Crystal Field Parameters Research Articles

Theoretical study of the local distortions and the EPR parameters for VO2+ and Cu2+ centers in oxyfluoroborate glasses

Theoretical calculations of the local structures and the electron paramagnetic resonance (EPR) parameters for the impurities VO2+ and Cu2+ centers in oxyfluoroborate glasses (OFG) were performed by using the perturbation formulas of the EPR parameters for tetragonally compressed octahedral [VO6]8- and tetragonally elongated octahedral [CuO6]10− clusters, respectively. In the calculations, the required crystal-field (CF) parameters are estimated from the superposition model which enables correlation of the CF parameters and hence the EPR parameters with the tetragonal distortion (characterized by the relative tetragonal elongation (or compression) ratio τ along the C4 axis) of [VO6]8- (or [CuO6]10−) cluster. Based on the calculations, the [VO6]8- and [CuO6]10− clusters are found to undergo the tetragonal compression (τ ≈ 1.7%) and elongation (τ ≈ 2.9%) along the C4 axis, the signs of A|| and A⊥ for VO2+ and Cu2+ centers in OFG were also determined. The above theoretical approach on local structural properties would enhance the understanding of the structural properties for other oxide glasses with transition-metal (TM) dopants.

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.

Studies of local structures for Cu2+ centers in M2Zn(SO4)2·6H2O (M = NH4 and Rb) crystals

Abstract The electron paramagnetic resonance (EPR) parameters (g factor g i , and hyperfine structure constants A i , with i = x, y, z) and local structures for Cu2+ centers in M2Zn(SO4)2·6H2O (M = NH4 and Rb) are theoretically investigated using the high order perturbation formulas of these EPR parameters for a 3d 9 ion under orthorhombically elongated octahedra. In the calculations, contribution to these EPR parameters due to the admixture of d-orbitals in the ground state wave function of the Cu2+ ion are taken into account based on the cluster approach, and the required crystal-field parameters are estimated from the superposition model which enables correlation of the crystal-field parameters and hence the studied EPR parameters with the local structures of the Cu2+ centers. Based on the calculations, the Cu–H2O bonds are found to suffer the axial elongation ratio δ of about 3 and 2.9% along the z-axis, meanwhile, the planar bond lengths may experience variation ratio τ (≈3.8 and 1%) along x- and y-axis for Cu2+ center in (NH4)2Zn(SO4)2·6H2O and Rb2Zn(SO4)2·6H2O, respectively. The theoretical results show good agreement with the observed values.

Atomistic simulation and spectroscopy study of the Eu-doped NaCdPO4 compound

Atomistic simulation based on lattice energy minimization was performed to study the defect process caused by europium in the NaCdPO4 structure. The calculations suggest that the most favorable intrinsic defect type is formed by cation anti-site defect, in which Na+ and Cd2+ ions exchange positions. The calculation also reveals that doping by Eu3+ on the Cd site is the most energetically favorable. Charge compensation by NaCdʼ anti-site defect is probable. Crystal field parameters are then calculated and used to obtain the 7F1 sublevels of the Eu3+, which we use to discuss the local symmetry of the Eu3+.

Photoluminescence and afterglow tuning in ultra-broadband near-infrared Mg3Y2Ge3O12:Cr3+ phosphor via cation substitution

Broadband near-infrared (NIR) long-persistent phosphors with adjustable optical property have been urgently demanded for practical applications in bio-imaging and plant cultivation. Herein, Cr3+ doped Mg3Y2Ge3O12 garnet phosphors with ultra-broadband NIR emissions (600−1200 nm) peaked at 776 nm were prepared via a high-temperature solid-state reaction method, and the cation sites for Cr3+ were well identified and assigned based on the Rietveld refinement and time-resolved emission spectra (TRES). Under the excitation at 436 nm, wide-range modulations in intensity, wavelength (763−834 nm) and bandwidth (143−227 nm) of 4T2→4A2 (4F) transition were achieved via Zn2+/Ca2+ substitution, which completely covers the maximum absorption ranges (715−1050 nm) of bacteriochlorophyll from anaerobic photosynthetic bacteria (PSB). According to the classic Tanabe-Sugano theory, the crystal field parameters around different Cr3+ centers were calculated to explain the spectral tailoring effect. Meanwhile, the afterglow duration was greatly improved from 2 min to 66 and 240 min after co-doping Zn2+/Ca2+, and the trap distribution and mechanism was elaborately explored via thermos-luminescence (TL) experiments. Above results provide an effective strategy for designing NIR-emitting phosphors with desired optical and afterglow performance, which well meets the light requirements of diverse applications especially in plant growth.

Spectroscopic study of the Eu3+ local symmetry in EuF3 crystal

We employed the simple overlap model (SOM) with the method of equivalent nearest neighbours (MENN) to indicate the local symmetry of the Eu3+ in the europium trifluoride (EuF3) compound through two analyses based on the emission spectra. In first situation, three lines were observed at the 5D0 → 7F1 (0-1) transition. Therefore, based on the amount of non-null crystal field parameters, we indicate that the Eu ion occupies a site with distorted inversion centre as a combination of C2 and C2h symmetries, with the following charge factors: g1 = 0.438, g2 = 0.292, g3 = 0.411, g4 = 0.201, g5 = 0.396 and g6 = 0.293. In second situation, with two lines at the 0-1 transition, we suggested the luminescent site as a combination of the D3d and D3h symmetries with g1 = 0.279 and g2 = 0.495. The 7F1 manifold splitting was obtained through Auzel-Malta equation. All theoretical calculations reproduced satisfactorily the 7F1, 7F2 and 5D1 energy sublevels in good agreement with experimentals data.

A broadband near-infrared phosphor BaZrGe3O9:Cr3+: luminescence and application for light-emitting diodes

A broadband NIR luminescence material BaZrGe3O9:Cr3+ was designed, the crystal field parameters and Huang–Rhys factor were calculated to evaluate the luminescence. The potential application for NIR pc-LED was demonstrated.

Regulating the magnetic dynamics of mononuclear β-diketone Dy(iii) single-molecule magnets through the substitution effect on capping N-donor coligands.

A series of five mononuclear β-diketonate-Dy(iii) complexes, with formulas Dy(ntfa)3(Br-bpy) (1), Dy(ntfa)3(Br2-bpy) (2), Dy(ntfa)3(5,5-(CH3)2-bpy) (3), Dy(ntfa)3(4,4-((CH3)3)2-bpy) (4) and Dy(ntfa)3(4,4-(CH3)2-bpy) (5) (ntfa = 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione, Br-bpy = 5-bromo-2,2'-bipyridine, Br2-bpy = 4,4'-dibromo-2,2'-bipyridine, 5,5-(CH3)2-bpy = 5,5'-di-methyl-2,2'-bipyridine, 4,4-((CH3)3)2-bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, and 4,4-(CH3)2-bpy = 4,4'-di-methyl-2,2'-bipyridine), have been prepared by altering the capping N-donor coligands. Dy(iii) ions in all complexes possess N2O6 octa-coordinated environments, displaying a distorted square antiprismatic D4d symmetry in complexes 1-4, as well as a triangular dodecahedron D2d symmetry in 5. Magnetic investigations evidence the SIM behavior in the five complexes with the energy barriers (Ueff) of 104.19 K (1), 122.93 K (2), 84.20 K (3), 64.16 K (4) and 80.23 K (5) under zero applied dc field. The potential QTM effects in the title complexes are successfully suppressed in the presence of the extra applied fields. The crystal field parameters and orientations of the magnetic easy axes were obtained from the simulation of the magnetic data and the electrostatic model calculation. The distinct electronic effects originating from the subtle changes of the substituents on the capping N-donor coligands induce varying coordination microenvironments and geometries on the Dy(iii) sites, further drastically impacting the overall magnetic properties of the title complexes. The disparities of the uniaxial anisotropy and the magnetic dynamics for 1-5 have been elucidated by ab initio calculations as well.

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.

COMPARATIVE CRYSTAL FIELD ANALYSIS OF THE NI2+ ENERGY LEVEL SCHEME IN CdCl2, CdBr2, CdI2 CRYSTALS

The exchange charge model of crystal field was used to calculate the crystal field splitting of all Ni2+ energy levels (3d8 electron configuration) in three structurally similar crystals of CdCl2, CdBr2, and CdI2. The influence of the nature of the ligands and interionic distances on the values of the crystal field parameters and location of the split energy levels and relative intervals between them was identified. Comparison of the calculated energy levels with the experimental results available in the literature yielded good agreement.

Study of the thermochromic phase transition in CuMo[formula omitted]W[formula omitted]O[formula omitted] solid solutions at the W L[formula omitted]-edge by resonant X-ray emission spectroscopy

Polycrystalline CuMo1−xWxO4 solid solutions were studied by resonant X-ray emission spectroscopy (RXES) at the W L3-edge to follow a variation of the tungsten local atomic and electronic structures across thermochromic phase transition as a function of sample composition and temperature. The experimental results were interpreted using ab initio calculations. The crystal-field splitting parameter Δ for the 5d(W)-states was obtained from the analysis of the RXES plane and was used to evaluate the coordination of tungsten atoms. Temperature-dependent RXES measurements were successfully employed to determine the hysteretic behaviour of the structural phase transition between the α and γ phases in CuMo1−xWxO4 solid solutions on cooling and heating, even at low (x<0.10) tungsten content. It was found that tungsten ions have octahedral coordination for x>0.15 in the whole studied temperature range (90-420 K), whereas their coordination changes from tetrahedral to octahedral upon cooling for smaller (x≤0.15) tungsten content. Nevertheless, some amount of tungsten ions was found to co-exists in the octahedral environment at room temperature for x<0.15. The obtained results correlate well with the color change in these solid solutions.

Quantum Entanglements in mixed-spin XY systems

In order to study the entanglement properties of mixed-spin systems, taking Negativity as a measure of quantum entanglement, it is studied in (1/2, 1) mixed-spin XY systems. It is found that both ferromagnetic or antiferromagnetic cases, the Negativity of systems decreases with increasing temperature and finally changes to zero smoothly. An interesting phenomenon is that the temperature T0 (defined as the characteristic temperature) at which the Negativity becomes zero is low when the value of the crystal field parameter D is large. The relation between the thermal entanglement(T→0) and ground-state entanglement with two and three sites is studied, respectively. It is also found that the mixed-state entanglement and the thermal entanglement at T→0 can be well corresponding in both systems studied; but the Negativity in pure state and that in mixed state are quite different in the system with two sites.

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.

On the incorporation of nickel into hexagonal barium titanate: magnetic properties and electron paramagnetic resonance (EPR)

Systematic measurements of the magnetic moment of hexagonal 6H-BaTiO3 + 0.04 BaO + x NiO (0.005 ≤ x ≤ 0.02) ceramics were performed to study the influence of Ni ions on the magnetic properties. By temperature-dependent measurements of the paramagnetic susceptibility at 90 kOe, the Ni2+ ion was identified as the majority defect in air-sintered Ni-doped hexagonal barium titanate. Q-band EPR investigations of a 2.0 mol% Ni-doped single crystal revealed three different Ni centers located at Ti sites: first, Ni3+ ions at Ti sites in intact oxygen octahedra, second, Ni3+ associated with an oxygen vacancy and third, the presence of Ni2+ centers leading to a forbidden transition. The Ni3+—VO associate characterized by a vacancy in the face-sharing oxygen plane of the oxygen octahedra can be ruled out. The crystal field parameters of the Ni2+ defect were estimated by a combined fitting of the paramagnetic susceptibility and the EPR fine structure parameter D to B_{0}^{4} ≈ −17,300 cm−1, left| {B_{0}^{2} } right| ≈ 2500 cm−1 and B_{3}^{4} ≈ 19,000 cm−1.

Magnetically tuning the loss tangent in La(Al1−xFex)O3 using low field electron paramagnetic resonance transitions

In 2012, we demonstrated that microwave loss in practical microwave dielectrics is dominated by electron paramagnetic resonance transitions at cryogenic temperatures. We later used this understanding to develop “smart” materials that switch Fe-doped Al2O3 (εr = 9.8) dielectric ceramics between a low-loss “on state” and a high-loss “off state” at frequencies of ∼12 and ∼19 GHz with a small magnetic field (&amp;lt;100 G). In this report, we extend our work on smart materials to the large dielectric constant (εr = 24) host La(Al1−xFex)O3 so that it can be used in compact resonator and filter designs operating at ∼4 GHz to ∼7 GHz. The Fe3+ ions' zero-field splitting energies are determined by the crystal-field parameters D = 1.55 GHz and E = 0 GHz, along with significant contributions from the higher-order terms, B40(−6.467 MHz) and B43 (160 MHz). These switchable dielectrics may have applications in future communication and Doppler technology.

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).

Statistical and analytical approaches to finite-temperature magnetic properties of the compound SmFe12

To investigate the magnetic properties of $\mathrm{Sm}{\mathrm{Fe}}_{12}$, we construct an effective spin model, where magnetic moments, crystal-field (CF) parameters, and exchange fields at 0 K are determined by first-principles calculations. Finite-temperature magnetic properties are investigated by using this model. We further develop an analytical method with strong mixing of states with a different quantum number of angular momentum $J$ ($J$-mixing), which is caused by a strong exchange field acting on the spin component of $4f$ electrons. Comparing our analytical results with those calculated by Boltzmann statistics, we clarify that the previous analytical studies for Sm transition-metal compounds overestimate the $J$-mixing effects. The present method enables us to perform a quantitative analysis of the temperature dependence of magnetic anisotropy (MA) with high reliability. The analytical method with model approximations reveals that the $J$-mixing caused by the exchange field increases the spin angular momentum, which enhances the absolute value of the orbital angular momentum and MA constants via spin-orbit interaction. It is also clarified that these $J$-mixing effects remain even above room temperature. Magnetization of $\mathrm{Sm}{\mathrm{Fe}}_{12}$ shows a peculiar field dependence known as the first-order magnetization process (FOMP), where the magnetization shows an abrupt change at a certain magnetic field. The result of the analysis shows that the origin of FOMP is attributed to competitive MA constants between positive ${K}_{1}$ and negative ${K}_{2}$. The sign of ${K}_{1(2)}$ appears due to an increase in the CF potential denoted by the parameter ${A}_{2}^{0}\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}$ (${A}_{4}^{0}\ensuremath{\langle}{r}^{4}\ensuremath{\rangle}$) caused by hybridization between $3d$-electrons of Fe on the $8i$ ($8j$) site and $5d$ and $6p$ valence electrons on the Sm site. It is verified that the requirement for the appearance of FOMP is given as $\ensuremath{-}{K}_{2}&lt;{K}_{1}&lt;\ensuremath{-}6{K}_{2}$.

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.