Synthesis, structural refinement and luminescence properties of green-emitting Gd₃GaO₆:Tb³⁺ phosphors for display applications

Abstract

Green-emitting Tb³⁺-doped Gd₃GaO₆ phosphors were fabricated through a low-temperature solution-combustion approach utilizing urea as the fuel. Powder XRD analysis suggested an orthorhombic structure with the Cmc2₁ space group. Crystallite size was determined using the Scherrer's formula and W–H plot. Surface topography was examined utilizing the FESEM technique, and EDX data confirmed the uniform incorporation of Tb³⁺ ions into the Gd₃GaO₆ lattice with proper stoichiometry. A bright green emission was exhibited by phosphors at 543 nm, attributed to the ⁵D₄ → ⁷F₅ transition upon irradiation with near ultraviolet light at 272 nm. The observed concentration quenching at 3 mol% Tb³⁺ in the Gd₃GaO₆ matrix was attributed to non-radiative energy transfer between the neighbouring Tb³⁺ ions, which became significant at higher doping levels. The quenching observed beyond the optimum concentration is primarily attributed to dipole–dipole interactions between adjacent Tb³⁺ ions. The CIE chromaticity coordinates are located the green region, and the calculated CCT value (4500–5500 K) lies in the cool temperature range, making this phosphor a suitable candidate for cool white-solid state lighting and display devices.

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1. Introduction

In the past decade, inorganic phosphors with tunable luminescent colors have attracted considerable attention owing to their wide applications in bioimaging, color display panels, light-emitting diodes (LEDs), optical thermometry and white light-emitting diodes (w-LEDs).^1–3 Recently, conventional fluorescent and incandescent lamps have been largely replaced by w-LEDs due to their eco-friendly nature, cost-effective production, compactness, high luminescence efficiency, long lifetime, excellent chemical and thermal stability and energy savings of approximately 70% compared with traditional sources.^4,5 For the synthesis of phosphors with desired applications, the suitable choice of a host lattice is very important. Gallates have emerged as promising host lattices for rare-earth ion doping because of their eco-friendly composition, structural stability, excellent optical properties and high resistance to oxygen, acids and bases.^6–9 Rare-earth-doped gallate matrices have thus found applications in waveguides, optical fibers, video displays, smart windows and control panels.^10–13

In particular, Gd₃GaO₆ stands out as an attractive host for phosphor synthesis. It possesses a relatively narrow band gap (E_g ≈ 3.60 eV) compared with other rare-earth gallates, such as RE₃GaO₆ (RE = Nd, Sm, and Eu), enabling efficient absorption and energy transfer. The orthorhombic structure (space group Cmc2₁) provides suitable cation sites for rare-earth substitution, and the material exhibits excellent thermal stability with no structural phase transitions over a wide temperature range (298–1273 K) along with favorable thermophysical properties.¹⁴ A key advantage is the presence of Gd³⁺ ions, which can serve as effective sensitizers by transferring energy from their excited ⁶P_7/2 state to dopant ions, enhancing luminescence efficiency and overcoming limitations associated with weak direct absorption in the far-UV region.^15,16

Trivalent lanthanide ions are preferred activators due to their sharp emission bands in the visible region arising from the 4f–4f transitions. By varying the dopant type and concentration, multicolor luminescence can be achieved.^17–19 Among these, the Tb³⁺ ion is a well-established green emitter, primarily due to the magnetic dipole-allowed ⁵D₄ → ⁷F₅ transition at ∼543 nm, which offers bright green luminescence ideal for w-LEDs, display backlights and optoelectronic devices. Tb³⁺ ions may also show blue emission from the ⁵D₄ → ⁷F₆ transition at ∼490 nm, but the relative intensity of this component depends on the host crystal field, Tb³⁺ ion concentration and phonon energy, which influence cross-relaxation and non-radiative processes.^20,21 While Tb³⁺-doped phosphors are common in hosts like garnets, silicates or phosphates, many suffer from issues such as thermal quenching, concentration quenching at high doping levels or insufficient color purity due to residual blue emission. Although Gd₃GaO₆ has been explored as a host for other activators, primarily Bi³⁺ (for persistent luminescence) and Er³⁺ (for upconversion or color-tunable emission), systematic investigations of Tb³⁺-activated Gd₃GaO₆ remain limited or absent in the literature.²² Tb³⁺ doping in Gd₃GaO₆ is interesting due to sensitization by the host Gd³⁺ ion, moderate band gap, low phonon energy and orthorhombic low-symmetry sites could promote intense, high-purity green emission. Such a phosphor could offer advantages for w-LEDs and displays, including better device integration, reduced reabsorption and improved performance under operational conditions.

Traditional solid-state synthesis of inorganic phosphors often involves high-temperature treatments and repeated grinding, which can damage particle surfaces, introduces defects and reduces emission intensity.²³ To address these limitations and produce uniform, high-efficiency nanophosphors, various wet-chemical methods have been developed including sol–gel, microwave-assisted, hydrothermal, precipitation and solution combustion synthesis.^24–27 The solution combustion method is particularly advantageous for Tb³⁺-doped Gd₃GaO₆, as it yields homogeneous products with large surface area, high purity, nanoscale morphology and short reaction times.²⁸ The present work, therefore, focuses on the synthesis of Tb³⁺-doped Gd₃GaO₆ nanophosphors via solution combustion. The structural and surface properties were characterized using XRD and FESEM, compositional analysis was performed by EDX and photoluminescence properties were investigated at varying dopant concentrations. These results aim to demonstrate the potential of Gd₃GaO₆:Tb³⁺ as an intense green-emitting phosphor for solid-state lighting and display applications.

2. Experimental

A series of Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) powdered phosphors was prepared by a facile solution combustion method. The raw materials used for the Gd, Ga and Tb sources were gadolinium nitrate [Gd(NO₃)₃·6H₂O], gallium nitrate [Ga(NO₃)₃·xH₂O] and terbium nitrate [Tb(NO3)₃·6H₂O], respectively. The precursors utilized in the solution combustion method were purchased from Sigma-Aldrich. The stoichiometric amount of metal nitrates and urea (organic fuel) was dissolved in a minimum amount of double-distilled water to obtain a homogeneous clear solution. This solution was further heated on a heating plate with uniform stirring and the temperature was maintained at 80 °C to form a gelatinous mixture. The entire solution was transferred to a crucible and placed in a muffle furnace set at 600 °C, where the resulting mixture underwent exothermic reaction, followed by decomposition and evolution of various gases. The crucible was drawn out of the furnace and the obtained voluminous foamy product was manually ground to a fine powder using an agate mortar and pestle. The samples were further calcined in an alumina crucible at 1450 °C for 4 h to improve crystallinity. Final products are brought to ambient temperature, crushed and used for characterizations. The preparation scheme of this method is illustrated in Fig. 1.

Fig. 1 Representation of the solution combustion synthesis employed in the nanophosphor fabrication.

3. Characterization techniques

Powder X-ray diffraction profiles of the phosphor material were analyzed to determine phase purity by using a Rigaku Ultima-IV X-ray diffractometer equipped with a Cu-Kα source and nickel filter. Scanning was performed between 10°–70°, employing a 0.02° step size and scanning speed of 2° per minute. Microstructural and surface characteristics of the particles were examined by FESEM. The micrographs were taken on a Carl Zeiss Sigma 360 instrument. An Ametek EDX spectrometer was employed for confirming the elemental composition of the sample. Photoluminescence spectral data were recorded on a Horiba-Jobin Fluorolog spectrophotometer using an Xe lamp as an excitation source.

4. Results and discussion

4.1 XRD measurements

Phase purity and crystallinity of pure and Tb³⁺-doped gadolinium gallium oxide samples were analyzed by means of XRD patterns. Fig. 2(a) shows the diffraction peaks of undoped Gd₃GaO₆ and the Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) phosphors. XRD data confirmed that all peaks were indexed to the orthorhombic phase, consistent with standard JCPDS card number 53–1225, and no additional peaks were present, confirming the non-existence of an impurity phase.²⁹ Fig. 2(b) shows an enlarged view of diffraction peaks, which revealed peak shifting toward higher 2θ values with increasing Tb³⁺ concentration.

Fig. 2 (a) XRD profiles of Gd₃GaO₆ and Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) and their (b) magnified view, highlighting key features of all the samples.

These results were consistent with lattice compression on doping smaller Tb³⁺ ions. Radius difference percentage (Δr) was evaluated to confirm the replacement of the Gd³⁺ ion with the Tb³⁺ ion by eqn (1).³⁰

Here, R_B(CN) = host cation radius, and R_D(CN) = dopant ion radius. The acceptable value of Δr for the successful inclusion of a particular cation should be lower than 15%, as reported in the literature.³¹ In the present study, the value of Δr comes out to be 2% by substituting the radius values for Gd³⁺ (R_B = 1 Å with CN = 7) and Tb³⁺ (R_D = 0.98 Å with CN = 7) ions. This suggests the Tb³⁺ ion successfully replaced the Gd³⁺ ion without altering the crystal structure of the GGO host.

Furthermore, replacement of a bigger host cation by a smaller Tb³⁺ ion was supported by calculations of interplanar d-spacing values determined from the main XRD peak corresponding to various Tb³⁺ doping levels using Bragg's eqn (2).³²

nλ = 2d sin θ (2)

The computed d-spacing values are summarized in Table 1. Crystallite size (D) was estimated by employing the Debye Scherrer eqn (3) based on XRD broadening.³³

In this formula, λ represents the wavelength of Cu-Kα radiations (1.54 Å), β refers to peak broadening (FWHM) and θ denotes Bragg's angle.

Table 1 XRD peak positions (2θ values) and corresponding interplanar spacings (d-values) of the Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) phosphors

Sample	2θ	d-Spacing (Å)
GGO:0 mol% Tb^3+	30.2062	2.9564
GGO:1 mol% Tb^3+	30.2416	2.9530
GGO:2 mol% Tb^3+	30.2633	2.9509
GGO:3 mol% Tb^3+	30.2952	2.9479
GGO:4 mol% Tb^3+	30.3052	2.9469
GGO:5 mol% Tb^3+	30.3269	2.9449
GGO:6 mol% Tb^3+	30.3491	2.9441

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Crystallite size values for the host and Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) samples were evaluated and are presented in Table 2. Additionally, the Williamson–Hall (W–H) approach was utilized for comprehensive estimation of crystallite size using relation (4).³⁴

Table 2 Computed XRD parameters (crystallite size and dislocation density) of the synthesized Gd_3−xGaO6:xTb³⁺ (x = 1–6 mol%) phosphors

Sample (GGO)	2θ	FWHM	Crystallite size (nm)		Dislocation density
			Scherrer's	W–H
GGO:0 mol% Tb^3+	30.2062	0.34724	24.75	41.25	1.63 × 10^−3
GGO:1 mol% Tb^3+	30.2416	0.30321	28.35	43.73	1.24 × 10^−3
GGO:2 mol% Tb^3+	30.2633	0.27125	31.69	46.14	9.96 × 10^−4
GGO:3 mol% Tb^3+	30.2952	0.37221	23.09	38.36	1.88 × 10^−3
GGO:4 mol% Tb^3+	30.3052	0.40812	21.06	35.15	2.25 × 10^−3
GGO:5 mol% Tb^3+	30.3269	0.32194	26.70	42.64	1.40 × 10^−3
GGO:6 mol% Tb^3+	30.3487	0.29326	29.32	44.91	1.16 × 10^−3

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This expression can be further modified as eqn (5).³⁵

In this context, K = constant, whose value depends on the shape of particles, ε = microstrain and all other variables have their common meanings as described earlier. Fig. 3 illustrates a linear fitted W–H plot with 4 sin θ plotted on the x-axis and β cos θ along the y-axis.

Fig. 3 Williamson–Hall (W–H) plots for the Gd₃GaO₆ host and Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) phosphors.

Crystallite sizes for different doped samples and the host, calculated from the intercept of the linear fitted data, are given in Table 2. Slight variation in crystallite size calculated from Scherrer's formula and W–H plot arises from the assumption of Scherrer's formula, where the strain component was taken as zero and peak broadening is solely attributed to grain size. Dislocation density represented as δ was also calculated for various samples by employing eqn (6), and values are given in Table 2.³⁶

4.2 Rietveld refinement and crystal structure

To further confirm the single-phase purity of the prepared phosphors, Rietveld refinement analysis was executed. The lattice constraints of representative samples were assessed via the FULLPROF software. Fig. 4(a) and (b) represent refinement plots of the undoped GGO host and the optimized Tb³⁺-doped GGO sample, respectively.

Fig. 4 Rietveld-refined XRD profiles of the (a) Gd₃GaO₆ and (b) Gd_2.97GaO₆:3 mol% Tb³⁺ samples.

The observed and calculated data are shown as black crosses and red/green solid lines, respectively. Vertical (brown) lines represent the Bragg's position and the difference between observed and calculated data is indicated at the bottom by a blue line in each figure. Refinement data for Gd₃GaO₆ and Gd_3−xGaO₆:xTb³⁺ (x = 3 mol%) are arranged in Table 3. The reliable factors for the GGO host are R_p = 4.192%, R_wp = 5.223% and χ² = 2.104 and for Gd_2.97GaO₆:3 mol% Tb³⁺ sample are R_p = 4.482%, R_wp = 5.613% and χ² = 2.248. As the reliability factors lie within the 10% range, signifying strong agreement between theoretical and experimental patterns.³⁷ The measured cell parameters for the optimized sample (a = 8.8742 Å, b = 11.3259 Å, c = 5.4636 Å, α = β = γ = 90° and V = 549.1371 ų) are slightly below the standard host values.

Table 3 Rietveld refinement constraints of Gd₃GaO₆ (GGO) and the Gd_2.97GaO₆:3 mol% Tb³⁺ phosphors

Refinement parameters	Refinement values of Gd3GaO6	Refinement values of Gd2.97GaO6:3 mol% Tb^3+
Formula weight	637.47	637.07
Crystal system	Orthorhombic	Orthorhombic
2θ interval	10–70°	10–70°
Bravais lattice	C	C
No. of space group	36	36
Hermann-Mauguin symbol	Cmc21	Cmc21
Hall symbol	C2c-2	C2c-2
Laue class	mmm	mmm
Point group	mm2	mm2
Lattice symbol	oC	oC
Formula unit (Z)	4	4
Unit cell parameters	a = 8.9278 Å, b = 11.3859 Å, c = 5.5060 Å	a = 8.8742 Å, b = 11.3259 Å, c = 5.4636 Å
	α = β = γ = 90°	α = β = γ = 90°
Volume (Å^3)	559.6906	549.1371
R-Factors	Rp = 4.192%, Rwp = 5.223%	Rp = 4.482%, Rwp = 5.613%
Density (g cm^−3)	7.565	7.714
χ^2	2.104	2.248

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This contraction in lattice constraints for Tb³⁺-doped samples is consistent with the substitution of larger Gd³⁺ ions by smaller Tb³⁺ ions. The crystal structure of the GGO lattice designed by Vesta software is depicted in Fig. 5. As represented in the structure, there are three different cationic sites: two corresponding to gadolinium [Gd(3) and Gd(4)] and one for gallium [Ga(6)]. [Gd(3), Gd(4) and Ga(6)] ions are located at the 4a, 8b and 4a Wyckoff positions, respectively. Gadolinium ions form the [Gd(3)/(4)O₇] polyhedra, in which the Gd(3)/(4) ions are surrounded by seven oxygen atoms, whereas gallium ions form a tetrahedron coordinated to four oxygen atoms. Furthermore, there are four different types of oxygen atoms, i.e., O(1), O(2), O(5) and O(7), located at the 4a, 4b, 4a and 4b crystallographic positions, respectively. Table 4 shows the atomic coordinates (x, y, z) and dislocation factors of the Gd₃GaO₆ host.

Fig. 5 Visualization of the crystal structure of Gd₃GaO₆, illustrating the coordinative environment of individual cations.

Table 4 Crystallographic parameters of the Gd₃GaO₆ (GGO) host, exhibiting an orthorhombic crystal system

Atoms	x	y	z	Occ.	Site	U	Sym.
Gd (3)	0.00000	0.39640	0.42369	1.000	4a	0.205	m
Gd (4)	0.19300	0.10567	0.46705	1.000	8b	0.205	1
O (1)	0.00000	0.04660	0.12500	1.000	4a	0.205	m
O (2)	0.00000	0.21400	0.67100	1.000	4a	0.205	m
O (5)	0.34000	0.01890	0.20600	1.000	8b	0.205	1
O (7)	0.15590	0.26010	0.17000	1.000	8b	0.205	1
Ga (6)	0.00000	0.26010	0.17000	1.000	4a	0.205	m

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4.3 Morphological insights

To study the structural and surface characteristics of the synthesized phosphor material, FESEM is an essential tool due to its ability to provide structural information at various magnifications. Fig. 6(a) exhibits the FESEM image of the optimized Gd_3−xGaO₆:xTb³⁺ (x = 3 mol%) sample. The phosphor synthesized via the combustion approach demonstrated a porous structure and slight agglomeration, reflecting typical features associated with the exothermic nature of combustion synthesis.

Fig. 6 (a) FESEM image of the Gd_3−xGaO₆:xTb³⁺ (x = 3 mol%) phosphor, showing its surface morphology. (b) Particle size distribution of the Gd_3−xGaO₆:xTb³⁺ (x = 3 mol%) sample.

Due to calcination of the sample at a higher temperature, some agglomeration was observed, and the obtained particles are slightly irregular in shape with an average particle size in the 20–50 nm range. Fig. 6(b) depicts the particle size distribution of the optimized sample.

4.4 EDX spectral analysis

EDX spectroscopy and elemental mapping of the as-prepared phosphor were conducted in order to ascertain the elemental composition.

The EDX spectra in Fig. 7(a) provide information about the presence of all expected elements: terbium (Tb), gadolinium (Gd), gallium (Ga) and oxygen (O). The elements are evenly distributed, and no indication of the presence of an extra element was observed in the synthesized sample, suggesting the successful preparation of the Tb³⁺-doped GGO phosphor. The inset of Fig. 7(a), along with the experimental data, illustrates each constituent element together with its atomic and weight percentages. The homogenous distribution of constituent elements was confirmed by X-ray mapping images of the Gd_3−xGaO₆:xTb³⁺ (x = 3 mol%) phosphor (Fig. 7(b)).

Fig. 7 (a) EDX spectrum of the optimized Gd_2.97GaO₆:3 mol% Tb³⁺ nanophosphor with an inset table summarizing elemental compositions by atomic % and weight %. (b) EDX mapping images for the Gd_2.97GaO₆:3 mol% Tb³⁺ nanophosphor, displaying the spatial distribution of the Gd, Ga, Tb and O elements.

4.5 Study of photoluminescence behaviour

4.5.1 Analysis of excitation and emission spectra. Fig. 8 depicts the photoluminescence excitation spectra of the Gd_3−xGaO₆:xTb³⁺ (x = 3 mol%) sample assessed at an emission wavelength of 545 nm.

Fig. 8 Excitation spectrum of the Gd_2.97GaO₆:3 mol% Tb³⁺ phosphor recorded at λ_em = 545 nm.

The trivalent gadolinium ion exhibits intra-configuration f–f transition ⁸S_7/2 → ⁶I_11/2 around 272 nm, which overlaps with the 4f⁸ → 4f⁷4d¹ transition of Tb³⁺ ions and is not evidently distinguishable in the spectra.^38,39 There are several other peaks observed in the spectra due to the 4fⁿ–4fⁿ transition of the Tb³⁺ ion, which are assigned to the electronic transitions (⁷F₆ → ⁵H₆) at 306 nm, (⁸S_7/2 → ⁶P_7/2) at 312 nm, (⁷F₆ → ⁵L₆) at 343 nm, (⁷F₆ → ⁵L₉) at 357 nm, (⁷F₆ → ⁵G₆) at 380 nm and (⁷F₆ → ⁵D₄) at 495 nm.^40–42 The emission spectra of varying concentrations of Tb³⁺-doped Gd₃GaO₆ phosphors measured at the 272 nm excitation wavelength lie in the range of 450–650 nm and is demonstrated in Fig. 9.

Fig. 9 Emission spectra of the Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) phosphors.

The photoluminescence emission profile is mainly composed of four emission lines due to ⁵D₄ → ⁷F₃, ⁵D₄ → ⁷F₄, ⁵D₄ → ⁷F₅ and ⁵D₄ → ⁷F₆ transitions positioned at 626 nm, 590 nm, 543 nm and 490 nm, respectively.⁴³ Among all the observed transitions, the green emission, originating from the ⁵D₄ → ⁷F₅ transition, exhibits the strongest intensity. The transition is distinguished as a magnetic dipole and follows the selection rule ΔJ = ±1.⁴⁴

The energy level diagram represented in Fig. 10 explains the mechanism of the possibility of energy transfer from Gd³⁺ to the Tb³⁺ ion. Upon UV excitation of Gd³⁺, in the ⁶I_11/2 level from the ⁸S_7/2 state, the system relaxes to lower-energy levels through non-radiative transitions, eventually populating the ⁶P_7/2 energy state. Energy from the ⁶P_7/2 excited state of Gd³⁺ may be partially transferred to the ⁵H₆ energy state of Tb³⁺ via phonon-assisted dipole–dipole interactions, which then rapidly dissipates to its ⁵D₄ levels, thereby enhancing the luminescence of Tb³⁺. This state subsequently undergoes radiative transition, generating four emission peaks located at 626 nm, 590 nm, 543 nm and 490 nm, corresponding to red, orange-red, green and blue light.

Fig. 10 Depiction of electronic transitions in Tb³⁺ and energy transfer from Gd³⁺ ions under UV excitation.

4.5.2 Concentration quenching. The emission intensity of phosphor materials is strongly influenced by the concentration of doped ions. Fig. 11 shows the luminescence intensity of the Gd₃GaO₆ phosphor at different dopant concentrations of the Tb³⁺ ion. The intensity of peaks in the emission spectrum first increases with an increase in Tb³⁺ ion concentration, reaches a maximum value at 3 mol% concentration and then decreases on further increasing the concentration of the dopant ion. This decrease is observed owing to the effect of concentration quenching. Thus, according to the above information, 3 mol% was considered the optimized Tb³⁺ ion doping concentration for the Gd₃GaO₆ lattice. The concentration quenching phenomenon is generally observed when the activator ion concentration is high, which causes reduced spacing between activator ions, leading to increased non-radiative transitions amongst luminescent centres and results in decreased luminescence intensity.⁴⁵ The mechanisms accountable for concentration quenching by non-radiative energy transfer can be radiation absorption, exchange interaction or electrical multipolar interaction.⁴⁶

Fig. 11 Emission intensity dependence on Tb³⁺ concentration in the Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) phosphors.

Due to a lack of significant overlap between the excitation and emission spectra of the Gd₃GaO₆:Tb³⁺ phosphor, concentration quenching does not occur via the radiation absorption mechanism. Critical distance (R_c) between Tb³⁺ ions in the host lattice was used to better understand the exchange interactions. It was calculated by Blasse and Grabmair utilizing the formula given in eqn (7).^47,48

Here, V, Z and x_c refer to unit cell volume, number of cations per unit cell and critical concentration of Tb³⁺ ions, respectively. Substituting the values of Z = 4, x_c = 0.03 and V = 549.1371 ų for the Gd₃GaO₆ system in eqn (7), the estimated R_c appears to be 20.596 Å. According to Van Uitert, exchange interactions are dominant between dopant ions only when the critical distance is close to 5 Å.⁴⁹ As the calculated value in the present scenario is beyond the critical limit of 5 Å, interaction via the exchange mechanism is less likely to occur. Therefore, multipole–multipole electrostatic interaction is responsible for non-radiative energy transfer. According to Dexter and Schulman's energy transfer theories, concentration quenching of activators is attributed to energy transfer from one dopant ion (donor) to another ion, finally reaching the energy sink within the host lattice. To understand the concentration quenching mechanism of Gd₃GaO₆:Tb³⁺ in detail, Dexter's eqn (8) was utilized.⁵⁰In this context, x denotes the dopant ion concentration, I represent the intensity of emission at different Tb³⁺ concentrations, k and β are constants and θ indicates the nature of multipolar interactions. The values of θ equal to 6, 8 and 10 correspond to dipole–dipole, dipole-quadrupole and quadrupole–quadrupole interactions, respectively. When β(x)^θ/3 ≫ 1, eqn (8) can be transformed to eqn (9) as follows.^51,52Here, k′ is a constant. A graph of log(I/x) versus log(x) was plotted and is represented in Fig. 12.

Fig. 12 Graph of the synthesized materials with linear fitting applied after concentration quenching.

The relation was found to be almost linear with a slope of −2.17. The computed θ value is 6.51, which is approximately equal to 6. As a consequence, concentration quenching of the Gd₃GaO₆:Tb³⁺ phosphor occurs via the electric dipole–dipole interaction mechanism.

4.6 Luminescence lifetime

The decay curve of the GGO:Tb³⁺ phosphor at 3 mol% concentration is shown in Fig. 13(a). Measurements were performed using excitation and emission wavelengths of 272 nm and 543 nm, respectively, to obtain the luminescence lifetime.

Fig. 13 (a) Decay lifetime curve of the Gd_2.97GaO₆:3 mol% Tb³⁺ phosphor. (b) Auzel fitting curve of all the prepared samples.

The decay curve was fitted with various exponential functions, and the best fit was provided by the bi-exponential eqn (10).⁵³

In this equation, τ₁ and τ₂ correspond to the fast and slow components, A₁ and A₂ serve as fitting parameters and I_t and I₀ refer to the luminous intensity at time t and t = 0, respectively. By substituting the values of A₁, A₂, τ₁ and τ₂ in eqn (11), the average lifetime can be calculated.⁵⁴

The fitted decay analysis reveals the average lifetimes for G_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) phosphors, which are tabulated in Table 5. The rapid quenching of the average lifetime with increasing Tb³⁺ concentration was attributed to enhanced non-radiative energy transfer processes. Additionally, Auzel's model (Fig. 13(b)) was applied to evaluate radiative lifetimes and interpret luminescence lifetime variations.⁵⁵

Here, τ_c is defined as the lifetime at concentration C, τ₀ is the intrinsic radiative lifetime, C₀ is a constant and N represents the number of phonons. The intrinsic lifetime was calculated to be 4.86 ms. Furthermore, the quantum efficiencies (η) of the activated phosphors were calculated using the ratio of the average lifetime to the radiative lifetime (eqn (13)).⁵⁶

η = τ_avg/τ₀ (13)

Table 5 Computed decay lifetimes and quantum efficiencies of the Gd_3−xGaO₆:xTb³⁺ (x = 1–6 mol%) phosphors

Sample	τavg (ms)	η (%)
GGO:1 mol% Tb^3+	1.657	83.86
GGO:2 mol% Tb^3+	1.457	73.73
GGO:3 mol% Tb^3+	1.396	70.65
GGO:4 mol% Tb^3+	1.170	59.21
GGO:5 mol% Tb^3+	1.055	53.39
GGO:6 mol% Tb^3+	0.909	46.00

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The quantum efficiency (η) values obtained for the synthesized nanophosphors are tabulated in Table 5, and a gradual decrease in the values of quantum efficiencies with increasing Tb³⁺ content was observed owing to non-radiative energy transfer.

4.7 Temperature dependent photoluminescence

Thermal stability plays a crucial role in determining the applicability of nanophosphors. Accordingly, the temperature dependent emission spectra of Gd_2.97GaO₆:3 mol% Tb³⁺ was measured in the temperature range of 298–473 K at intervals of 25 K, as illustrated in Fig. 14(a). The data indicate a steady decrease in luminescence intensity with increasing temperature, which can be attributed to thermal quenching, a phenomenon in which the phosphor's emission intensity diminishes at an elevated temperature range. As illustrated in Fig. 14(b), the emission intensity retains 74.29% of its original value at 473 K relative to the initial temperature, confirming the good thermal stability of the developed phosphor.

Fig. 14 (a) Temperature-dependent photoluminescence of the Gd₃GaO₆:3 mol% Tb³⁺ phosphor. (b) Variations in the relative emission intensity with temperature for the Gd_2.97GaO₆:3 mol% Tb³⁺ phosphor.

To further investigate the thermal quenching behavior of the synthesized material, the activation energy of the optimized GGO:3 mol% phosphor was obtained using the approach given in eqn (14).^57,58

In this expression, A is a constant, K_B represents Boltzmann's constant (8.617105 × 10⁻⁵ eV K⁻¹) and I_T and I₀ correspond to the emission intensities at measured and room temperatures, respectively. Eqn (14) can be modified to eqn (15) as follows.^59,60

The plot of 1/K_BT against ln(I₀/I_T), as shown in Fig. 15, exhibits a linear fit with slope −0.1767 and activation energy of 0.1767 eV, which is consistent with moderate quenching behavior. Table 6 entails a comparison of the Gd₃GaO₆:Tb³⁺ phosphor with other well-known Tb³⁺-doped and Ce³⁺-co-doped green phosphors in terms of lifetime, quantum efficiency and thermal stability.^61–66

Fig. 15 Linear fit plot for the calculation of activation energy.

Table 6 Comparative optical performance of the synthesized GGO:Tb³⁺ with Tb³⁺-activated phosphors

Host lattice	Emission peak	Luminescence lifetime	Quantum efficiency	Thermal stability	References
Gd3GaO6:Tb^3+	543 nm	1.396 ms	70.6%	74.6% at 473 K	This work
YAl3(BO3)4:Tb^3+	543 nm	2.248 ms	—	70% at 473 K	61
Li2NaBP2O8:Tb^3+	544 nm	0.440 ms	34.8%	89% at 423 K	62
SrLaAlO4:Tb^3+	548 nm	1.399 ms	48%	61.2% at 398 K	63
BiOCl:Tb^3+	543 nm	0.012 ms	—	24.6% at 423 K	64
BaY4Si5O17:Ce^3+, Tb^3+	544 nm	20.29 ns	94.4%	96.1% at 423 K	65
Na3Sc2(PO4)3:Ce^3+, Tb^3+	542 nm	18.58 ns	65%	85.6% at 423 K	66

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4.8 Colorimetric investigations

To study the color purity and accurate emission color of the synthesized green GGO:Tb³⁺ phosphor, the XY chromaticity coordinate diagram (XY graph) based on CIE 1931 (Commission International de I'Eclairage) was plotted. CIE triangle comprises a triangular area composed of green, red and blue colors and the location of any color within the graph is referred to as its chromaticity point.^67,68 The chromaticity coordinates were evaluated from the photoluminescence emission spectra for different samples of Tb³⁺-doped Gd₃GaO₆ phosphors. Fig. 16(a) and (b) represent the CIE color coordinate chart, and the calculated values are summarized in Table 7. It is clearly observed from Fig. 16 that all the coordinates of emitted color lie in the greenish region of the triangle, signifying the single color-emitting property of the GGO:Tb³⁺ phosphor. The obtained CIE chromaticity coordinates (x = 0.3419, y = 0.5962) for the optimized sample are consistent with the Tb³⁺-activated green emission dominated by the ⁵D₄ → ⁷F₅ transition (∼543 nm) and are comparable to reported Tb³⁺-doped oxides, such as CsAlSi₂O₆:Tb³⁺, Sr₂Ga₂SiO₇:Tb³⁺ and MgAl₂O₄:Tb³⁺.^69–71 CIE 1976 was additionally provided for the validation of luminous characteristics of the prepared samples. According to CIE 1976, the u′, v′ coordinates (Fig. 17(a) and (b)) are computed by following eqn (16).⁷²

Fig. 16 (a) Representation of chromaticity coordinates (x, y) plotted on the CIE 1931 diagram. (b) Enlarged view, showing detailed spread of coordinates.

Table 7 CIE (x, y and u′, v′) coordinates, CCT (K) and color purity for the GGO:Tb³⁺ phosphors at different Tb³⁺ doping concentrations

Sample	CIE Co-ordinates				Color purity (%)	CCT (K)
	x	y	u′	v′
GGO:1 mol% Tb^3+	0.3345	0.5882	0.1425	0.5638	64.24	5471.81
GGO:2 mol% Tb^3+	0.3373	0.5864	0.1441	0.5637	63.81	5424.37
GGO:3 mol% Tb^3+	0.3419	0.5962	0.1444	0.5666	66.51	5351.04
GGO:4 mol% Tb^3+	0.3389	0.5835	0.1454	0.5632	63.39	5395.69
GGO:5 mol% Tb^3+	0.3374	0.5826	0.1449	0.5628	63.11	4646.13
GGO:6 mol% Tb^3+	0.3432	0.5952	0.1452	0.5665	66.30	5329.68

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Fig. 17 (a) Representation of chromaticity coordinates (u′, v′) plotted on the CIE 1976 diagram. (b) Enlarged view, showing detailed spread of coordinates.

In order to characterize light sources accurately, the color purity of the emitted color was determined using eqn (17).⁷³

In the above equation, x_s, y_s represent the CIE coordinates of the sample, x_i, y_i correspond to illuminant points and x_d, y_d signify chromaticity coordinates of the dominant wavelength. The calculated values of color purity are listed in Table 7, which shows that the color purity of the prepared GGO:3 mol% Tb³⁺ sample is 66.51%. These findings demonstrate the potential use of the Tb³⁺-doped GGO material as an excellent green-emitting phosphor for designing WLEDs and photonic devices. Another key parameter that determines the color quality, along with the temperature characteristics of the light source, is correlated color temperature (CCT). Warm light is generally characterized by CCT values less than 3000 K and values more than 4000 K are associated with cool light.⁷⁴

CCT values for various Tb³⁺-doped samples were calculated by employing the polynomial eqn (18) given by McCamy.^75,76

CCT = −437n³ + 3601n² − 6861n + 5514.31 (18)

Here, n signifies the anti-slope line connecting (x, y) and (x_e, y_e), and x_e and y_e represent chromaticity epicenters. The CCT values evaluated for all the samples are given in Table 7, which lie in the range of 4600 K–5500 K. The results confirm the cool green light nature of the prepared samples, suggesting their applications for outdoor illumination.

5. Conclusion

This study includes the synthesis of Gd₃GaO₆ phosphors doped with varying concentrations of trivalent terbium via an energy-efficient solution combustion approach. Successful inclusion of the Tb³⁺ ion within the Gd₃GaO₆ matrix was confirmed by X-ray diffractogram patterns. The GGO:Tb³⁺ phosphors form a crystalline orthorhombic structure belonging to Cmc2₁ symmetry as demonstrated by Rietveld refinement analysis. FESEM micrograph, supported by the particle size distribution plot, confirmed that the synthesized phosphor possesses slightly irregular particles having a size in the nano-range (20–50 nm). EDX spectra and mapping provided insights into the distribution of Gd, Ga, O and Tb within the host lattice. Excitation at 272 nm results in multiple emission peaks equivalent to ⁵D₄ → ⁷F_3,4,5,6, with the strongest being the green emission at 543 nm due to the ⁵D₄ → ⁷F₅ electronic transition. The emission intensity increases with the rise in the Tb³⁺ doping concentration, reaching a maximum value at x = 3 mol% and then decreases. This decrease in intensity is caused by concentration quenching. Dipole–dipole interactions are the primary reason for the increased transfer of energy between nearby ions, leading to reduced luminescence. In addition, CIE coordinates and CCT values suggest Gd₃GaO₆:Tb³⁺ phosphors as efficient green-emitting materials for use in lighting and display technologies.

Author contributions

Reshu Kajal: data curation, writing – original draft, investigation, methodology; Devender Singh: writing – review & editing, resources, supervision; Rinki Jangra: validation; Pawan Kumar: project administration; Varun Kumar: software; Ramesh Kumar: visualization; Harish Kumar: formal analysis.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgements

The author (Reshu Kajal) wants to thank University Grants Commission, New Delhi, for providing the Junior Research Fellowship [221610025278].

References

1. B. M. Manohara, H. Nagabhushana, D. V. Sunitha, K. Thyagarajan, B. D. Prasad, S. C. Sharma, B. M. Nagabhushana and R. P. Chakradhar, J. Alloys Compd., 2014, 592, 319–327,  DOI:10.1016/j.jallcom.2014.01.003.
2. S. Singh and D. Singh, J. Mater. Sci.: Mater. Electron., 2020, 31, 5165–5175,  DOI:10.1007/s10854-020-03076-5.
3. R. Kokate, P. Rohilla, S. Kaur, A. S. Rao and V. Singh, Optik, 2021, 243, 167391,  DOI:10.1016/j.ijleo.2021.167391.
4. P. Kumar, S. Singh, I. Gupta, V. Kumar and D. Singh, J. Mol. Struct., 2022, 1265, 133362,  DOI:10.1016/j.molstruc.2022.133362.
5. S. Cho, J. Kim, D. Hwang and S. W. Cho, Korean J. Mater. Res., 2016, 26, 67–72,  DOI:10.1016/j.jallcom.2014.07.100.
6. P. Kumar, D. Singh, I. Gupta, S. Singh and V. Kumar, Chem. Phys. Lett., 2023, 812, 140277,  DOI:10.1016/j.cplett.2022.140277.
7. G. Souadi, Ü. H. Kaynar, M. E. Ayvacikli and N. Can, Appl. Radiat. Isot., 2023, 199, 110905,  DOI:10.1016/j.apradiso.2023.110905.
8. R. Kajal, D. Singh, R. Jangra, P. Kumar, V. Kumar, H. Kumar and R. Kumar, J. Alloys Compd., 2025, 1050, 185688,  DOI:10.1016/j.jallcom.2025.185688.
9. T. P. Jyothi, K. Gopal, D. V. Sunitha, D. Kavyashree and A. G. Prakash, Ceram. Int., 2025, 51(26, Part C), 50644–50657,  DOI:10.1016/j.ceramint.2025.08.293.
10. B. Mari, K. C. Singh, P. Cembrero-Coca, I. Singh, D. Singh and S. Chand, Displays, 2013, 34, 346–351,  DOI:10.1016/j.displa.2013.07.003.
11. I. Gupta, P. Kumar, S. Singh, S. Bhagwan, S. K. Chhikara and D. Singh, Inorg. Chim. Acta, 2022, 543, 121183,  DOI:10.1016/j.ica.2022.121183.
12. S. Raoux, S. Anders, K. M. Yu, I. C. Ivanov and I. G. Brown, MRS Online Proc. Libr., 1995, 392, 241–246,  DOI:10.1557/PROC-392-241.
13. Y. Gong, L. Li, J. Chen and H. Guo, Materials, 2023, 16, 4711,  DOI:10.1016/j.jallcom.2022.164016.
14. O. N. Kondrat'eva, M. N. Smirnova, G. E. Nikiforova, A. V. Tyurin, V. A. Rassulov and V. A. Ketsko, J. Alloys Compd., 2024, 982, 173703,  DOI:10.1016/j.jallcom.2024.173703.
15. R. Kajal, R. Jangra, P. Kumar, D. Singh, R. Kumar and H. Kumar, J. Mol. Struct., 2026, 1359, 145495,  DOI:10.1016/j.molstruc.2026.145495.
16. M. M. Rodríguez-García, J. G. Williams and I. R. Evans, J. Mater. Chem. C, 2019, 7, 7779–7787,  10.1039/c9tc02336d.
17. I. Gupta, S. Singh, S. Bhagwan and D. Singh, Ceram. Int., 2021, 47, 19282–19303,  DOI:10.1016/j.ceramint.2021.03.308.
18. J. He, A. A. Haider, H. Jiang, C. Liu, J. Li, W. Qian, J. Zhang and J. Zhu, Inorg. Chem. Commun., 2025, 13, e01767,  DOI:10.1002/adom.202501767.
19. S. Malik, K. Jakhar, D. Singh, S. Redhu, V. Aggarwal, S. Kumar, R. S. Malik and P. Kumar, Inorg. Chem. Commun., 2025, 181, 115231,  DOI:10.1016/j.inoche.2025.115231.
20. L. A. Rocha, R. L. Siqueira, J. Esbenshade, M. A. Schiavon and J. L. Ferrari, J. Alloys Compd., 2018, 731, 889–897,  DOI:10.1016/j.jallcom.2017.10.084.
21. P. Kumar, S. Singh, I. Gupta, V. Kumar and D. Singh, Optik, 2022, 267, 169709,  DOI:10.1016/j.ijleo.2022.169709.
22. Z. Li, Z. Lyu, P. Luo, S. Wei, C. Zhuo, D. Sun, S. Shen and H. You, Inorg. Chem. Front., 2023, 10, 6746–6753,  10.1039/D3QI01525D.
23. D. Singh, V. Tanwar, A. P. Simantilleke, B. Mari and P. S. Kadyan, J. Mater. Sci.:Mater. Electron., 2016, 27, 2260–2266,  DOI:10.1007/s10854-015-4020-1.
24. L. A. Kolahalam, I. K. Viswanath, B. S. Diwakar, B. Govindh, V. Reddy and Y. L. Murthy, Mater. Today Proc., 2019, 18, 2182–2190,  DOI:10.1016/j.matpr.2019.07.371.
25. S. Singh, D. Singh, P. Siwach, I. Gupta and P. Kumar, Appl. Res., 2025, 4, e202400190,  DOI:10.1002/appl.202400190.
26. M. Farooq, H. Rafiq and M. H. Rasool, ECS J. Solid State Sci. Technol., 2023, 12, 126002,  DOI:10.1149/2162-8777/ad1062.
27. D. Singh, S. Sheoran, V. Tanwar and S. Bhagwan, J. Mater. Sci.:Mater. Electron., 2017, 28, 3243–3253,  DOI:10.1007/s10854-016-5914-2.
28. R. Kajal, D. Singh, R. Jangra, B. Dahiya, M. Kadian, P. Kumar, R. Kumar and H. Kumar, Opt. Mater., 2026, 174, 117942,  DOI:10.1016/j.optmat.2026.117942.
29. S. Choi, K. Kim, S. Nahm and H. K. Jung, Opt. Mater., 2009, 31, 1684–1687,  DOI:10.1016/j.optmat.2009.04.007.
30. V. Tanwar, S. Singh, I. Gupta, P. Kumar, H. Kumar, B. Mari and D. Singh, J. Mol. Struct., 2022, 1250, 131802,  DOI:10.1016/j.molstruc.2021.131802.
31. P. Barik, A. K. Verma, R. Kumar, V. Kumar, P. Dewangan, M. Sahu and I. P. Sahu, Appl. Phys. A, 2024, 130, 391,  DOI:10.1007/s00339-024-07525-4.
32. P. Kumar, S. Singh, I. Gupta, K. Nehra, V. Kumar and D. Singh, Mater. Chem. Phys., 2023, 295, 127035,  DOI:10.1016/j.matchemphys.2022.127035.
33. T. Ghrib, I. Massoudi, A. L. Al-Otaibi, A. Al-Malki, A. Kharma, E. Al-Hashem, R. A. Al-Ghamdi and R. A. Al-Zuraie, J. Inorg. Organomet. Polym. Mater., 2021, 31, 239–250,  DOI:10.1007/s10904-020-01761-w.
34. P. Kumar, D. Singh and I. Gupta, RSC Adv., 2023, 13, 7703–7718,  10.1039/D3RA00636K.
35. A. S. Rao, Sens. Actuators, A, 2024, 372, 115336,  DOI:10.1016/j.sna.2024.115336.
36. B. M. Manohara, H. Nagabhushana, D. V. Sunitha, K. Thyagarajan, B. D. Prasad, S. C. Sharma, B. M. Nagabhushana and R. P. Chakradhar, J. Alloys Compd., 2014, 592, 319–327,  DOI:10.1016/j.jallcom.2014.01.003.
37. I. Gupta, D. Singh, S. Singh, P. Kumar, S. Bhagwan and V. Kumar, Chem. Phys. Lett., 2023, 814, 140350,  DOI:10.1016/j.cplett.2023.140350.
38. R. E. Muenchausen, L. G. Jacobsohn, B. L. Bennett, E. A. McKigney, J. F. Smith, J. A. Valdez and D. W. Cooke, J. Lumin., 2007, 126, 838–842,  DOI:10.1016/j.jlumin.2006.12.004.
39. P. Kumar, S. Singh, I. Gupta, A. Dalal, V. Kumar and D. Singh, J. Mater. Sci. Eng. B, 2023, 288, 116189,  DOI:10.1016/j.mseb.2022.116189.
40. P. K. Jisha, S. C. Prashantha and H. Nagabhushana, J. Sci.:Adv. Mater. Devices, 2017, 2, 437–444,  DOI:10.1016/j.jsamd.2017.10.001.
41. P. Kumar, D. Singh and H. Kumar, Mater. Sci. Semicond. Process., 2024, 174, 108162,  DOI:10.1016/j.mssp.2024.108162.
42. I. Gupta, D. Singh, S. Singh, P. Kumar, S. Bhagwan, V. Kumar, H. Kumar and S. K. Chhikara, Luminescence, 2023, 38, 585–599,  DOI:10.1002/bio.4486.
43. M. Ayvacıklı, A. Canimoglu, Y. Ü. Karabulut, Z. Kotan, L. K. Herval, M. P. De Godoy, Y. G. Gobato, M. Henini and N. Can, J. Alloys Compd., 2014, 590, 417–423,  DOI:10.1016/j.jallcom.2013.12.135.
44. P. Kumar, D. Singh and I. Gupta, Mater. Res. Bull., 2023, 167, 112413,  DOI:10.1016/j.materresbull.2023.112413.
45. Y. S. Vidya, K. Gurushantha, H. Nagabhushana, S. C. Sharma, K. S. Anantharaju, C. Shivakumara, D. Suresh, H. P. Nagaswarupa, S. C. Prashantha and M. R. Anilkumar, J. Alloys Compd., 2015, 622, 86–96,  DOI:10.1016/j.jallcom.2014.10.024.
46. Z. C. Wu, J. X. Shi, J. Wang, M. L. Gong and Q. Su, J. Solid State Chem., 2006, 179, 2356–2360,  DOI:10.1016/j.jssc.2006.04.030.
47. S. Singh and D. Singh, J. Mater. Sci.: Mater. Electron., 2021, 32, 17674–17685,  DOI:10.1007/s10854-021-06303-9.
48. C. Liu, Y. Wang, R. Li, G. Zhou, A. A. Haider, X. Huang, X. Zhao, Y. Qin, H. Li, H. Jiang and J. Li, Aggregate, 2025, 6, e70125,  DOI:10.1002/agt2.70125.
49. P. Kumar, S. Singh, I. Gupta, V. Kumar and D. Singh, Luminescence, 2022, 37, 1932–1941,  DOI:10.1002/bio.4377.
50. T. A. Safeera and E. I. Anila, J. Lumin., 2019, 205, 277–281,  DOI:10.1016/j.jlumin.2018.09.033.
51. P. Kumar, D. Singh, I. Gupta, S. Singh, S. Nehra and R. Kumar, RSC Adv., 2023, 13, 7752–7765,  10.1039/D3RA00735A.
52. T. T. Chanu and N. R. Singh, Spectrochim. Acta, Part A Mol. Biomol. Spectrosc., 2018, 191, 539–546,  DOI:10.1016/j.saa.2017.10.060.
53. D. Singh and S. Sheoran, J. Mater. Sci.:Mater. Electron., 2016, 27, 12707–12718,  DOI:10.1007/s10854-016-5405-5.
54. Z. Li, P. Wang, T. Yang, H. Yu, B. Xiao and M. Zhang, J. Phys. Chem. C, 2015, 119, 27688–27694,  DOI:10.1021/acs.jpcc.5b09328.
55. P. Kumar, D. Singh, S. Singh, H. Kumar and R. Kumar, RSC Adv., 2024, 14, 18716–18729,  10.1039/D4RA03833A.
56. M. Luo, X. Sha, B. Chen, X. Zhang, H. Yu, X. Li, J. Zhang, S. Xu, Y. Cao, Y. Wang and X. Wang, J. Am. Ceram. Soc., 2022, 105, 3353–3363,  DOI:10.1111/jace.18299.
57. P. Kumar, D. Singh and H. Kumar, RSC Adv., 2024, 14, 755–770,  10.1039/D3RA07175H.
58. F. L. Joaquim, L. A. Rocha, C. S. Júnior, J. Esbenshade, M. A. Schiavon and J. L. Ferrari, J. Alloys Compd., 2019, 774, 694–699,  DOI:10.1016/j.jallcom.2018.09.148.
59. P. Kumar, D. Singh and H. Kumar, Mater. Chem. Phys., 2024, 320, 129418,  DOI:10.1016/j.matchemphys.2024.129418.
60. H. Li, A. A. Haider, Z. Xie, C. Liu, H. Zhang, H. Jiang, J. Li and J. Zhu, Adv. Sci., 2025, 12, 2415989,  DOI:10.1002/advs.202415989.
61. J. Hakami, Ü. H. Kaynar, M. E. Ayvacikli, M. B. Çoban, J. Garcia-Guinea, P. D. Townsend, M. Oglakci and N. Can, Ceram. Int., 2022, 48, 32256–32265,  DOI:10.1016/j.ceramint.2022.07.167.
62. J. Zhu, J. Xiang and Y. Mao, Optik, 2021, 240, 166920,  DOI:10.1016/j.ijleo.2021.166920.
63. X. Huang, C. He, X. Wen, Z. Huang, Y. Liu, M. Fang, X. Wu and X. Min, Opt. Mater., 2019, 95, 109191,  DOI:10.1016/j.optmat.2019.109191.
64. X. Huang, B. Li and H. Guo, J. Alloys Compd., 2017, 695, 2773–2780,  DOI:10.1016/j.jallcom.2016.11.224.
65. Z. Huang, Z. Lyu, S. Shen, S. Wang, Z. Yang, C. Chen and H. You, Inorg. Chem., 2024, 63, 6362–6369,  DOI:10.1021/acs.inorgchem.4c00123.
66. H. Guo, B. Devakumar, B. Li and X. Huang, Dyes Pigm., 2018, 151, 81–88,  DOI:10.1016/j.dyepig.2017.12.051.
67. D. Singh, V. Tanwar, A. P. Samantilleke, B. Mari, S. Bhagwan, P. S. Kadyan and I. Singh, J. Electron. Mater., 2016, 45, 2718–2724,  DOI:10.1007/s11664-015-4318-z.
68. V. R. Panse, N. S. Kokode, A. Shukla, A. N. Yerpude and S. J. Dhoble, Optik, 2016, 127, 5860–5864,  DOI:10.1016/j.ijleo.2016.04.031.
69. D. Singh, V. Tanwar, S. Bhagwan, P. S. Kadyan and B. Mari, Adv. Sci. Lett., 2014, 20, 1726–1729,  DOI:10.1166/asl.2014.5736.
70. Z. Liu, S. Wang, Y. Xu, J. Li, Y. Du, J. Hong and Z. Chen, Appl. Phys. A, 2025, 131, 449,  DOI:10.1007/s00339-025-08518-7.
71. S. Zhongxiang, S. Jun, W. Jing, L. Yang and L. Yongfu, Spectroscopy, 2023, 38, 28–34,  DOI:10.56530/spectroscopy.xr3287v2.
72. P. Kumar, D. Singh, S. Kadyan, H. Kumar and R. Kumar, RSC Adv., 2024, 14, 16560–16573,  10.1039/D4RA02571G.
73. K. Maheshwari and A. S. Rao, Opt. Mater., 2023, 137, 113533,  DOI:10.1016/j.optmat.2023.113533.
74. J. Deng, Z. Wang, W. Zhou, M. Yu, J. Min, X. Jiang, Z. Xue, C. Ma, Z. Cheng and G. Luo, Ceram. Int., 2023, 49, 14478–14486,  DOI:10.1016/j.ceramint.2023.01.036.
75. H. E. Sekrafi, E. Hergli, S. Ferdov, P. J. Coutinho and L. Rebouta, J. Alloys Compd., 2025, 1010, 177389,  DOI:10.1016/j.jallcom.2024.177389.
76. P. Kumar, D. Singh, I. Gupta, S. Singh, V. Kumar, H. Kumar and S. K. Chhikara, Inorg. Chem. Commun., 2022, 145, 110064,  DOI:10.1016/j.inoche.2022.110064.

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