Racah Parameters Research Articles

Influence of Mo or/and Co ions on the structural and optical properties of phosphate zinc lithium glasses

Impact of Molybdenum or/and Cobalt ions on the network and the optical transition of Phosphate Zinc Lithium (PZL) glass was explored. Melt quenching method was used to prepare the studied glasses. X-ray diffraction profiles assure the amorphously of the prepared materials. The density shows insensible decreasing behavior with low concentration addendum of Mo or/and Co ions. This leads to minimal tightly packed in glass network and confirms the modifier role of these ions in the studied network. Fourier Transform Infrared spectroscopy results showed that a small amount of Mo or/and Co caused minor irregular changes in the structural units of phosphate glasses. The optical absorption spectra revealed that molybdenum ions occupy the two valence states Mo3+ and Mo5+, while cobalt ions exist in Co2+ octahedral and tetrahedral forms and Co3+ tetrahedral form. The crystal field parameter (10Dq), Racah parameters, the nephelauxetic ratio, optical band gap and Urbach energy were discussed.

Spectroscopic features of chromium ions in Li2O-BaO-B2O3 glasses

Abstract Li2O-BaO-B2O3: Cr2O3 glass samples were prepared by adapting the Melt-quenching method. The pre prepared glass samples were characterized by the XRD and confirm the amorphous nature of the samples. Various physical parameters were estimated employing experimental densities and refractive index. The bands observed in the Optical absorption spectra were assigned and described based on the electronic transitions by using the TS diagrams. Crystal field (Dq) and Racah parameters (B and C) were calculated by the crystal field relations. Direct and Indirect optical band gaps were evaluated using Davis and Mott relation from the absorption edges. Correlate these results with Infrared and ESR studies. Finally, discussed the structural behavior of the Chromium ions in Li2O-BaO-B2O3glass samples was analyzed.

Optical studies of chromium doped zinc oxy fluoro borate glasses – A possible disordered material for tunable LASERS

Melt-quenching technique adapted to prepare the (10-x)ZnO-30ZnF2-60B2O3: xCr2O3 (x: 0.2, 0.4, 0.6, 0.8, 1.0) (mol %) glasses. Various physical parameters are estimated with the help of experimental densities and refractive index. Optical absorption studies reveal that the chromium ions exist in Cr3+ state at very low concentrations, whereas the concentration increases chromium ions occupies both Cr3+ and Cr6+ state. Crystal field (Dq) and Racah parameters (B and C) are calculated by the crystal field relations. Direct and Indirect optical band gaps are calculated from the absorption edges. Finally, by means of optical studies, the behaviors of Cr ions in present glasses are discussed.

Optical and Mössbauer spectroscopy of lithium tetraborate glass doped with iron oxide

The glass system with composition [80Li 2 B4O 7 +(20-x) ZnO + xFe 2 O 3 ] where x = (0, 2, 4, 6, 8 mol. %), was prepared by traditional melt quenching method. The impact of iron ions on the structural and optical properties of the investigated samples was studied by a set of commentary techniques including density, infrared, optical absorption, electron spin resonance (ESR) and Mössbauer measurements. The obtained results indicate that both the measured density and calculated molar volume increase with Fe 2 O 3 content. IR spectra revealed the characteristic absorption bands of BO 3, and BO 4 structural unites. The optical absorption spectra of the glass samples featured four bands which are characteristic of Fe 3+ in tetrahedral (Td) symmetry. The ligand field strength (10Dq), Racah parameters (B and C) and nephelauxetic effect ( h ) were calculated. ESR spectra were consistent with the optical findings, showing a broad resonance signal at g ~2.1, sharp signal at g ~4.1 and tiny shoulder (kink) at g ~ 6.7. Mössbauer spectra show two phases of iron ions existing within the glass network, the first is related to Fe 3+ ions in octahedral (Oh) sites with the isomer shift (0.675–0.553) mm s −1 , the second is related to Fe 3+ ions in Td sites with the isomer shift below 0.34 mm s −1 . • The research reports on the identification of near-edge optical transitions originating from Fe ions embedded in B 2 O 3 glass. • The impact of Fe ions on the structural and optical properties of the samples was studied by a set of commentary techniques. • The ligand field strength (10Dq), Racah parameters (B and C) and nephelauxetic effect ( h ) were calculated.

Spectroscopy of Cr3+ activator: Tanabe−Sugano diagram and Racah parameter analysis

The Racah parameters in conjunction with the Sugano−Tanabe energy-level diagram are used as a means of describing the effects of electron−electron repulsion within the metal complexes in coordination chemistry. A determination of the excitation energies in the 3d3-configuration ions like Cr3+ and Mn4+ is possible from a set of Racah (B and C) and crystal-field splitting parameters (Dq), and vice versa. However, it is very difficult to exactly determine some excitation-state energies that leads to a large error in the Racah parameter determination. The present study focuses on this problem and proposes a new model that introduces an optional Racah parameter ratio of C/B ≡ r to make it possible without requiring an uncertain excited-state energy in the Racah parameter calculation. As a typical example, spectroscopic data of MgO:Cr3+ phosphor are investigated based on the present analysis model, where Cr3+ ions can be situated in cubic, tetragonal, and rhombic site of MgO lattice. The Racah parameters and related excited-state energies determined here are found to fall very well on each Tanabe−Sugano diagram curve (2Eg, 4T2g, 2T1g, 4T1g,a, 2T2g, and 2A2g). The present analysis model can be expanded to other 3d3-activated phosphor systems such as Mn4+ ion or fluoride hosts.

Impact of sunlight on the optical properties and ligand field parameters of nickel borosilicate glasses

In this paper, the impact of sunlight on stability and ligand field parameters of borosilicate glasses doped with NiO synthesized via melt quenching approach is reported. XRD analysis confirms the absence of any crystalline nature for the glass matrices. The average density (D), molar volume (Vm) and microstructural parameters were investigated. Optical features of these glasses were determined by measuring their absorption spectra in the visible-near infrared region. The absorption spectra of Ni2+ show four fundamental absorption bands that are associated with the wavenumbers ν1, ν2, ν3 and ν4 which attributed to; allowed electronic transitions ν1: 3A2g(F) → 3T2g(F), ν2: 3A2g(F) → 3T1g(F), ν3: 3A2g(F) → 3T1g(P) and forbidden transition ν4: 3A2g(F) → 1Eg(D). These absorption bands were availed to obtain Racah parameter (B) and ligand field splitting (Δ0). It was found that the average density and molar volume were decreased with further NiO doping. The impact of NiO doping on the glass color, B and Δ0 was determined. Additionally, the influence of sunlight on the glass % degradation, B and Δ0 was determined. All the investigated glass samples are stable under sunlight irradiation meanwhile stability has decreased with NiO increment. The glass sample with 1 NiO content has the lowest degradation % under sunlight and so is the most stable one; no change has been detected in the ligand field parameters. This study paves the way for understanding the impact of sunlight (solarization) on the stability, optical properties and ligand field parameters of glasses used for solar cells and smart windows.

First-Principles Study of the Crystal-Field and Racah Parameters of Cr3+ in Alumina

The development of blue LEDs [1] enabled one to construct white LEDs by combining them with appropriate phosphors. Recently Mn4+-doped fluorides have been put into commercial use as the red phosphors for high color-rendering white LEDs [2]. However, in order to suppress the degradation due to high humidity and high temperature, a special surface treatment is required for these phosphors. Therefore, the search of other red phosphors activated by Mn4+ is still desired. For theoretical design of novel phosphors, non-empirical prediction of optical parameters such as the crystal-field and Racah parameters is quite important. The purpose of this study is to establish a first-principles approach to predict these parameters of transition metal ions in crystals.In this work, the crystal-field and Racah parameters of ruby (Cr3+ doped α-Al2O3) were evaluated based on the first-principles calculations using the relationship between the ligand-field theory and the molecular orbital (MO) theory derived by Sambe and Felton [3]. Two types of first-principles methods were adopted. One is the discrete-variational (DV)-Xα method [4], which is an MO calculation method based on the one-electron approximation. The other is the discrete variational multi-electron (DVME) method [5], which is a configuration-interaction (CI) calculation method based on the explicit many-electron wave functions. The estimation of the crystal-field and Racah parameters were performed in the following three approaches.(1) Estimation based only on the DV-Xα method(2) Estimation based only on the DVME method(3) Estimation based on the combination of the DVME method and the DV-Xα methodThe results show that the values of the crystal-field and Racah parameters obtained by the approach (3) are in the best agreement with the values obtained by fitting to the experimental data [6], indicating that CI calculations using some corrections based on the one-electron MO calculations are effective for the non-empirical estimation of the crystal-field and Racah parameters [7]. These results also provide theoretical basis for the configuration-dependent correction and the correlation correction introduced in our previous CI calculations of ruby [5]. This approach could be applied to prediction of d-d transitions of Mn4+ in crystals to find a suitable host for novel Mn4+-activated red phosphors for white LEDs.[1] I. Akasaki and H. Amano, Jpn. J. Appl. Phys. 45, 9001 (2006).[2] J. E. Murphy, F. Garcia-Santamaria, A. A. Setlur, and S. Sista, SID 2015 DIGEST, 927 (2015).[3] H. Sambe and R. Felton, Int. J. Quant. Chem., 10, 155 (1976).[4] H. Adachi, M. Tsukada, and C. Satoko, J. Phys. Soc. Jpn. 45, 875 (1978).[5] K. Ogasawara T. Ishii, I. Tanaka, and H. Adachi, Phys. Rev. B 61, 143 (2000).[6] W. M. Fairbank, Jr., G. K. Klauminzer, and A. L. Schawlow, Phys. Rev. B 11, 60 (1975).[7] K. Ogasawara, K.C. Mishra, and J. Collins, ECS J. Solid State Sci. Technol. 9, 016011 (2020).

Deep-red emitting Mg2TiO4:Mn4+ phosphor ceramics for plant lighting

In this study, deep-red emitting Mg2TiO4:Mn4+ phosphor ceramics were synthesized by the high temperature solid-state reaction method. The ceramics can be excited by the 465 nm blue light and had a narrow emission with a full width at half maximum (FWMH) value of 31 nm. The peak wavelength was located at 658 nm, which matched the demanded wavelength for photosynthesis. The crystal field strength (Dq) and the Racah parameters (B and C) were estimated by the Tanabe-Sugano diagram. The thermal conductivity of the Mg2Ti(0.999)O4:0.001Mn4+ ceramic was 7.535 W/(m·K) at room temperature, which was one order of magnitude higher than that of the traditional packaging method using the silicone gel. A set of phosphor converted LEDs were fabricated by mounting the phosphor ceramics onto the 460 nm blue LED chips and the CIE coordinates can move from the blue region to the purple light region with the thickness of the ceramic increasing. These results indicated that the Mg2TiO4:Mn4+ phosphor ceramic was suitable for plant lighting when combined with a blue LED chip.

Influence of Na2O addition on the alkali borochromate glasses: structure and ligand field

Trivalent chromium is a very common ion used for coloring in the industry of glass. As glasses are melted under high oxidizing circumstances, hexavalent chromium Cr(VI) can be fixed and found in certain advertising applications of UV-protection receptacles. Melt quenching technique is employed to prepare oxide glasses with a general formula xNa2O–(30 − x) Li2O–10K2O–10ZnO–49.7B2O3–0.3Cr2O3 (where x = 0, 5, 10 and 15 mol%). The amorphous nature of the samples is confirmed using X-ray diffraction. The optical absorption spectra have been recorded, and the resulting data were used for the ligand field theory analysis, from which the crystal-field energy (10Dq), Racah parameters (B and C) and nephelauxetic function (h) were evaluated. The variations in the optical bandgap and the band tail with Na2O content have been discussed in relation to the glass structure. FTIR of these glasses revealed that the borate network is affected by the replacement of Na2O by Li2O content. It showed also reduction in N4 with increasing Na2O. This result indicates the increase of non-bridging oxygens (NBOs) which in turn reduces the rigidity of the glass matrix and opens up the structure. The density and the molar volume (VM) measurements were also employed to investigate the structure. These measurements show a linear increase as Na2O content increases. This can be attributed to the differences in the ionic radii of sodium and lithium ions, being larger for sodium. UV–Visible optical absorption spectra show three characteristics bands around ~ 588, 435 and 360 nm which are assigned to the transitions 4A2g → 4T2g, 4A2g → 4T1g and 4A2g → 2A1g, respectively. Racah parameters (B and C) show an opposite behavior. The obtained ratio (10Dq/B) reveals a strong ligand field around chromium ions. Also, the nephelauxetic parameter of the ligand shows the same 10Dq behavior. The increase of chromium ions in the trivalent state shows that it acts as modifiers, yielding the higher concentration of NBOs in the glass matrix and confirming the FTIR and optical results. Electron spin resonance spectroscopy was measured and studied to confirm the existence of Cr3+ and Cr6+.

Correlation of mechanical properties and crystal field parameters of europium-doped magnesium–zinc–sulfophosphate glasses

Europium-doped magnesium–zinc–sulfophosphate glasses of molar composition (65 − x) P2O5–20MgO–15ZnSO4–xEu2O3 (x = 0.0, 0.5, 1.0, 1.5 and 2.0) were prepared via melt-quenching method. The synthesized glasses were characterized at room temperature to determine the relationship between Racah and mechanical properties. The FTIR spectra of glasses in the range of 4000–400 cm−1 revealed the presence of characteristic functional groups. The Young’s, shear and bulk moduli of glasses were observed to enhance with the increase in Eu3+ contents. The ratio increased from 0.710 to 0.747 which confirms the increase in the degree of centrality of the glasses. Racah parameters evaluated from first three major absorption bands were reduced from 72.3906 to 60.1111 cm−1 with the increase in Europium contents, thus signifying the weakening in f–f electrons repulsion. Crystal field parameters decrease from 42.2445 to 35.2397 cm−1 denoting strong covalent character of ligand bond of the synthesized glass system. The synthesized glasses have potential for the development of hard surface engineering.

Influence of BaO on spectral studies of Cr2O3 doped titanium-boro-tellurite glasses

Abstract Barium-titanium-boro-tellurite glasses containing the transition metal (TM) Cr3+ ions with different BaO mol% were prepared using the melt quenching method. From the XRD, SEM and EDS studies, it was confirmed that the prepared glasses were impurity-free and their atomic arrangement is non-crystalline in nature. The UV–visible spectra exhibited a dominant high absorption band corresponding to the enrgy of ~15974 cm−1 and three weak low-intensity bands around 23641 cm−1, 14881 cm−1 and 14306 cm−1. The identified bands are assigned to 4A2g→4T2g (F), 4A2g→4T1g (F), 4A2g→2T1g (G) and 4A2g→2Eg (G) transitions respectively. The optical band gap values are increasing while the refractive index values are decreasing with increasing BaO content. The crystal-field parameter Dq values and Racah parameter (B) values were found to change non-linearly with the barium oxide content which suggests a change in the environment around Cr3+ ions. From the value of Dq/B, it can be concluded that the Cr3+ ions are localized in the weak crystal field sites. The EPR spectra which were recorded at X-band frequencies exhibited four resonance signals at g 1 ≈ 1.97, g 2 ≈ 2.38, g 3 ≈ 4.25 and g 4 ≈ 5.23. The theoretical g-values were evaluated from the Macfarlane's perturbation formulae. The FTIR and Raman spectra show broad peaks which are the characteristic features of the prepared glass component vibrating modes.

A new double perovskite CaY0.5Ta0.5O3:Mn4+ deep-red phosphor: Synthesis, optical properties, and potential applications in plant-growth LEDs

Deep-red emitting CaY0.5Ta0.5O3:Mn4+ phosphors were obtained by optimizing the reaction temperature. The structure of CaY0.5Ta0.5O3:Mn4+ consisted of a double perovskite with the Pnma (62) space group. The luminescence intensity of the synthesized CaY0.5Ta0.5O3:0.3%Mn4+ at 1550 °C was twice as strong as that at 1500 °C. The photoluminescence spectra of the synthesized CaY0.5Ta0.5O3:Mn4+ phosphor exhibited a deep-red broadband luminescence from 650 nm to 750 nm with a 680 nm dominated peak due to the 2Eg→4A2g transition of Mn4+ in TaO6 octahedral structures. The optimal concentration for the CaY0.5Ta0.5O3:xMn4+ phosphor was 0.3 mol.%. The concentration quenching effect was ascribed to the dipole–quadrupole interaction. The decay time (0.398–0.171 ms) indicated that the forbidden character of 3d transitions existed among Mn4+. The crystal field strength and the Racah parameters were discussed to estimate the nephelauxetic effect of Mn4+ in the CaY0.5Ta0.5O3 host. Besides, the red light-emitting diodes (LEDs) were packaged using the CaY0.5Ta0.5O3:0.3%Mn4+ phosphor and a 368 nm near-ultraviolet chip. The emission spectrum of the red LED overlapped with the absorption curves of chlorophylls a and b. Results indicated that the CaY0.5Ta0.5O3:Mn4+ can serve as a component of the deep-red light in plant-growth LEDs.

Effect of SiO2 Addition on Chromium Transitions in Borate Glasses

Melt-quenching technique was used to prepare borosilicate glasses of composition xSiO2- (75-x) B2O3–24.7Li2O-0.3Cr2O3 (x = 0, 10, 20, 30, 40 and 50 mol%). With increasing the former content, both Cr6+ and Cr3+ optical transitions undergo strong intensity variations. The crystal field strength (10Dq) is increased, while Racah parameters (C,B) are decreased with increasing SiO2 content. The ratio estimated Dq/B values confirmed a weak crystal field environment for Cr3+ions, and further suggest a more covalent bond character. The ESR results revealed that Si-contained samples exhibit a strong microwave absorption signal, and justified the presence of both Cr3+ and Cr6+(charge transfer into 3d0 2p6+ = 3d1Cr5+) oxidation states. The fingerprints of borate and silicate relevant structural groups were clearly identified from FTIR spectroscopy.

The role of NiO on the structural and optical properties of sodium zinc borate glasses

Abstract Nickel-doped sodium zinc borate (NZB) glasses of composition [15 ZnO – (65 – x) B2O3 –x NiO – 20 Na2O], where (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %), were prepared following the melt quenching technique. The effect of NiO additives on the structural and optical properties of NZB glasses has been studied by a set of complementary techniques. The amorphous nature of the prepared samples was verified by both X-ray diffraction pattern analysis (XRD) and scanning electron microscopy (SEM). Additional structural information and physically relevant parameters such as density, molar volume, inter ionic distance, the average Boron–Boron separation, and ion concentration, have been calculated. Furthermore, the optical basicity and electronic polarizability of the system were empirically determined. Boron structural units and both Ni2+ and Zn2+ specific vibrations were identified using FTIR spectroscopy. The direct comparison of the vibrational spectra of all samples showed a slight reduction in N4 ratio, from 32% to 29%, with increasing NiO content. The detailed deconvolution process of optical spectra revealed specific absorption bands characteristic for Ni2+ ions, in the visible and NIR regions, preferentially occupying octahedral sites. Ligand field parameters such as crystal field and Racah parameters were estimated, and the covalent nature between Ni2+ ions and the surrounding ligand is indicated from the nephelauxetic effect. The optical band gap energy as calculated from Tauc's model was observed to decrease from 3.88 to 3.49 eV with increasing NiO content, while the refractive index follows the opposite behavior. Urbach energy values and metallization criterion factor confirmed the semiconductor character of the present Ni-doped NZB glasses.

Enhanced luminescence performances of Mn4+-activated Sr4Al14O25 red phosphors by doping with Sc3+ ions

A series of non-rare earth Mn4+-activated strontium aluminate phosphors Sr4Al14O25:Mn4+ co-doped with Sc3+ ions were successfully synthesized by a high-temperature solid-state reaction method. XRD result reveals that there is no introduction of additional phase but expansion of lattice with incorporation of Sc3+ ions. Excitation and emission spectrum measurement shows that the synthesized phosphors can be efficiently excited by near-ultraviolet and blue light, and a deep red emission centered at 652 nm with a narrow full width at half maximum (FWHM) can be obtained, which is attributed to the transition 2E→4A2 of Mn4+ ions. In addition, the crystal field strength parameter (Dq) and Racah parameters (B, C) and energies of states were calculated based on experimental data. Moreover, the luminous intensity of Sr4Al14-xScxO25:Mn4+ is enhanced and increased by 60% compared with Mn4+ single incorporated sample at x = 0.06. A phenomenon of redshift is observed in the excitation spectrum and discussed systematically. Finally, the mechanism of the positive effects with Sc3+ ions incorporated into lattice is discussed in detail. All the results suggest that the Sr4Al13.94Sc0.06O25:Mn4+ phosphor will become one of the great candidates for backlight of LCD.

Synthesis and luminescence properties of a novel deep red phosphor Li2ZnTi3O8:Mn4+ for plant-cultivation.

In this thesis, a novel ultraviolet-activated Li2ZnTi3O8:Mn4+ red emitting phosphor is prepared by the traditional high-temperature solid-state reaction method in air. The crystal structure, luminescence properties and decay curves of the phosphor are studied in detail. The sample belongs to spinel cubic crystal structure and there are abundant [TiO6]8- octahedron sites can be occupied by Mn4+. The Li2ZnTi3O8:Mn4+ phosphor shows the red emission in region of 600-800nm with a maximum at ~681nm under 330nm excitation, which can promote the plant growth. The optimal Mn4+ doping concentration is ~0.3mol%, which is higher than that of other hosts. By calculating the crystal field strength (Dq) and Racah parameter B, we can evaluate that there is a strong crystal environment of Mn4+ in the Li2ZnTi3O8 host lattice, which major comes from the 2Eg→4A2g transition of Mn4+ ion. Furthermore, the characteristics of thermal quenching are also studied, poor thermal quenching performance may be due to high doping concentration of Mn4+. All data suggest that Li2ZnTi3O8:Mn4+ phosphor can be a potential application in plant-cultivation.

Luminescence properties and energy transfer of Mn4+-doped double perovskite La2ZnTiO6 phosphor

A series of Mn4+-doped La2ZnTi1-xO6 red-emitting phosphors were synthesized by a high-energy deformation process. The crystal structures were analyzed and refined with the aid of the Rietveld structure refinement on the basis of XRD patterns. The morphologies and luminescence properties of the phosphors were investigated using field emission scanning electron microscopy, photoluminescence spectroscopy, and decay curves. These phosphors present effective excitation bands in the UV, near-UV, and blue regions, that originate from the charge transfer transition between O2− and Mn4+, and the 4A2g → 4T1g, 4A2g → 2T2g, and 4A2g → 4T2g transitions of the Mn4+ ion. The emission spectra under 390 nm excitation revealed the highest red emission peak at 708 nm attributable to the spin-forbidden 2Eg → 4A2 transition of the Mn4+ ion. The optimized Mn4+-doping concentration in the La2ZnTiO6:Mn4+ phosphor was found to be 0.2 mol%, with higher levels, leading to concentration quenching. The optical characteristics of the La2ZnTiO6:Mn4+ phosphor is described by a Tanabe-Sugano energy-level diagram and a configuration coordinate diagram. The crystal field strength (Dq) and the Racah parameters (B, C) are calculated to approximate the nephelauxetic effect for Mn4+ caused by the La2ZnTi1-xO6 host.

Luminescent properties of a red-emitting oxyfluoride phosphor Rb2WO2F4:Mn4+

One new kind of red-emitting oxyfluoride phosphors Rb2WO2F4 doped with different Mn4+ contents were synthesized via a facile co-precipitation method at an ambient condition. Meanwhile, the luminescent behaviors of Mn4+ in Rb2WO2F4 were investigated in detail. The sample prepared with the WO3/K2MnF6 molar ratio of 40:1 has a broad excitation band around 465 nm and emits a series of sharp-emitting peaks around 631 nm. The crystal-field strength Dq as well as Racah parameters B and C were calculated to be 2151, 583 and 3872 cm−1, meaning that Mn4+ in Rb2WO2F4 experiences a strong crystal field. The luminescence decay time was also determined to be 2.15 ms. Finally, a warm white LED was obtained using Rb2WO2F4:Mn4+ as red component. It's color rendering index was improved to 81.4 and color temperature was decreased to 3588 K.

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (VI) shows the selective formation of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This similitude is reinforced by comparable ligand-field strengths (∆oct) and nephelauxetic effects (as measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+ within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of [Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand arm wrestling match operating in bound L5, which prevents the contraction of the coordination sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in [Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic compensation. The decrease of the stability constants by five orders of magnitude observed in going from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this effect, which had been not foreseen by the authors.

Luminescent properties of Li(Ga1-xCrx)5O8 (LGCO) phosphors

In this work, we investigate the synthesis of LiGa5O8 ceramic powders through a polyvinyl alcohol-based sol-gel technique and their optical properties when doped with high Cr3+ concentrations (5, 25 and 50 mol% with respect to the Ga3+ sites). The results indicate that the main crystalline phase, LiGa5O8, is obtained after calcining the samples at 1000 °C/2 h in a static air atmosphere. Via X-ray photoelectron spectroscopy, Cr is confirmed to exist in its trivalent oxidation state and the evaluation of the optical properties is performed via photoluminescence excited from visible to vacuum ultraviolet energy range and with X-ray excited optical luminescence, indicating the typical Cr3+ emission at the near-infrared energy range. The crystal field and Racah parameters are calculated and the influence of Cr3+ concentration in the host material indicates luminescence suppression/quenching and a redshift for a higher amount of these ions.