000 cm–1. The intensities of some of the VIFe3+-related bands can be increased through exchange-coupled interactions with next nearest IVFe2+ neighbors. The colors of various doped gahnites and the end-member galaxite are analyzed using their single-crystal absorption spectra in the visible region. Their dominant wavelength, λk, and hue saturation, pc, values are given on the CIE 1931 color-space-chromaticity diagram and are discussed. The Hex colors of all crystals are calculated and can be compared to those of the studied crystals.

Modifications of Tanabe-Sugano d6 Diagram Induced by Radical Ligand Field: Ab Initio Inspection of a Fe(II)-Verdazyl Molecular Complex.

Quantum entanglement between the spin states of a metal center and radical ligands is suggested in an iron(II) [Fe(dipyvd)2]2+ compound (dipyvd = 1-isopropyl-3,5-dipyridil-6-oxoverdazyl). Wave function ab initio (Difference Dedicated Configuration Interaction, DDCI) inspections were carried out to stress the versatility of local spin states. We named this phenomenon excited state spinmerism, in reference to our previous work (see Roseiro et al., ChemPhysChem 2022, e202200478) where we introduced the concept of spinmerism as an extension of mesomerism to spin degrees of freedom. The construction of localized molecular orbitals allows for a reading of the wave functions and projections onto the local spin states. The low-energy spectrum is well-depicted by a Heisenberg picture. A 60 cm-1 ferromagnetic interaction is calculated between the radical ligands with the Stotal = 0 and 1 states largely dominated by a local low-spin SFe = 0. In contrast, the higher-lying Stotal = 2 states are superpositions of the local SFe = 1 (17%, 62%) and SFe = 2 (72%, 21%) spin states. Such mixing extends the traditional picture of a high-field d6 Tanabe-Sugano diagram. Even in the absence of spin-orbit coupling, the avoided crossing between different local spin states is triggered by the field generated by radical ligands. This puzzling scenario emerges from versatile local spin states in compounds which extend the traditional views in molecular magnetism.

Shortwave Infrared Luminescence of Tetravalent Chromium and Divalent Nickel: Phosphor Design Principles and Applications

Shortwave infrared radiations (SWIR, 900–1700 nm) emitting inorganic luminescent materials are key components for multiple applications, which range from optical communication and anticounterfeiting purposes to chemical and medical diagnosis. The blue light excitable SWIR phosphors activated with the transition metal ions Cr4+/Ni2+ are considered prospective light sources for an upcoming compact modern device using phosphor converter technology. In this review, the crystal field theory and Tanabe-Sugano diagram are briefly discussed to understand the splitting of energy levels and the concurrent effect. Several possible strategies to achieve and enhance the luminescence efficiency of Cr4+/Ni2+ SWIR emission are disclosed as is the reported chemical system. Finally, the domains for the potential practical application of SWIR phosphors are introduced. This review will provide information on the recent advances and the luminescence mechanism of Cr4+/Ni2+-activated SWIR phosphors and will also assist researchers in developing next-generation SWIR phosphors and exploring new applications.

Manganese Luminescent Centers of Different Valence in Yttrium Aluminum Borate Crystals.

We present an extensive study of the luminescence characteristics of Mn impurity ions in a YAl3(BO3)4:Mn crystal, in combination with X-ray fluorescence analysis and determination of the valence state of Mn by XANES (X-ray absorption near-edge structure) spectroscopy. The valences of manganese Mn2+(d5) and Mn3+(d4) were determined by the XANES and high-resolution optical spectroscopy methods shown to be complementary. We observe the R1 and R2 luminescence and absorption lines characteristic of the 2E ↔ 4A2 transitions in d3 ions (such as Mn4+ and Cr3+) and show that they arise due to uncontrolled admixture of Cr3+ ions. A broad luminescent band in the green part of the spectrum is attributed to transitions in Mn2+. Narrow zero-phonon infrared luminescence lines near 1060 nm (9400 cm-1) and 760 nm (13,160 cm-1) are associated with spin-forbidden transitions in Mn3+: 1T2 → 3T1 (between excited triplets) and 1T2 → 5E (to the ground state). Spin-allowed 5T2 → 5E Mn3+ transitions show up as a broad band in the orange region of the spectrum. Using the data of optical spectroscopy and Tanabe-Sugano diagrams we estimated the crystal-field parameter Dq and Racah parameter B for Mn3+ in YAB:Mn as Dq = 1785 cm-1 and B = 800 cm-1. Our work can serve as a basis for further study of YAB:Mn for the purposes of luminescent thermometry, as well as other applications.

Synthesis, structure and photoluminescence properties of NIR phosphor LiAlSi2O6:Cr3+

A novel Cr3+-doped LiAlSi2O6 phosphor was synthesized by a solid-state reaction. Details of first-principles calculation, structural studies, spectroscopic, crystal field calculations along with photoluminescence thermal stability for Cr3+ doped LiAlSi2O6 are given in the present paper. First-principles calculations show β-LiAlSi2O6 has the lowest formation free energy in α1-, α2-, β- and γ-LiAlSi2O6 and is the indirect band gap. The XRD pattern indicates that the sample is β-LiAlSi2O6. The excitation spectrum of LiAlSi2O6:Cr3+ peaking at 254, 411 and 561 nm corresponded to three spin-allowed Cr3+ d–d intra-shell transitions of 4A2g-4T1g(4P), 4A2g-4T1g(4F) and 4A2g-4T2g(4F), respectively. The reason for the change of the emission peak observed between the ultra-trace doping sample and the actively doped sample is discussed. Cr doping increases the disorder degree of cations in LiAlSi2O6 and makes the possible emission of Cr due to mainly N line emission. The Tanabe–Sugano diagram shows that Cr doping increases the strength of LiAlSi2O6 crystal field. The emission intensity of LiAlSi2O6:0.15%Cr3+ at 423 K (150 °C) maintains ≈ 70.0% of its initial value at 303 K (30 °C). Only the sharp emission line of the 2Eg-4A2g related transitions even at high temperatures (T = 523 K) and no large increase of the PL intensity caused by the odd-parity phonon gaining over the wide range of temperature from T = 303 K–523 K.

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.