Optical Spectroscopy and Crystal Field Analysis of Ni²⁺ Centres in Lithium Tetraborate Glass

Abstract

The Ni-doped Li₂B₄O₇ (Li₂O–2B₂O₃) glass was prepared by the conventional melt-quenching method. Structural and spectroscopic properties of the glass were investigated using X-ray diffraction (XRD), optical absorption, and photoluminescence spectroscopy, including excitation, emission, and decay kinetics. The absence of sharp diffraction peaks in the XRD pattern confirms the disordered glassy structure of the investigated glass. The optical absorption spectrum shows three broad bands associated with spin-allowed transitions of Ni²⁺ (3d⁸) ions. The excitation and emission spectra of the Li₂B₄O₇:Ni glass reveal several spin-allowed and spin-forbidden 3d–3d transitions of Ni²⁺ ions. In particular, the Li₂B₄O₇:Ni glass exhibits broad green and red emissions centred at approximately 525 and 687 nm, respectively, as well as weak near-infrared emission bands in the 900–1100 and 1500–1750 nm regions. The analysis of the crystal field splitting for Ni²⁺ ions in the Li₂B₄O₇:Ni glass is carried out, and the Racah parameters (B and C) and the crystal field strength (Dq) are calculated. The Tanabe–Sugano diagram for the octahedrally coordinated Ni²⁺ ions in the Li₂B₄O₇:Ni glass is built and discussed.

1. Introduction

Borate glasses are glassy materials that contain boron oxide (glass former) and various other oxides (glass modifiers). These glasses have interesting optical properties and are used in various applications such as solid-state lighting, laser materials, scintillation detectors, and optical fibres [1,2,3,4]. The preparation of borate glasses is simple and cost-effective compared with complicated methods for growing the corresponding single crystals. Besides this, the properties of borate glasses can be easily modified by doping with rare-earth and/or transition metal impurities. The presence of nickel (Ni) impurity in borate glasses is useful for many technological applications [5,6,7]. In particular, the Ni-doped borate glasses can be used as (1) luminescent materials for solid-state lighting; (2) the active medium in optical fibre amplifiers; (3) the gain medium in laser systems; (4) the transparent layers in photovoltaic devices, such as solar cells, to enhance the efficiency of light absorption.

This work is focused on studies of Ni-doped borate glass with the Li₂B₄O₇ (Li₂O–2B₂O₃) chemical composition, which is equivalent to the well-known lithium tetraborate crystal [8,9,10]. The Li₂B₄O₇:Ni glass can be easily fabricated into a variety of shapes and sizes, which is practically impossible for the crystalline analogue. The spectroscopic terms of transition metal ions, including nickel ions, are split in the crystal field according to the ligand field theory, depending on the symmetry of the local environment of 3d ions. It should be noted that a full understanding of the optical and structural properties of Ni-doped Li₂B₄O₇ glass is necessary to exploit its potential for various optical and electronic applications.

In this work, we use X-ray diffraction (XRD) and optical spectroscopy (absorption and luminescence) techniques to investigate the spectroscopic properties of the Ni-doped Li₂B₄O₇ glass. XRD is a powerful technique for the structural characterisation of a material. In this work, XRD was used to confirm the disordered glassy structure of the investigated material. Optical absorption and luminescence spectroscopy was used to obtain valuable information about the electronic and local structure of nickel ions.

The Ni impurity is typically incorporated into the network of oxide glasses as Ni²⁺ (3d⁸) ions, which can be detected by optical absorption spectroscopy due to the presence of three intense spin-allowed absorption bands in the visible to near-infrared (Vis–NIR) spectral range. The optical spectra of Ni²⁺ ions are dependent on their local surroundings. The four-, five-, and/or six-coordinated Ni²⁺ ions can be observed in glasses depending on the glass composition and the content and nature of modifiers. In particular, Ni²⁺ ions in the tetrahedral geometry (four-fold coordination) have been observed in potassium silicate glasses [11]. The Ni²⁺ ions in the trigonal bipyramidal geometry (five-fold coordination) have been observed in calcium and magnesium silicate and various borosilicate glasses [11,12]. The Ni²⁺ ions in the octahedral geometry (six-fold coordination) are dominant in alkali borate glasses [12,13].

The position of the optical absorption bands of Ni²⁺ ions is dependent on the crystal field strength, which decreases with lowering of coordination number. As a consequence, the optical absorption bands of Ni²⁺ in the tetrahedral configuration are considerably red-shifted compared to those in the octahedral configuration. It has been summarised in [14] that the colour of a glass sample can indicate the coordination number of the oxygen anions around the Ni²⁺ cation. Glasses with Ni²⁺ ions in the four-fold coordination reveal a blue colour because the most intense absorption band is located in the red spectral range [14]. Glasses containing Ni²⁺ ions in the five-fold coordination have an intense brown colour because a very broad absorption band is located in the blue-green spectral range [14]. The most intense absorption band of Ni²⁺ ions in the six-fold coordination is located in the blue spectral region, resulting in a yellow colour for the corresponding glasses [14]. Therefore, the positions and shapes of the Ni²⁺ absorption bands provide valuable information about the coordination geometry and degree of structural distortion surrounding the impurity ion.

Several studies have reported the spectroscopic properties of Ni-doped borate glasses with chemical compositions similar to the investigated glass. For instance, the optical absorption of Ni-doped borate glasses with 20R₂O–80B₂O₃ composition (where R = Li, Na and K) was studied in [15]. The absorption spectrum of the Ni-doped Li₂O–4B₂O₃ glass showed a band at 416 nm, which was assigned to the ³A_2g (F)–³T_1g (P) transition of Ni²⁺ ions in the octahedral coordination [15]. However, article [15] did not examine the optical absorption in the NIR spectral region, perform luminescence studies, and analyse crystal field parameters. Another study [16] reported the optical absorption of Ni-doped sodium zinc borate glass with 20Na₂O–15ZnO–64B₂O₃–1NiO composition. The observed two bands with maxima at 424 nm and 799 nm were assigned to spin-allowed transitions of Ni²⁺ ions in the octahedral field [16]. In bismuth borate glass with 29Bi₂O₃–70B₂O₃–1NiO composition, an absorption band at 423 nm was observed, which was due to the spin-allowed ³A_2g (F) → ³T_1g (P) transition of Ni²⁺ ions in the octahedral geometry [17]. However, analysis of the crystal field splitting of Ni²⁺ ions is missing in [17]. Three broad absorption bands at 425, 793, and 1264 nm assigned to the ³A_2g(³F) → ³T_1g(³P), ³T_lg(³F), and ³T_2g(³F) spin-allowed transitions of Ni²⁺ ions in the octahedral coordination were observed in the lead borate glass with 49.5PbO–50B₂O₃–0.5NiO chemical composition [18]. However, study [18] did not analyse the crystal field or perform detailed luminescence studies.

The available referenced data indicate that the luminescent properties and crystal field splitting in Ni-doped borate glasses have not been thoroughly studied. In particular, a comprehensive spectroscopic description of the Ni-doped Li₂B₄O₇ glass is still lacking. Accordingly, the present work aims to provide a comprehensive structural and spectroscopic characterisation of the Li₂B₄O₇:Ni glass. XRD was employed to examine the glassy structure of the investigated material. UV–Vis–NIR absorption and photoluminescence spectroscopy were used to identify the oxidation state and the electronic transitions of Ni impurity ions. The luminescence decay kinetics were analysed to assess the influence of multiple radiative channels and structural disorder on the excited-state relaxation. Finally, the experimental transition energies were used to determine the crystal-field strength and Racah parameters and to construct a Tanabe–Sugano diagram. The combination of the obtained results provides valuable insight into the electron structure, ligand crystal-field environment, and spectroscopic behaviour of the Ni impurity centres in the Li₂B₄O₇:Ni glass.

2. Materials and Methods

2.1. Preparation of the Ni-Doped Li₂B₄O₇ Glass

The Ni-doped lithium tetraborate glass with the nominal composition Li₂B₄O₇ (or Li₂O–2B₂O₃) was prepared by the conventional melt-quenching method under the technological conditions described in [19]. The starting materials used for the preparation of the Li₂B₄O₇ host composition were lithium carbonate (Li₂CO₃, purity—≥99.9%, Sigma-Aldrich) and boric acid (H₃BO₃, purity—99.97%, Sigma-Aldrich), in amounts of 0.1 and 0.4 mol, respectively. The reagents were weighed in stoichiometric proportions, thoroughly mixed and loaded into a 60 mL corundum crucible. The solid-state synthesis of the Li₂B₄O₇ compound was carried out using the following multi-step chemical reaction:

$$\begin{array}{c}{\mathrm{Li}}_2{\mathrm{CO}}_3{+4\mathrm{H}}_3{\mathrm{BO}}_3{\displaystyle \underset{170°\mathrm{C}}{\overset{{4\mathrm{H}}_2\mathrm{O}\uparrow }{\to }}}{\mathrm{Li}}_2{\mathrm{CO}}_3+4(\alpha \text{-}{\mathrm{HBO}}_2){\displaystyle \underset{250°\mathrm{C}}{\overset{{2\mathrm{H}}_2\mathrm{O}\uparrow }{\to }}}\\ {\mathrm{Li}}_2{\mathrm{CO}}_3{+2\mathrm{B}}_2{\mathrm{O}}_3{\displaystyle \underset{600°\mathrm{C}}{\overset{{\mathrm{CO}}_2\uparrow }{\to }}}x{\mathrm{Li}}_2{\mathrm{B}}_4{\mathrm{O}}_7+(1-x)\left[{\mathrm{Li}}_2{\mathrm{CO}}_3+2{\mathrm{B}}_2{\mathrm{O}}_3\right]{\displaystyle \underset{800°\mathrm{C}}{\overset{{\mathrm{CO}}_2\uparrow }{\to }}}{\mathrm{Li}}_2{\mathrm{B}}_4{\mathrm{O}}_7\end{array}$$

(1)

The nickel impurity was introduced in the form of nickel (II) oxide (NiO, purity—99.99%, Sigma-Aldrich) at a nominal concentration of 1.0 mol.%. The Li₂B₄O₇:Ni glass was obtained by a quick cooling of the corresponding melt. To avoid the crystallisation processes, the melt was heated approximately 100 K above the melting point of Li₂B₄O₇ (T_melt = 917 °C, or 1190 K) [19]. The process of glass preparation was carried out in the air. The resulting glass samples were cut and polished to sizes of approximately 10 × 6 × 2 mm³ for optical absorption and photoluminescence measurements. Meanwhile, small pieces were used for the XRD studies.

2.2. Experimental Techniques

The XRD studies were carried out with usage of a computer-controlled X-ray diffractometer (model DRON-3, NPO “Burevestnik”, Leningrad, USSR) with Cu Kα radiation source. The XRD diffractogram was obtained in the 2θ angle range of 5–145° with a scanning speed of 2.0°/min. The optical absorption spectrum of the Li₂B₄O₇:Ni glass was measured using the Agilent Cary 5000 UV–Vis–NIR spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA). The spectrophotometer includes a double-beam optical system, deuterium and tungsten halogen lamps, a double-grating monochromator, an R928 photomultiplier tube, and a cooled PbS detector.

The luminescence emission and excitation spectra in the UV–Vis spectral range, as well as slow decay kinetics, were registered using the Horiba FluoroMax–4 spectrofluorimeter (HORIBA, Ltd., Kyoto, Japan). The spectrofluorimeter includes a 150 W xenon lamp, excitation and emission grating monochromators, and an R928P photomultiplier tube. The luminescence emission spectra in the NIR spectral region were registered using the Dongwoo Optron DM711 monochromator (Dongwoo Optron Co., Ltd., Gwangju-si, Republic of Korea) and an InGaAs detector (Hamamatsu, Japan). The NIR luminescence was excited using a high-power LED at λ_exc = 445 nm. The fast decay kinetics was registered using an experimental setup containing a Continuum Surelite optical parametric oscillator (OPO) pumped by the third harmonic of a pulsed Nd:YAG laser, a GDM 1000 grating monochromator (Carl Zeiss Jena, Jena, Germany), a Hamamatsu R955 photomultiplier tube (Hamamatsu Photonics K.K., Hamamatsu City, Japan), and a Tektronix MSO44 oscilloscope (Tektronix, Inc., Beaverton, OR, USA). All investigations were performed at room temperature.

3. Results and Discussion

3.1. XRD Characterisation

Figure 1 shows the experimental X-ray diffraction pattern of the Li₂B₄O₇:Ni glass together with the reference diffraction profile of crystalline Li₂B₄O₇ calculated from the crystallographic data reported in [20]. The experimental XRD pattern of the Li₂B₄O₇:Ni glass is characterised by a broad diffuse scattering halo and the absence of sharp Bragg reflections, indicating the absence of detectable long-range crystalline order and confirming the disordered glassy (vitreous) structure of the investigated sample. The position of the main diffuse maximum is consistent with the region of the strongest reflections of crystalline Li₂B₄O₇, suggesting that the short-range structural properties of the glass retain some similarity to those of the crystalline phase.

Figure 1. The experimental XRD profile of the Li₂B₄O₇:Ni glass (black curves (a)) and the diffraction peaks of the Li₂B₄O₇ crystal (blue curves (b)).

3.2. Optical Absorption and Crystal-Field Analysis

The UV–Vis–NIR optical absorption spectrum of the Li₂B₄O₇:Ni glass is shown in Figure 2. The fundamental absorption edge of the glass host is manifested by a steep increase in the absorbance approximately between 370 nm and 320 nm. In addition to the host absorption edge, three broad absorption bands are observed at 428 nm (23,364 cm⁻¹), 815 nm (12,270 cm⁻¹), and 1417 nm (7057 cm⁻¹). The absorption band at 428 nm is the strongest, whereas the absorption band at 1417 nm is the weakest. After careful examination and verification, the observed absorption bands are related to the 3d–3d transitions of Ni²⁺ ions.

Figure 2. Optical absorption spectrum of the Li₂B₄O₇:Ni glass registered in the UV–Vis–NIR spectral range.

The Ni²⁺ ions contain 8 electrons in the unfilled 3d shell and have five LS terms: two spin-triplets (³F and ³P) and three spin-singlets (¹D, ¹G, and ¹S). The relative positions of the spectroscopic terms can be expressed in terms of the Racah parameters B and C [21]. The ^2S+1L terms are split under the influence of the crystal field. In particular, in the octahedral crystal field the ³F ground term splits into the ³A_2g, ³T_2g, and ³T_1g states, the ³P term transforms to the ³T_1g state, the ¹D term splits into the ¹E_g and ¹T_2g states, the ¹G term splits into the ¹A_1g, ¹T_1g, ¹E_g, and ¹T_2g states, and the ¹S term transforms to the ¹A_1g state. The crystal field splitting can be analysed using the Tanabe–Sugano diagram [22], which depicts the dependence of E/B on Dq/B for a fixed C/B ratio, where E is the energy of the crystal field state and Dq is the strength of the crystal field.

The observed three broad absorption bands peaking at 428 nm, 815 nm, and 1417 nm are assigned to the spin-allowed transitions ³A_2g (³F) → ³T_2g (³F), ³A_2g (³F) → ³T_1g (³F), and ³A_2g (³F) → ³T_1g (³P), respectively. The relatively large bandwidths of these transitions are consistent with the distribution of local crystal-field environments expected for Ni²⁺ centres in a structurally disordered glass. Spin-forbidden triplet-singlet transitions are not observed in Figure 2 due to their low intensity caused by the spin-forbidden character. Besides this, the inhomogeneous broadening of spin-allowed absorption bands caused by the disordered structure of the Li₂B₄O₇:Ni glass can hide weak triplet-singlet transitions.

The energies of the three triplet crystal field states can be expressed analytically as functions of the crystal-field parameter Dq and the Racah parameter B by solving the corresponding Tanabe–Sugano matrices. The explicit expressions used in the present analysis are the following:

E(³T_2g(³F)) = 10Dq

(2)

$$E{(}^3{T}_{1g}{(}^{3}F))=15\mathit{Dq}+7.5B-0.5\sqrt{100D{q}^2-180\mathit{DqB}+225B^2}$$

(3)

$$E{(}^3{T}_{1g}{(}^{3}P))=15\mathit{Dq}+7.5B+0.5\sqrt{100D{q}^2-180\mathit{DqB}+225B^2}$$

(4)

Using the energies obtained from the optical absorption spectrum, it is possible to calculate the Dq and B values from different pairs of transitions. It is possible to obtain three sets of Dq and B values using Equations (2) and (3), Equations (2) and (4), and Equations (3) and (4). As a result, slightly different values of the Dq and B parameters were obtained depending on the equations used for the calculation. In particular, Dq = 706 cm⁻¹ was obtained using Equations (2) and (3), and Equations (2) and (4), B = 943 cm⁻¹ was obtained using Equations (2) and (4) as well as Dq = 727 cm⁻¹ and B = 921 cm⁻¹ were obtained using Equations (3) and (4). The differences between the obtained values are expected for a structurally disordered glass host, where the local crystal field is not perfectly described in the framework of an ideal octahedral model.

A weak shoulder near 520 nm is visible on the long-wavelength side of the strongest absorption band at 428 nm. This shoulder can be associated with Ni²⁺ ions in the trigonal bipyramidal geometry (five-fold coordination) according to [11,12]. However, this assignment cannot be considered unambiguous because the shoulder lies close to the intense 428 nm absorption band and may also contain contributions from weak spin-forbidden transitions or from the splitting and broadening of crystal-field states in a distorted octahedral environment. Moreover, additional absorption bands that could be unambiguously assigned to the five-coordinated Ni²⁺ ions were not observed or were hidden by the broad absorption of the six-coordinated Ni²⁺ ions. On the one hand, the attribution of the shoulder around 520 nm to the five-coordinated Ni²⁺ ions has sense, but on the other hand, it can also be associated with several inhomogeneously broadened spin-forbidden transitions of Ni²⁺ ions or with the distortion of the octahedral field (Jahn-Teller effect). Therefore, in the further analysis only the three well-defined absorption maxima related exclusively to the octahedral geometry of the ligand field are considered.

To obtain a single consistent set of crystal-field parameters, the three experimental transition energies were fitted simultaneously using a least-squares procedure. The Dq and B parameters were determined by minimising the root mean square deviation between the experimental energies obtained from the optical absorption spectrum and the theoretical energies calculated from Equations (2)–(4). Using this method, it is possible to obtain Dq = 720.3 cm⁻¹ and B = 928.7 cm⁻¹. Consequently, the Dq/B ratio equals 0.776 and indicates a weak crystal field in the Li₂B₄O₇:Ni glass. A strong crystal field condition usually corresponds to the relation Dq/B > 1.6, in which the first excited crystal field state is ¹E_g (¹D) instead of ³T_2g (³F) for the weak crystal field.

It should be noted that the theoretical energies given by Equations (2)–(4) are non-linearly dependent on the Dq and B parameters. As a result, obtaining uncertainties of the Dq and B values is a very complicated task. Therefore, the Monte Carlo method according to the procedure described in [23] was used to estimate the uncertainties of the fitted parameters. Two hundred sets of simulated transition energies were generated by randomly varying the experimental energies within their respective uncertainties. For each simulated data set, Dq and B were recalculated using the same least-squares fitting procedure as for the experimental data. The resulting distributions of Dq and B were approximately Gaussian, with standard deviations of 7.3 and 4.9 cm⁻¹, respectively. The 95% confidence intervals give Dq = 720 ± 15 cm⁻¹ and B = 929 ± 10 cm⁻¹. The obtained Dq/B = 0.776 ± 0.024 value in the Li₂B₄O₇:Ni glass will be used below for the identification of Ni²⁺ transitions in the luminescence spectra using the Tanabe–Sugano diagram [22].

3.3. Luminescence Excitation and Emission Spectra in the UV–Vis Range

The strongest Ni²⁺ absorption band is at 428 nm; therefore, excitation in the violet-blue spectral region is particularly effective for investigating the luminescence of the Ni-doped Li₂B₄O₇ glass. The photoluminescence excitation and emission spectra of the Li₂B₄O₇:Ni glass are presented in Figure 3. First, a series of emission spectra was registered under excitation at different wavelengths in the violet-blue spectral region in order to determine the excitation wavelength providing the highest emission intensity. The emission spectrum of the Li₂B₄O₇:Ni glass was registered upon excitation at 419 nm and is given by the red curve in Figure 3.

Figure 3. Luminescence excitation (blue dashed curve) and emission (red solid curve) spectra of the Li₂B₄O₇:Ni glass, registered under excitation at λ_exc = 419 nm and by monitoring at λ_mon = 525 nm, respectively.

The emission spectrum exhibits a strong broad band at 525 nm (19,048 cm⁻¹) and a much weaker band near 684 nm (14,620 cm⁻¹). The full width at half maximum (FWHM) of the green emission band equals 77 nm (2787 cm⁻¹). The emission band peaked at 525 nm belongs to the ³T_1g (³P) → ³A_2g (³F) transition of Ni²⁺ ions. It should be noted that the ¹A_1g (¹G) → ³A_2g (³F) and ¹T_2g (¹D) → ³A_2g (³F) spin-forbidden transitions can also contribute to the green emission of the Li₂B₄O₇:Ni glass, as the ¹A_1g (¹G) and ¹T_2g (¹D) states lie in the vicinity of the ³T_1g (³P) state at Dq/B = 0.776. The weak emission band peaked near 684 nm belongs to the ¹E_g (¹D) → ³A_2g (³F) transition of Ni²⁺ ions.

The excitation spectrum of the Li₂B₄O₇:Ni glass given in Figure 3 was registered by monitoring the intensity of the emission peak at 525 nm. The excitation spectrum consists of an intense band at 419 nm, a weak band at 467 nm, a very intense band at 334 nm, and a shoulder near 280–290 nm. The intense band at 419 nm (23,866 cm⁻¹) belongs to the ³A_2g (³F) → ³T_1g (³P) transition of Ni²⁺ ions. The ³A_2g (³F) → ¹A_1g (¹G) transition can also contribute to the band at 419 nm because the ³T_1g (³P) and ¹A_1g (¹G) states are very close at Dq/B = 0.776. The weak band at 467 nm (21,413 cm⁻¹) is related to the ³A_2g (³F) → ¹T_2g (¹D) transition of Ni²⁺ ions. The very intense band at 334 nm cannot be related to nickel impurity, because Ni²⁺ ions in the Li₂B₄O₇:Ni glass do not exhibit any spin-allowed transitions in the UV region within the present crystal-field model. The band at 334 nm agrees well with the fundamental absorption edge of the glass host and can be attributed to the band-to-band transition of the Li₂B₄O₇:Ni glass. The shoulder near 280–290 nm can be assigned to the ³A_2g (³F) → ¹E_g (¹G) and ³A_2g (³F) → ¹T_2g (¹G) transitions of Ni²⁺ ions.

The emission and excitation spectra of the intrinsic luminescence caused by the band-to-band electron-hole recombination are given in Figure 4. The results obtained for the Li₂B₄O₇:Ni glass are very similar to those reported by us for undoped borate glasses in [24]. In particular, the undoped borate glasses with Li₂B₄O₇, LiKB₄O₇, CaB₄O₇, and LiCaBO₃ compositions exhibit a luminescence band in the 360–400 nm spectral region with very fast decay kinetics upon excitation at 310–340 nm [24]. The emission spectrum of the Li₂B₄O₇:Ni glass given in Figure 4 shows the intrinsic host-related emission band at 374 nm and the Ni²⁺ emission band at 525 nm. The presence of Ni²⁺ luminescence under excitation in the host absorption region is consistent with energy transfer from the glass network to the nickel impurity.

Figure 4. Luminescence excitation (blue dashed curve) and emission (red solid curve) spectra associated with the intrinsic luminescence in the Li₂B₄O₇:Ni glass.

In addition to the green luminescence, the Li₂B₄O₇:Ni glass also exhibits a weaker red emission. The excitation spectrum of the ¹E_g (¹D) → ³A_2g (³F) emission band of Ni²⁺ ions in the Li₂B₄O₇:Ni glass is shown in Figure 5. It can be seen that the excitation spectrum of the red emission in Figure 5 is different from the excitation spectrum of the green emission given in Figure 3. The excitation spectrum given in Figure 5 consists of an intense broad band at 296 nm, an intense band at 381 nm, and three weak bands in the spectral range of 405–475 nm. The intense broad band with a maximum at 296 nm (33,784 cm⁻¹) and FWHM of 78 nm (8665 cm⁻¹) corresponds to the ³A_2g (³F) → ¹E_g (¹G) and ³A_2g (³F) → ¹T_2g (¹G) transitions of Ni²⁺ ions. The broad nature of this band can also be caused by the influence of the band-to-band transition of the glass host. The band peaked at 381 nm (26,247 cm⁻¹) with FWHM of 22 nm (1547 cm⁻¹) belongs to the ³A_2g (³F) → ¹T_1g (¹G) transition of Ni²⁺ ions. Finally, the three weak bands in the spectral range of 405–475 nm belong to the ³A_2g (³F) → ³T_1g (³P), ³A_2g (³F) → ¹A_1g (¹G), and ³A_2g (³F) → ¹T_2g (¹D) transitions of Ni²⁺ ions. The relative enhancement of the red emission under UV excitation compared with violet-blue excitation is consistent with the increased contribution of higher-lying singlet states to the excitation pathways of the red emission.

Figure 5. Luminescence excitation (blue dashed curve) and emission (red solid curve) spectra of the Li₂B₄O₇:Ni glass, registered under excitation at λ_exc = 296 nm and by monitoring λ_mon = 687 nm, respectively.

The CIE chromaticity diagram of the Li₂B₄O₇:Ni glass is given in Figure 6. The chromaticity coordinates were determined using the CIE 1931 standard colourimetric system [25] with the 2° standard observer. First, the CIE tristimulus values (X, Y, and Z) were calculated by integrating the spectral data multiplied by the colour-matching functions. Then, the chromaticity coordinates (x, y) were calculated from these tristimulus values. Further details of the calculation procedure can be found elsewhere [26]. The emission colour of the investigated glass under excitation at 419 nm and 296 nm is represented by the symbol “•” marked by (a) and (b), respectively. It can be seen that the emission colour of the Li₂B₄O₇:Ni glass under the UV and violet-blue excitation is quite similar. Only a slight shift from the green to the yellowish-green spectral region can be observed due to the increase in intensity of the red Ni²⁺ emission under UV excitation.

Figure 6. CIE diagram showing the emission colour coordinates of the Li₂B₄O₇:Ni glass upon excitation at 419 nm (marked with “•” and labelled as (a)) and 296 nm (marked with “•” and labelled as (b)).

3.4. Luminescence Emission Spectra in the NIR Range

The NIR emission spectrum of the Li₂B₄O₇:Ni glass registered upon excitation at 445 nm using an LED source is shown in Figure 7. The spectrum consists of two bands in the spectral ranges of 900–1100 nm and 1500–1750 nm. The first band, with a peak at 1020 nm and a shoulder near 940 nm, can be ascribed to the spin-allowed ³T_1g (³F) → ³A_2g (³F) transition. The second band with a main peak at 1677 nm and an additional peak at 1591 nm can be ascribed to the spin-allowed ³T_2g (³F) → ³A_2g (³F) transition.

Figure 7. Luminescence emission spectrum of the Li₂B₄O₇:Ni glass, registered in the NIR spectral range under excitation at λ_exc = 445 nm using an LED source.

Considering the shapes of the absorption and emission bands, one can observe varying degrees of band splitting, which is particularly pronounced for some bands, while others show only weak evidence of this feature. Two main mechanisms may account for this splitting: (1) distortion of the octahedral coordination environment (O_h symmetry) to shortened and/or elongated octahedra with lower D_4h symmetry, and (2) spin–orbit coupling of double (E) and triple (T) degenerate states. Among the observed absorption and emission bands, the NIR emission bands exhibit the most pronounced deviations from a simple Gaussian profile. In our opinion, the shape of the NIR emission bands is associated with distortion of the octahedral coordination environment rather than with spin–orbit splitting. In particular, for the shortened and/or elongated octahedra, the ³A_2g (³F) ground state transforms to the ³B_1g state, the ³T_2g (³F) state splits into the ³E_g and ³B_2g states, and the ³T_1g (³F) state splits into the ³A_2g and ³E_g states. Thus, the considered splitting of the ³T_2g (³F) and ³T_1g (³F) states into two tetragonal states well describes the shape of the NIR emission bands consisting of two spectral components. In the case of spin–orbit splitting, three components are expected for the triply degenerate ³T_2g (³F) and ³T_1g (³F) crystal field states, which do not correlate well with the doublet structure of the observed NIR emission bands. Nevertheless, the contribution of spin–orbit splitting in the Li₂B₄O₇:Ni glass cannot be entirely excluded, taking into account the relatively large spin–orbit coupling constant of Ni²⁺ ions and the possibility that inhomogeneous broadening may obscure the individual spin–orbit components. It is quite possible that the electronic states initially split by the lowering of symmetry may undergo additional splitting due to spin–orbit coupling. Similar to observed NIR emission bands, the splitting or asymmetry observed in some of the other absorption and emission bands may also arise primarily from distortions of the octahedral coordination environment, although contributions from spin–orbit coupling may also be present.

The intensity of the observed luminescence in the NIR spectral region is significantly weaker than that in the visible spectral region. However, a quantitative comparison is very complicated because the luminescence shown in Figure 3, Figure 5 and Figure 7 was registered using different instruments with different detector types. The weaker intensity of the NIR emission can be attributed to the increasing of non-radiative processes caused by multiphonon relaxation.

Near-infrared emission from Ni²⁺ centres in glasses has been reported only in a limited number of studies. An intense Ni²⁺ emission near 1300 nm was observed in glass ceramics containing LiGa₅O₈:Ni²⁺ nanocrystals [27]. The broadband near-infrared emission band with a peak at 1400 nm and a bandwidth exceeding 300 nm attributed to the ³T_2g (³F) → ³A_2g (³F) transition of Ni²⁺ ions was registered in Ni-doped zinc–alumino–silicate glass by the scientists from Japan in [28]. The crystal field strength in the ZnO–Al₂O₃–SiO₂ glass was Dq = 932 cm⁻¹ [28]. This value is higher than the Dq in the Li₂B₄O₇:Ni glass, and consequently the corresponding emission band in [28] has a higher energy in comparison with our result. It was concluded in [28] that Ni-doped glass has the potential to serve as an amplification medium for broadband optical amplifiers and tuneable lasers. In our opinion, further studies are needed in order to evaluate the potential of Ni-doped Li₂B₄O₇ glass for use in the field of optoelectronics.

3.5. Luminescence Decay Kinetics

The luminescence decay kinetics of the green Ni²⁺ emission with a maximum at 525 nm in the Li₂B₄O₇:Ni glass is shown in Figure 8. The decay curve exhibits a pronounced non-exponential character and cannot be adequately described by a single-exponential function. A bi-exponential fit also gives an unsatisfactory description of the experimental data. But, the decay curve is well fitted by a tri-exponential function with lifetimes of 1.6 μs, 22 μs, and 116 μs. The weighted amplitudes (also called fractional or relative amplitudes) of this tri-exponential fit are 0.432, 0.327, and 0.241, respectively. The amplitude-weighted average lifetime (τ_avg) was calculated using the following formula:

$${\tau }_{\mathrm{avg}}={\displaystyle \frac{{\displaystyle \sum _i{A}_i{\tau }_i}}{{\displaystyle \sum _i{A}_i}}},$$

(5)

where A_i is the amplitude of the τ_i lifetime. The evaluated τ_avg lifetime equals 36 μs.

Figure 8. Luminescence decay kinetics of the green Ni²⁺ emission in the Li₂B₄O₇:Ni glass. The experimental decay curve and tri-exponential fit are shown together with the corresponding distribution of luminescence lifetimes in the inset.

The pronounced non-exponential decay behaviour of the green emission band may be attributed to several factors: (1) contributions from different Ni²⁺ electronic transitions with different radiative and non-radiative relaxation rates; (2) local symmetry lowering, which lifts the degeneracy of Ni²⁺ electronic states and may lead to their splitting; (3) the distribution of local coordination environments in the glass network. In our opinion, all three decay components of the tri-exponential fit are primarily associated with the spin-allowed ³T_1g (³P) → ³A_2g (³F) transition. The nearly spectrally located spin-forbidden ¹A_1g (¹G) → ³A_2g (³F) and ¹T_2g (¹D) → ³A_2g (³F) transitions may affect the measured decay kinetics, causing the lifetime of the ³T_1g (³P) → ³A_2g (³F) transition to be longer. Nevertheless, it is not possible to unambiguously assign individual components of the tri-exponential fit to these spin-forbidden transitions. Regarding the effect of local symmetry lowering, distortion of the ideal octahedral coordination environment through shortening or elongation may reduce the symmetry from O_h to D_4h. This distortion can lift the degeneracy of the ³T_1g (³P) emitting state, resulting in two tetragonal states, ³A_2g and ³E_g. Their potentially different relaxation rates may give slightly different decay components. In addition, variations in Ni–O bond lengths and local crystal-field strength in the distorted octahedra can produce a distribution of relaxation rates and, consequently, a distribution of lifetimes.

Figure 9 shows the luminescence decay kinetics of the red Ni²⁺ emission peaked at 687 nm in the Li₂B₄O₇:Ni glass. The decay curve is also non-single exponential, but its deviation from single-exponential behaviour is considerably weaker than that observed for the green Ni²⁺ emission. The decay curve was well fitted by a bi-exponential function with lifetimes of 485 μs and 2465 μs. The weighted amplitudes of this bi-exponential fit are 0.312 and 0.688, respectively. The amplitude-weighted average lifetime calculated using the Formula (5) equals 1847 μs. The red Ni²⁺ emission is assigned to the spin-forbidden ¹E_g (¹D) → ³A_2g (³F) transition. Its decay is more than 50 times slower than that of the green emission, which is dominated by the spin-allowed ³T_1g (³P) → ³A_2g (³F) transition.

Figure 9. Luminescence decay kinetics of the red Ni²⁺ emission in the Li₂B₄O₇:Ni glass. The experimental decay curve and bi-exponential fit are shown together with the corresponding distribution of luminescence lifetimes in the inset.

Since there are no other crystal field states located near the ¹E_g (¹D) state at Dq/B = 0.776 in the Li₂B₄O₇:Ni glass, the contribution of other electronic transitions to the kinetics of the red Ni²⁺ emission is insignificant. The non-exponential decay behaviour of the red emission may be associated with local symmetry lowering and the distribution of local coordination environments in the glass network. In a tetragonally distorted octahedral coordination environment, the ¹E_g (¹D) emitting state splits into the ¹A_1g and ¹B_1g states. Transitions from these tetragonal states may exhibit slightly different relaxation rates, potentially giving two decay components with slightly different lifetimes. In addition, variations in bond lengths and local crystal-field strength in the distorted octahedra formed by Ni²⁺ ions and O²⁻ ligands can lead to a distribution of relaxation rates and, consequently, to a distribution of observed lifetimes.

The luminescence lifetimes of the NIR emission bands in the Li₂B₄O₇:Ni glass were found to be in the microsecond range, but the low emission intensity of the NIR luminescence did not allow registration of decay curves of good quality.

To obtain additional information on the distribution of decay kinetics, a continuous lifetime-distribution analysis was performed. It was assumed that the total emission is the result of a superposition of luminescence from a considerable number of Ni²⁺ luminescence centres, each localised in sites with slightly different local surroundings. Besides this, the continuous distribution of the local crystal field in the Li₂B₄O₇:Ni glass causes a continuous distribution of the lifetimes. The calculated emission intensity can be expressed as follows [29]:

$$I(t)={\displaystyle \int \frac{A(\tau )}{\tau }}{e}^{-{\displaystyle \frac{t}{\tau }}}\mathit{d\tau }={\displaystyle \int A(\tau )}{e}^{-{\displaystyle \frac{t}{\tau }}}d(\mathit{ln\tau }),$$

(6)

where A(τ) is the distribution function of lifetime τ. The continuous lifetime distribution was approximated by a discrete set of logarithmically spaced lifetimes in order to go from integration to summation and to facilitate the calculation. The amplitudes A_i(τ_i) associated with logarithmically spaced lifetimes were determined by fitting the calculated decay curve to the experimental data using a least-squares procedure.

The lifetime distributions obtained for the green and red Ni²⁺ emission are given as insets in Figure 8 and Figure 9, respectively. The obtained distributions support the appropriateness of the tri- and bi-exponential fitting models for the green and red Ni²⁺ emissions, respectively. The maxima of the distributions correlate very well with the corresponding fitted lifetimes. From the obtained lifetime distribution of the green Ni²⁺ emission, it can be seen that the luminescence lifetimes are distributed in the time range of 0.7–700 μs. Such wide lifetime distribution is caused by three effects: first, the green emission band contains contributions from both spin-allowed and spin-forbidden Ni²⁺ transitions with different relaxation rates; second, local symmetry lowering causes splitting of Ni²⁺ octahedral states into tetragonal states with potentially slightly different relaxation rates; third, structural disorder of the glass produces a distribution of local crystal-field environments and, consequently, a distribution of Ni²⁺ relaxation rates. For the red emission, the lifetime distribution extends approximately from 200 to 6000 μs and is substantially narrower than that obtained for the green emission. Since the red emission is caused by the single spin-forbidden ¹E_g (¹D) → ³A_2g (³F) transition, this distribution is attributed to the local symmetry lowering and positional disorder of the glass network, which produces a distribution of Ni²⁺ relaxation rates.

3.6. Tanabe–Sugano Diagram of Ni²⁺ Ions in the Li₂B₄O₇:Ni Glass

The Tanabe–Sugano diagram describes the dependence of the state energies on the crystal-field strength as the relationship between E/B and Dq/B for a fixed C/B ratio. For the Li₂B₄O₇:Ni glass, the analysis of the optical absorption spectrum give Dq = 720 ± 15 cm⁻¹, B = 929 ± 10 cm⁻¹, and Dq/B = 0.776 ± 0.024. In order to build the Tanabe–Sugano diagram, the C parameter and the C/B ratio must also be determined. The C parameter cannot be calculated from the positions of the spin-allowed transitions used to obtain Dq and B, because their energies are independent of C (see Equations (2)–(4)). To find the C parameter, the energy of a spin-forbidden transition must be used. Obtaining the energy from the emission spectrum is less reliable because the emission energy can be affected by the Stokes shift between absorption and emission. Consequently, obtaining the energy from the excitation spectrum is a better option. Analysis of the excitation spectra given in Figure 3 and Figure 5 suggests that the ³A_2g (³F) → ¹T_1g (¹G) transition is the best candidate for calculation of the C parameter. The corresponding band with a maximum at 381 ± 1 nm (26,247 ± 69 cm⁻¹) has a relatively high intensity and does not overlap with other identified Ni²⁺ transitions. From the Tanabe–Sugano theory, the theoretical energy of the ¹T_1g (¹G) state can be analytically determined to be equal to:

E(¹T_1g(¹G)) = 10Dq + 12B + 2C.

(7)

As a result, the Racah parameter C and the C/B ratio in the Li₂B₄O₇:Ni glass equal 3950 ± 170 cm⁻¹ and 4.25 ± 0.23, respectively. The uncertainties were calculated using the law of uncertainty propagation.

It is important to compare the obtained Racah parameters B = 929 ± 10 cm⁻¹ and C = 3950 ± 170 cm⁻¹ with the corresponding values B₀ and C₀ for free Ni²⁺ ions. According to reference [30], free Ni²⁺ ions exhibit B₀ = 1030 cm⁻¹ and C₀ = 4850 cm⁻¹. The B/B₀ and C/C₀ ratios equal 0.9019 ± 0.0097 and 0.814 ± 0.035, respectively, and indicate a reduction in the interelectronic repulsion parameters in the glass relative to the free Ni²⁺ ion. The obtained β = B/B₀ ratio is typical for oxygen ligands in the context of the nephelauxetic effect. It should be noted that in [31] slightly different values B₀ = 1068 cm⁻¹ and C₀ = 4457 cm⁻¹ for free Ni²⁺ ions were reported. In this case, the ratios B/B₀ = 0.8699 ± 0.0094 and C/C₀ = 0.886 ± 0.038 are very similar to each other. Although the absolute ratios depend on the selected free-ion reference values, both sets of parameters indicate a reduction in the Racah parameters relative to the free-ion case.

The Tanabe–Sugano diagram for Ni²⁺ ions in the Li₂B₄O₇:Ni glass plotted for C/B = 4.25 is shown in Figure 10. This diagram allows us to find theoretical E/B values (see Table 1) at the obtained above Dq/B = 0.776. The uncertainty of the E/B ratio was calculated individually for each state from the Tanabe–Sugano diagram plotted, taking into account the uncertainty of the C/B and Dq/B ratios. The dimensionless energies (E/B) of the relevant crystal-field states can be converted into absolute energies E and wavelengths λ using the value of B.

Figure 10. The Tanabe–Sugano diagram for Ni²⁺ ions in the Li₂B₄O₇:Ni glass calculated for C/B = 4.25 and plotted in the wide (A) and narrow (B) ranges of the Dq/B values. The vertical position corresponding to the experimentally determined Dq/B = 0.776 is used to identify the relevant Ni²⁺ crystal-field states and transition energies.

Table 1. The observed wavelength (λ_obs) and energy (E_obs) of the transitions between the ground and excited states, as well as the calculated relative energy (E/B), absolute energy (E_cal), and wavelength position (λ_cal) of the excited Ni²⁺ crystal field states in the Li₂B₄O₇:Ni glass evaluated using the Tanabe–Sugano model with Dq/B = 0.776 and C/B = 4.25. The superscripts ^(abs), ^(exc), and ^(em) denote transitions observed in the absorption, excitation, and emission spectra, respectively.

Crystal Field State	λobs [nm]	Eobs [cm−1]	E/B	Ecal [cm−1]	λcal [nm]
1A1g (1S)	—	—	62.67 ± 1.94	58,200 ± 2420	172 ± 7
1T2g (1G)	296 ± 1 (exc)	33,784 ± 114 (exc)	34.22 ± 0.92	31,780 ± 1190	315 ± 12
1Eg (1G)	—	—	33.72 ± 0.92	31,310 ± 1190	319 ± 12
1T1g (1G)	381 ± 1 (exc)	26,247 ± 69 (exc)	28.26 ± 0.70	26,240 ± 930	381 ± 13
3T1g (3P)	428 ± 1 (abs)	23,364 ± 55 (abs)	25.17 ± 0.54	23,380 ± 750	428 ± 14
1A1g (1G)	—	—	25.10 ± 0.42	23,310 ± 640	429 ± 12
1T2g (1D)	467 ± 1 (exc)	21,413 ± 46 (exc)	23.06 ± 0.73	21,410 ± 900	467 ± 20
1Eg (1D)	687 ± 1 (em)	14,556 ± 21 (em)	15.80 ± 0.48	14,680 ± 600	681 ± 28
3T1g (3F)	815 ± 1 (abs)	12,270 ± 15 (abs)	13.11 ± 0.37	12,170 ± 470	821 ± 32
3T2g (3F)	1417 ± 1 (abs)	7057 ± 5 (abs)	7.76 ± 0.24	7210 ± 300	1388 ± 58

The calculated theoretical energies and wavelengths of all crystal field states of Ni²⁺ ions are listed in Table 1. The uncertainty of theoretical energies and wavelengths was calculated using the law of uncertainty propagation. Table 1 also presents the energies and wavelengths of Ni²⁺ transitions observed in the Li₂B₄O₇:Ni glass. One can notice a quite good correlation between the observed and calculated wavelengths. Some deviations can be related to the distortion of octahedral geometry in the Li₂B₄O₇:Ni glass.

The Ni²⁺ ions occupy the Li⁺ sites of the Li₂B₄O₇ glass. The ionic radius of the B³⁺ ions is very small, being equal to 0.01 Å and 0.11 Å for three- and four-coordinated B³⁺ ions, respectively [32]. The ionic radii of Li⁺ and Ni²⁺ in the six-fold coordination are quite similar and equal approximately 0.76 Å and 0.69 Å [32], respectively, making substitution of Li⁺ by Ni²⁺ structurally plausible. The statement about the incorporation of Ni²⁺ ions into the Li⁺ well agrees with work [33], in which it was revealed by extended X-ray absorption fine structure (EXAFS) spectroscopy that rare-earth ions occupy the Li⁺ sites in the Li₂B₄O₇ glass. The excess positive charges resulting from Ni²⁺ → Li⁺ heterovalent substitution in the investigated glass can be compensated by cationic vacancies as a characteristic feature of the glass network.

Based on the obtained results of optical spectroscopy, it is possible to state that the octahedral geometry around Ni²⁺ ions in the Li₂B₄O₇:Ni glass is significantly distorted. In particular, the considerable spread of Li–O interatomic distances due to the positional disorder of the glass network manifests itself in the formation of distorted octahedra. This distortion is evident in the non-Gaussian shape with an additional weakly observed splitting of some observed optical absorption and emission bands, as well as the non-single-exponential luminescence decay kinetics of the Ni²⁺ emission.

The comparison of the experimentally observed wavelengths of the spin-allowed Ni²⁺ transitions in the optical absorption spectrum, the calculated Racah parameters, and the crystal field strength in the investigated Li₂B₄O₇:Ni glass and materials with similar composition is presented in Table 2. The values obtained in the present study are in good agreement with those reported previously for Ni-doped borate glasses [15,16,17,18,34,35,36,37]. It should be noted that Refs. [15,16,17,18,35] report only the results of optical absorption without the analysis of crystal field splitting or determination of the corresponding Racah parameters, which underlines the significance of this work. Hence, the results of this comprehensive study provide valuable information about the structural, optical, and luminescent properties as well as the crystal field analysis of the Ni-doped Li₂B₄O₇ glass.

Table 2. The comparison of the observed wavelengths (λ₁, λ₂, λ₃) of the ³A_2g (³F) → ³T_1g (³P), ³A_2g (³F) → ³T_1g (³F), and ³A_2g (³F) → ³T_2g (³F) spin-allowed transitions in the optical absorption spectrum, the calculated Racah parameters (B and C) and the crystal field strength (Dq) determined for the Li₂B₄O₇:Ni glass and reported for selected Ni-doped materials. A dash (—) indicates that the corresponding parameter was not reported or could not be determined from the cited study.

Composition	λ1 [nm]	λ2 [nm]	λ3 [nm]	B [cm−1]	C [cm−1]	Dq [cm−1]	Reference
Li2B4O7:NiO(1 mol.%) or 33Li2O–66B2O3–1NiO glass	428 ± 1	815 ± 1	1417 ± 1	929 ± 10	3950 ± 170	720 ± 15	this work
19.537Li2O–80B2O3–0.463NiO glass	416	—	—	—	—	—	[15]
20Na2O–15ZnO–64B2O3–1NiO glass	424	799	—	920	—	743	[16]
29Bi2O3–70B2O3–1NiO glass	423	—	—	—	—	—	[17]
49.5PbO–50B2O3–0.5NiO glass	425	793	1264	—	—	—	[18]
LiF–B2O3 glass with 1 mol.% Ni(II)	440	840	1471	925	3790	680	[34]
30Li2O–10BaO–59.6B2O3–0.4NiO glass	443	810	1310	—	—	—	[35]
69.5B2O3–25Na2O–5PbO–0.5NiO glass	428	808	—	912	—	715.5	[36]
β-BaB2O4:Ni(6.86wt.%) nanopowder	400	685	1100	850	3500	900	[37]

4. Conclusions

Based on the analysis of the obtained experimental results, the following conclusions can be proposed:

•

The Li₂B₄O₇ (Li₂O–2B₂O₃) glass doped with 1.0 mol.% NiO was successfully synthesised by the conventional melt-quenching method and has been studied in detail using XRD, optical absorption, and photoluminescence (emission and excitation spectra, decay kinetics) techniques.

•

The absence of sharp crystalline diffraction peaks in XRD pattern confirms the disordered glassy (vitreous) structure of the investigated material.

•

The optical absorption spectrum exhibits three broad bands at 428 nm, 815 nm, and 1417 nm corresponding to the spin-allowed ³A_2g (³F) → ³T_2g (³F), ³A_2g (³F) → ³T_1g (³F), and ³A_2g (³F) → ³T_1g (³P) transitions of Ni²⁺ (3d⁸) ions in the ligand field of octahedral geometry.

•

In the luminescence spectra, the green emission band with a maximum at 525 nm was assigned to the ³T_1g (³P) → ³A_2g (³F) transitions with possible contributions from nearby spin-forbidden ¹A_1g (¹G) → ³A_2g (³F), and ¹T_2g (¹D) → ³A_2g (³F) transitions, whereas the red emission band with a maximum at 687 nm (14,556 cm⁻¹) was ascribed to the ¹E_g (¹D) → ³A_2g (³F) transition of Ni²⁺ ions. Additionally, two weak emission bands related to the ³T_1g (³F) → ³A_2g (³F) and ³T_2g (³F) → ³A_2g (³F) transitions were observed in the spectral ranges of 900–1100 nm and 1500–1750 nm, respectively.

•

The excitation spectra of the Li₂B₄O₇:Ni glass reveal several spin-allowed and spin-forbidden 3d–3d transitions of Ni²⁺ ions.

•

The luminescence decay curves of the green and red emission bands are non-single-exponential and are characterised by lifetimes distributed over a relatively wide time range, which is caused by the superposition of Ni²⁺ transitions, local symmetry lowering, as well as the positional disorder of the Li₂B₄O₇:Ni glass network.

•

The octahedral geometry around Ni²⁺ ions in the Li₂B₄O₇:Ni glass is distorted due to the positional disorder of the glass network. The non-Gaussian shape of some observed optical absorption and emission bands, as well as deviations between the observed and calculated energies of some transitions, are related to the distorted octahedral ligand field.

•

Analysis of the crystal field splitting for Ni²⁺ ions in the Li₂B₄O₇:Ni glass was carried out, and the Racah parameters and the crystal field strength were calculated to be B = 929 ± 10 cm⁻¹, C = 3950 ± 170 cm⁻¹, and Dq = 720 ± 15 cm⁻¹.

•

The Tanabe–Sugano diagram for Ni²⁺ ions in the Li₂B₄O₇:Ni glass was built, and quite good agreement between the observed and calculated wavelengths of the Ni²⁺ transitions was obtained.

•

Overall, the valuable information gathered in this research is useful for understanding the structural and optical properties of Ni²⁺ ions in borate glasses.