White LEDs are the most used practical white light source since they can be used in various technologies and offer the most significant advantages, including high luminous efficiency, reliability, and chemical stability. The combination of host lattice and activators (Host: activator) known phosphors plays an essential role in developing optically driven white LEDs (Duke et al., 2018; Guo and Huang, 2018; Huang and Guo, 2018; Hakeem et al., 2019; Zhang Q. et al., 2021; Li et al., 2022; Xu et al., 2022). The combination of yellow-emitting Y₃Al₅O₁₂: Ce³⁺ (YAG: Ce³⁺) phosphors and blue-emitting (440–480 nm) InGaN chips are currently used in commercial w-LEDs for solid-state lighting. The white LEDs obtained because of this combination generate poor-quality white light due to excellent light generation because of the deficiency of the red color component. The poor color quality limits the use of this combination for general illumination (Geng et al., 2018; Wu et al., 2019). Another technique involves covering a UV-emitting chip with phosphors emitting red, green, and blue light. This combination produces white light with high color quality. However, the cyan emission color gap means white light is still behind ideal. The quality of light produced by white LEDs in artificial lighting is significantly determined by the lighting efficacy (LE), color rendering index (CRI), and correlated color temperature (CCT)

The phrase refers to evaluating a light source’s ability to faithfully replicate the colors of diverse objects compared to a perfect reference light (such as incandescent or natural light). The CRI values range from 0 to 100. The color quality of the white light produced will be lower when the CRI is lower. Similarly, CRI values over 80 are typically required for general lighting. There is a substantial demand for high-CRI (Ra>90) light sources in numerous fields, including photography, movies, museums, and art galleries. The emission spectrum of a light source must be broad enough to achieve a high CRI value. “Full-visible spectrum illumination” has been suggested as a fresh idea for solid-state white lighting (Zhang et al., 2016; Huang, 2019a; Huang, 2019b; Yuan et al., 2019; Khan et al., 2021; Cao et al., 2022). It aims to generate a light source that matches natural sunlight regarding CRI and color temperature. It is challenging to depict colors adequately since there is still a cyan gap between blue and green emissions, peaking at 470 and 500 nm. The general illumination obtained because of the approaches mentioned above is likewise not appropriate in this region. To provide white light of the highest quality, novel cyan-emitting phosphors must be developed in this spectral range. To boost the optical performance of phosphor-converted LEDs (Pc-LEDs), the emission spectra of the devices are modified using a narrow-band, cyan-emitting phosphor with a slight Stokes shift or a broadband phosphor covering both the cyan and green spectral region (Wang et al., 2016; Strobel et al., 2018; Ding et al., 2021; Li et al., 2022; Wu et al., 2022). The narrow-band cyan-emitting phosphors compensate for the peak valley between blue and green emissions, which is crucial for boosting color reproduction.

Designing very stable UV/blue excitable cyan-producing phosphors is essential to attain a full visible light spectrum. An efficient cyan emission with a peak at 495 nm and a full width at half maximum (FWHM) of 32 nm has recently been reported for a narrow band BaSi₂O₂N₂:Eu²⁺ phosphor (Wu et al., 2022). However, its layered crystal structure makes it chemically and thermally unstable. Another oxonitridoberylate phosphor was registered with a narrow-band emission peaking at 495 nm and a full width at half maximum (FWHM) of 35 nm for Sr[Be₆ON₄]: Eu²⁺ (Strobel et al., 2018). However, the phosphor’s toxicity and harsh synthetic conditions pose severe challenges to its implementation. It is required to create novel stable, non-toxic phosphors having narrow-band cyan emission to enhance the color rendering of pc-LEDs.

Another family of phosphor compositions is A₃B₂C₃O₁₂, which has different cation feasibility for A, B, and C ions and garnet type structure with different surrounding environments for Ce³⁺ and Eu²⁺ doped ion occupation. Because these dopants (Ce³⁺ and Eu²⁺) exhibit a 5d - 4f spin-permitted transition with tunable emission throughout the visible range, the Ce³⁺ or Eu²⁺ ions are frequently used as activators in various phosphors. Furthermore, this type of composition has high stability against moisture and heat. More interestingly, the Ce³⁺-activated garnet phosphors may exhibit much lower near-UV and blue light absorption, making them suitable for use with the most commercially available near-UV and blue excitable white LEDs. For example, the BaLu₂Al₄SiO₁₂:Ce³⁺ garnet phosphors emit green emission peaking at 513 nm under 450 nm blue light excitation (Qiang et al., 2018).

This review article thoroughly discussed the development of various rare-earth ion-activated A₃B₂C₃O₁₂ garnet phosphors that emit highly efficient narrow-band cyan emission under UV and blue light excitation. First, we shall discuss the formation of various compositions by substituting various cations and their effects on the structure and luminescence features. Next, we shall shed light on cyan and green emission formation with co-doping of Ce³⁺/Tb³⁺ ions in the garnet-type structure to develop a single phase with wide-range tunable emission in the cyan and green spectral region. These phosphors’ composition in white LED fabrication was also discussed for practical applications in solid-state lighting and display application.

Jaffe studied the role of the yttrium ions in garnet crystals and foresaw the double substitution of Y³⁺-Al³⁺ for Mn²⁺-Si⁴⁺ in a Mn₃Al₂Si₃O₁₂ garnet in 1951 (Jaffe, 1951). Yoder et al. also proposed the development of [Mn_1-xY_x]₃[Al]₂[Si_1-xAl_x]₃O₁₂ solid solutions in the same year. This resulted in the final composition of Y₃[Al]₂[Al]₃O₁₂, which has a garnet-like structure (Yoder and Keith, 1951). In today’s world, it is common knowledge that in addition to the naturally occurring silicate minerals, there are other fabricated garnets with compositions including aluminate, gallate, and germanates.

The coordinated Ca²⁺/Lu³⁺ ions and crystal structure of Ca₂LuHf₂Al₃O₁₂ (CLHAO) are shown in Figures 1A, B. One can see the occupation of Ca²⁺/Lu³⁺, Hf⁴⁺, and Al³⁺ in Wyckoff sites 24c, 16a, and 24d. There was strong octahedral and tetrahedral coordination between Hf⁴⁺ and Al³⁺, in which the octahedrons [HfO₆] and the tetrahedrons [AlO₄] were joined by O^2- ions. As a result, eight O^2- anions were coordinated with the Ca²⁺/Lu³⁺ cations, yielding a [(Ca/Lu)–O₈] dodecahedron. This compound may exhibit good thermal stability because of the structure’s compactness and rigidity. Figure 1C depicts the band structure of Ca₂LuHf₂Al₃O₁₂ calculated from the refined crystal structure. The CLHAO compound was found to have a direct bandgap of 4.15 eV at the G point of the Brillouin zone, located between the maximum valence band and the minimum conduction band. The calculations show that CLHAO is an appropriate host material because it offers enough band gaps to occupy Ce³⁺ to serve as emission centers. As a result of the absence of phonons in the transition, the direct band gap is also more likely to be advantageous to luminescence than the indirect band gap (Kireev and Samokhvalov, 1978).

Figure 1D shows the total and partial densities of states (DOS and PDOS) of the developed CLHAO compound. Anti-bonding orbitals of the Hf-3d and O-2p states dominated the conduction band. The orbital O-2p contributes the most to the valence band out of all the atoms. The O-2p states were almost filled in the valence band, but the Ca-s and Lu-p states were less concentrated. This suggests that the Ca/Lu-O bond was ionic. The wide band of Hf, Al, and O states, which corresponds to the hard polyhedron of [HfO₆] and [AlO₄], indicates the covalent connection between the Hf-O and Al-O bonds (Zhao et al., 2016).

Zhang and others developed Ce³⁺-doped garnet phosphors that emit cyan under 400 nm (near-UV) light irradiation (Zhang Z. J. et al., 2021). The Ce³⁺ ions were doped in the 0.01, 0.02, 0.03, 0.06, and 0.08 ranges. The phase purity and crystal structure of the developed CLHAO:xCe³⁺ phosphor was confirmed by x-ray diffraction (XRD). The surface morphology of the synthesized materials was checked with field-emission scanning electron microscopy (FE-SEM). Under near-UV (400 nm) light irradiation, a broadband emission in the cyan spectral region (477–493 nm) is highly efficient.

Interestingly, the thermal stability of the synthesized CLHAO:xCe³⁺ phosphors was excellent. To find the potential of the synthesized phosphors in the generation of white LEDs, a prototype of white LEDs was developed with a CRI ranging from 83.2 to 89.4.

The XRD pattern of the undoped CLHAO and Ce³⁺-doped CLHAO (CLHAO:0.02Ce³⁺) phosphors is illustrated in Figure 2A. These two samples show identical peak positions to Ca₂GdZr₂Al₃O₁₂ (COD ID-4338781). There is an impurity peak of HfO₂ at around 30.36°. All the remaining peaks match well with those in the standard PDF card number. This result meant that the phases of samples are independent of the Ce³⁺ doping. The minute amount of the HfO₂ impurity phase should not impact the optical characterization (Fischer et al., 2018). A FE-SEM image was used to examine the prepared phosphors. Figure 2B is an FE-SEM representation of the CLHAO as it was produced with a 0.02Ce³⁺ concentration, demonstrating the particles’ size range from 0.3 to 1.2 μm. Figure 2C shows the EDS spectra of CLHAO:0.02Ce³⁺ garnet phosphors. The distribution of the six components Ca, Lu, Al, Hf, and Ce on the produced phosphor was uniform, as shown in Figure 2D from the elemental mapping of CLHAO:0.02Ce³⁺ phosphors. As a result, CLHAO:Ce³⁺ phosphors were successfully prepared.

Aside from developing broadband cyan emission to fulfill the cyan gap in RGB phosphors converted white-LEDs, the emission spectrum of CLHAO:Ce³⁺ garnet phosphors can be efficiently tuned in the cyan and green color for the desired spectral region. More specifically, phosphor materials with extraordinary photoluminescence capabilities must be created for the subsequent development of high-quality solid-state white illumination. The broadband cyan-emitting phosphor is crucial to achieve “full-visible-spectrum lighting” and close the spectral gap since the emission spectrum of conventional phosphor-converted (w-LEDs) comprises a blue-green cavity. The synthesis of the thermally stable cyan-emitting Ca₂YHf₂Al₃O₁₂:Ce³⁺ garnet phosphor was doped with Ce³⁺. The prepared CYHAO: xCe³⁺ phosphor excitation band spans a wide range of wavelengths, from 360 to 460 nm, with a maximum peak at 408 nm. As a result, it can work with an LED chip that produces near-ultraviolet (NUV), which has a wavelength shorter than 400 nm and is produced by an LED chip. The best sample of CYHAO: 0.03Ce³⁺ showed robust broadband cyan emission when irradiated at 408 nm. The wavelength and bandwidth of the emission were 493 nm and 100 nm, respectively. The sample has a high internal quantum efficiency (IQE) of 89.5% despite having a low external quantum efficiency (EQE) of just 69.1% (Jia et al., 2020).

The normalized photoluminescence emission spectrum of CYHAO:xCe³⁺ garnet phosphors stimulated at 408 nm was plotted in Figure 9A to show the influence of Ce³⁺ ions in the PL characteristics. Naturally, the emission peak wavelength and the bandwidth (FWHM) varied as the concentration of Ce³⁺ doping increased. The dominant peak of the photoluminescence (PL) was observed at 485 nm at x = 0.005 and moved to 504 nm at x = 0.10, moving the peak point 19 nm toward the longer wavelength. The following is an explanation for the phenomenon of redshift.

The smallest Y³⁺ ions (r = 1.019 Å) were replaced by the bigger Ce³⁺ ions (r = 1.143 Å) in the phosphors CYHAO:xCe³⁺. The Ce³⁺-O^2- bond may be compressed in the hard garnet structure when the Ce³⁺ concentration increases due to a possible reduction in the distance between ligands and light centers. An increase in Ce³⁺ 5d crystal field splitting thus caused the red shift in emission spectra. There was also the possibility that the Ce³⁺ activators might transfer energy, which would explain the redshift. An increase in low-energy emission and a shift of the emission maxima to a longer wavelength were ultimately caused by a greater energy transfer from higher 5d level Ce³⁺ ions to lower-level Ce³⁺ ions as the Ce³⁺ concentration enhanced. The emission bands of CYHAO:xCe³⁺ phosphors also grew wider when the Ce³⁺ doping concentration enhanced from x = 0.005 to x = 0.10. Figure 9B displays the CIE chromaticity diagram of the CYHAO:xCe³⁺ garnet phosphors. By increasing the concentration of Ce³⁺ ions from x = 0.005 to x = 0.10, it is possible to modify the emission colors from cyan to green with CIE chromaticity coordinates ranging from (0.1756, 0.2936) to (0.1756, 0.2936), (0.2591, 0.4438), and so on. The inset of Figure 9B depicts digital photographs of these phosphors acquired under 365 nm UV light, showcasing their robust emission and range of emission colors.

Similarly, a potential CaLa_1-x Lu _x HAO:Ce³⁺ garnet phosphor has been created based on the solid-solution design of chemical cation substitution in Ca₂La_1-x Lu _x Hf₂Al₃O₁₂:Ce³⁺ garnet phosphors (Chan et al., 2023). The strategy of cationic substitution discussed here can create a new path towards developing high-efficiency luminescent materials by modifying the crystal structure. This approach will also significantly and broadly impact solid-state white lighting (Li et al., 2008; Tolhurst et al., 2017; Yan et al., 2017; Leaño et al., 2018; Wang et al., 2019; Amachraa et al., 2020; Ding et al., 2021; Viswanath et al., 2021; Chan et al., 2022a; Chan et al., 2022b; Wang et al., 2022; Yang et al., 2022; Li et al., 2023).

The trivalent Lu³⁺ ions may significantly improve luminous performance in CLa_1-x Lu _x HAO:Ce³⁺ phosphors when La³⁺ ions are substituted for Lu³⁺ ions. Thus, CLa_1-x Lu _x HAO:Ce³⁺ phosphors have been thoroughly investigated for their luminous characteristics. The PLE and PL spectra of the CLaHAO:Ce³⁺ phosphor sample are shown in Figure 10A without Lu³⁺ doping. In the region of 300–350 nm wavelengths, with a peak at 326 nm, the PLE spectrum obtained at 517 nm contains a weak excitation band. In the region 350–480 nm spectral range, there is an intense broad excitation band with a peak at 408 nm. This transition might be attributed to Ce³⁺ ions’ transition from the ground state 4f to their excited ⁵d₂ and ⁵d₁ states, which permit spin and parity (Hakeem et al., 2018; Li et al., 2022).

With an excitation wavelength of 408 nm, the CaLaHAO:Ce³⁺ phosphor emits a strong, broad-band green emission up to 517 nm with a full width at half maximum (FWHM) of 118.7 nm. Two Gaussian sub-bands in the CaLaHAO:Ce³⁺ phosphor’s PL emission band correspond to ion transitions caused by Ce³⁺ ions ⁵d→²F_5/2 and ⁵d→²F_7/2, respectively, (Leaño et al., 2018; Chan et al., 2022a). The energy difference (∆k) between the two Gaussian bands has been calculated to be 2018 cm⁻¹, which is close to the theoretical difference of 2000 cm⁻¹ and indicates that the Ce³⁺ ions only have one site in the host lattice of Ca₂LaHf₂Al₃O₁₂ (Yan et al., 2017; Chan et al., 2022b).

The PL spectra of CaLa_1-x Lu _x HAO: Ce³⁺ (0 ≤ x ≤ 0.5) phosphors upon 408 nm excitation are shown in Figure 10B. In all these samples, cyan-green emission bands are brilliant and broad in the range of 425–750 nm, with a slight blue shift occurring at the emission peak location when Lu³⁺ concentration increases (517 nm at x = 0–506 nm at x = 0.5). Figure 10C shows Lu³⁺ concentration-dependent integrated PL intensity of CaLa_1-x Lu _x HAO:Ce³⁺ phosphors. Figure 10D shows the PLE spectra of CaLa_1-x Lu _x HAO:Ce³⁺ (0 ≤ x ≤ 0.5) phosphors. Each of them consists of two bands of excitation. With a peak at 408 nm (caused by the Ce³⁺ ion’s 4f–5d¹ transition), these samples exhibit a broad and strong excitation band in the 350–480 nm spectral range. The intensity of excitation increases as the Lu³⁺ content (x) increases. This indicates that near-UV LED chips can function effectively in the excitation of CaLa_1-x Lu _x HAO:Ce³⁺ phosphors.

Moreover, all these CaLa_1-x Lu _x HAO:Ce³⁺ phosphors samples exhibit a relatively weak excitation band at 300–350 nm wavelengths (due to the 4f–5d² transition of Ce³⁺ ions), and the intensity of this band increases with increasing Lu³⁺ content (x), but the excitation peak position red-shifts from 326 nm for x = 0–332 nm for x = 0.5. The CaLa_0.5Lu_0.5HAO:Ce³⁺ solid solution sample has the maximum emission intensity among the CaLa_1-x Lu _x HAO: Ce³⁺ (0 ≤ x ≤ 0.5) garnet phosphors. Figure 10E shows the PLE and PL spectra of the optimized CaLa_0.5Lu_0.5HAO:Ce³⁺ phosphors. As the PLE spectrum shows, Ce³⁺ ions exhibit spin-and-parity-allowed electronic transitions of 4f–5d¹ and 4f–5d² in the 300–480 nm regions. A prominent cyan-green emission band was observed upon stimulation at 408 nm in the CaLa_0.5Lu_0.5HAO:Ce³⁺ phosphors sample. In addition, the emission band can be split into two Gaussian-fitting bands at 493 nm and 542 nm, corresponding to the electronic transitions of Ce³⁺ ions at 5d→2F_5/2 and 5d→2F_7/2. According to the experimental results, the energy difference between 2F_5/2 and 2F_7/2 levels is close to the theoretically calculated value of 1583 cm^-1 in CaLa_0.5Lu_0.5HAO:Ce³⁺ phosphors.

Figure 10F presents the CaLa_1-x Lu _x HAO:Ce³⁺ phosphors and their CIE chromaticity diagram. CIE chromaticity coordinates show a blue shift as Lu³⁺ concentration increases, going from (0.2979, 0.4802) for x = 0 to (0.2664, 0.4519) for x = 0.5. As x increases, the emission color of CaLa_1-x Lu _x HAO:Ce³⁺ becomes cyan, green, with the cyan component deepening. The PLE and PL characteristics of Ce³⁺-activated phosphor materials could be fine-tuned by adjusting their coordination environment.

For near-UV-pumped full-visible spectrum white LEDs with ultra-high color rendering indices (Ra = 98, R₉ = 95.9, and R₁₂ = 94.3), novel cyan-green phosphors with a superior quantum efficiency (76.4%) and significantly higher thermal stability have been developed. Due to the induced highly symmetric crystal structure, solid-solution phosphors synthesized from Ca₂La_1-x Lu _x Hf₂Al₃O₁₂:Ce³⁺ exhibit enhanced cyan-green emission with enhanced thermal stability for full-spectrum white LEDs, as shown in Figures 11A, B.

In our most recent study, Ca₂YTaO₆ confirmed that the different colors of light obtained from Ca₂YTaO₆:Bi³⁺ double perovskite oxide phosphors are caused by several luminescence centers. The smooth change in the emission spectrum from blue to cyan and green indicates several light sources. To investigate how the amount of Bi³⁺ concentration affects the emission of Ca₂YTaO₆:Bi³⁺ phosphors, we examined the PLE and PL spectra of the as-prepared samples at several monitored emission and excitation wavelengths at room temperature. Figure 12A compares the PLE spectra of the Bi³⁺ doped (monitored at 480 nm) and un-doped (424 nm) samples. It is clear from the comparison that the un-doped sample displayed a broadband excitation at the monitored wavelength of 424 nm, ranging from 200 nm to 400 nm, with the dominant peak around 315 nm on the high energy side.

In contrast, the PLE spectra of the Bi³⁺-doped Ca₂YTaO₆ phosphors show impressive broadness. In particular, the PLE spectra of the Bi³⁺-doped material expanded to longer wavelengths (at least 50 nm red-shifted). They gained an additional peak at 361 nm on the lower energy side because of the absorption of ¹S₀ to ³P₁ in the activated Bi³⁺ ions. We investigate whether the presence of several luminescence centers in the host lattice of Ca₂YTaO₆ phosphors may explain the occurrence of multiple excitation peaks, in addition to varied excitation, samples with and without Bi³⁺ doping yield phosphors with distinct emissions (Khan et al., 2021).

The increase of Bi³⁺ concentration causes a noticeable broadening of the dominant emission peak from 424 nm to 480 nm–500 nm as illustrated in Figure 12B. To understand the reason behind the varied color emission of Ca₂YTaO₆:Bi³⁺ phosphors, we looked at the room-temperature photoluminescence (PL) of optimized Ca₂YTaO₆:0.02Bi³⁺ phosphors with various excitations (250–430 nm with a 20 nm spacing). The optimized Ca₂YTaO₆:0.02Bi³⁺ phosphors sample’s normalized PL spectra can be shown in Figure 12C, and they demonstrate that the emission spectrum has successfully been changed from blue to cyan and green, demonstrating the existence of many luminescence centers. The prepared Ca₂YTaO₆:0.02Bi³⁺ double perovskite oxide phosphors’ digital images acquired at 254 and 365 nm showed that the color tuning was effective at the two excitation wavelengths (Figure 12D).

In conclusion, phosphors with the general chemical formula Ca₂LuHf₂Al₃O₁₂ (CLHAO) are a significant component of the inorganic material family, where numerous cation substitutions may be performed to produce optimal compositions for application in various sectors of illumination. The concepts and techniques of (a) fulfilling of cyan gap in the full spectral region of white-LEDs, (b) cations substitution to accomplish efficient tuning of the emission color and (c) the growing and tuning abilities of sensitizer emission because of the efficient energy-transfer phenomena via doping using different rare-earth (RE) and transition metal (TM) ions, including Ce³⁺, Cr³⁺, Tb³⁺, and Bi³⁺, were thoroughly examined in this review. The choice of host materials is essential in producing w-LED phosphors and the doped activators. The interaction between the host materials and the doped activators alters the luminous features of this garnet phosphor.

A major challenge in garnet ceramics for solid-state lighting (SSL) is distinguishing between concentration quenching, thermal quenching, and optical excitation quenching (Yan et al., 2017; Khan et al., 2022; Khan et al., 2023a; Ali et al., 2023; Khan et al., 2023b). Investigating the multiple coupling effects among these mechanisms will drive future research. However, addressing the quenching effect of red-emitting ceramics with a longer decay time under high-power density laser excitation remains difficult. Selecting an excitation source is critical to pursuing high-quality and healthy light sources like sunlight and avoiding the dangers of blue light for the human eye. The near-ultraviolet (n-UV, ∼400 nm) LED chips are emerging in SSL technology. However, the n-UV excited color converters with high spectrum-matching degrees with n-UV chips, high efficiency and stability, and broad practical application need more research. This progress would guide future research on Ce-doped garnet phosphors and help develop new ceramic photo-convertors with tailored luminous properties.

This review article highlights the development of other cyan-emitting phosphors to fulfil the cyan gap in the emission spectrum and fabricate a white LED with high thermal and moisture stability to generate a highly efficient white light source. Developing narrow-band cyan emission will also be promising for the high color gamut displays.