Structural confinement-induced highly efficient deep-red emission and negative thermal quenching performance in Mn⁴⁺-activated Ca₇Mg₂Ga_6−yAl_yO₁₈:Mn⁴⁺ phosphors

Abstract

High quantum efficiency (QE) and thermally stable emission are indispensable for the application of phosphors. Owing to the strong coupling between the lattice and naked d-orbitals of Mn⁴⁺, Mn⁴⁺-activated oxide red-emitting phosphors usually exhibit a low QE and serious thermal quenching (TQ), which strictly inhibit their prospective application. Herein, we rationally designed a novel phosphor series Ca₇Mg₂Ga_6−yAl_yO₁₈:0.02Mn⁴⁺ (CMGA_yO:Mn⁴⁺, 0 ≤ y ≤ 1.5), which adopts a high structural symmetry of the space group F432. Highly efficient and thermally stable deep-red emissions (λ_em = 721 nm) were achieved simultaneously by structurally confining the Mn⁴⁺-emitting centres at the isolated octahedrally coordinated sites. Specifically, the highest internal and external quantum efficiencies of 90.2 and 75.9%, respectively, were obtained for CMGA_1.5O:Mn⁴⁺ due to Al³⁺-alloying-induced local structure modification and suppression of non-radiative transitions. Excellent thermal stabilities of 99, 102.2, and 87.9% of room-temperature photoluminescence intensities were retained at 423 K for CMGA_yO:Mn⁴⁺ with y = 0, 0.5, and 1.5, respectively. CMGA_0.5O:Mn⁴⁺ exhibits an abnormal negative TQ behaviour over the measured temperature range (298–473 K), which is attributed to the energy gain via the electron–phonon interactions of the isolated MnO₆ octahedra. Moreover, in situ high-pressure emission spectra of CMGO:0.02Mn⁴⁺ showed that the emission position is highly sensitive to the external pressure with redshift coefficients of 2.8(2) and 1.16(4) nm GPa⁻¹ in different pressure regions. These excellent photoluminescence properties of CMGA_yO:Mn⁴⁺ phosphors signify their great application potential in red LEDs and optical pressure sensors. The findings of this work have paved an avenue for the design of high-performance Mn⁴⁺-activated phosphors.

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1. Introduction

Phosphor-converted white light emission diodes (pc-WLEDs) have been considered as the most promising next-generation artificial lighting source because of their attractive advantages, including low energy consumption, small size, environmental friendliness, high luminescence efficiency, and lasting durability.^1–5 The prevalent and currently commercialized method for achieving white light emission is the combination of a blue InGaN chip with the yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺).⁶ However, the insufficient red component in the spectrum inevitably results in a poor colour rendering index (CRI) and high correlated colour temperature (CCT), which thus stimulated the development of other rare-earth-activated red phosphors.⁷ Unfortunately, these red phosphors still have several shortcomings, including the high cost of rare-earth dopants, low utilization efficiency, and incomplete colour display.⁸ Moreover, red to deep-red light (620–735 nm) is also indispensable for plant growth in prompting photosynthesis, phototropism, and photomorphogenesis.^9,10 Hence, in terms of providing plants with customized light to replenish sunlight, pc-LEDs have shown overwhelming advantages of controllable illumination intensity, quality, and cycle to natural light, which is critical for improving agricultural outputs and tuning plant growth. Therefore, it is imperative to develop red phosphors that coincide with the application requirements of high-quality WLEDs and plant cultivation.

Recently, Earth-abundant and low-cost Mn⁴⁺-activated red phosphors have attracted substantial attention because of the intensive excitation spectrum covering the n-UV to blue light region and the red emission spanning from 600 to 780 nm.¹⁰ Mn⁴⁺ ions favour an octahedrally coordinated environment and their emission wavelength is highly dependent on the covalent Mn⁴⁺-ligand bonding.¹¹ Consequently, the Mn⁴⁺ ion in fluorides usually exhibits sharp line-shape red emissions, whereas Mn⁴⁺-doped oxide phosphors usually give rise to broad-band red emissions due to the much stronger covalency of the Mn⁴⁺–O²⁻ bond than that of Mn⁴⁺–F⁻. This distinct difference in chemical bonding also makes Mn⁴⁺-doped fluorides exhibit much higher quantum efficiency than Mn⁴⁺-activated oxides, making Mn⁴⁺-doped fluorides particularly attractive for modern-day WLEDs.^12–24 However, the moisture and high-temperature lability as well as the heavy use of toxic and corrosive HF during the synthesis process strongly restrict their widely practical application. Alternatively, Mn⁴⁺-activated oxide phosphors have outstanding thermal and chemical stability and eco-friendly preparation procedures, signifying their broad prospective applications. Consequently, numerous Mn⁴⁺-activated oxides, in particular B-site-ordered double perovskites, have been investigated as red-phosphors.^25–34 However, the luminescence performances of Mn⁴⁺ ions in oxide matrixes are significantly affected by the degree of cationic ordering, structural distortion, and defect-induced self-reduction of Mn⁴⁺ to Mn²⁺, which usually results in low quantum efficiency and severe thermal quenching (TQ). Therefore, it is urgent and essential to discover novel Mn⁴⁺-activated oxide phosphors exhibiting extraordinary luminescence thermal stability and extremely high efficiency.

Herein, in an attempt to explore highly efficient and thermally stable red-emitting phosphors, we rationally designed a new phosphor series CMGA_yO:Mn⁴⁺, which has not been reported yet. The structure confinement effect led to the formation of isolated Mn⁴⁺-emitters with Mn⁴⁺–Mn⁴⁺ distances >10.7 Å. These isolated Mn⁴⁺-emitters exhibit highly efficient deep-red emissions (λ_em = 721 nm) with IEQ and EQE values of 90.2 and 75.9%, respectively, achieved by Al³⁺-alloying. Such high quantum efficiencies reach up to the top values of hitherto documented Mn⁴⁺-activated oxide phosphors. More impressively, negative TQ, which is rarely observed for Mn⁴⁺-activated oxide phosphors, was also achieved by Al-alloying-induced suppression of non-radiative transitions. The findings of this research indicate that the structural confinement of isolated Mn-emitters can serve as a powerful strategy for the design of high-performance Mn⁴⁺-activated oxide red phosphors.

2. Experimental section

2.1 Synthesis

Polycrystalline powder samples of Ca₇Mg₂Ga₆O₁₈:xMn⁴⁺ (x = 0.01, 0.02, 0.03, 0.0375, 0.05, 0.0625, and 0.10) and CMGA_yO:Mn⁴⁺ (y = 0.25, 0.5, 1.0, and 1.5) were synthesized via conventional high-temperature solid-state reactions. Raw materials of calcium carbonate (CaCO₃, Alfa Aesar, 99.99%), magnesium oxide (MgO, Alfa Aesar, 99.99%), gallium oxide (Ga₂O₃, Alfa Aesar, 99.99%), aluminum oxide (γ-Al₂O₃, Alfa Aesar, 99.95%), and manganese oxide (MnO₂, Alfa Aesar, 99.95%) were used without further purification. First, all these reagents except for MnO₂ were dried at 500 °C for 10 h to remove the absorbed moisture before being weighed. Then, these raw materials were mixed in a stoichiometric ratio and preheated at 1000 °C to decompose the carbonates. Finally, all resulting powders were reground and pelleted and then calcinated at 1220 °C for 30 h with intermediate regrinding and re-pelleting.

2.2 Characterization

The phase purity of the phosphors was evaluated by lab powder X-ray diffraction (PXRD) using a PANalytical Empyrean diffractometer in the Bragg–Brentano geometry with Cu K_α diffraction. The working voltage and current were 40 kV and 40 mA, respectively. High-quality PXRD data used for Rietveld refinements were collected in the 2θ-range of 5–120° with a step size of 0.0131° and a counting time of 200 s. Rietveld refinements were performed using the TOPAS-Academic V7 software.³⁵ Room temperature photoluminescence (PL) spectra and PL excitation (PLE) spectra for Ca₇Mg₂Ga₆O₁₈:xMn⁴⁺ (denoted as CMGO:xMn⁴⁺) and Ca₇Mg₂Ga_6−yAl_yO₁₈:0.02Mn⁴⁺ (denoted as CMGA_yO:Mn⁴⁺) were recorded using a Hitachi F-7100 fluorescence spectrophotometer with a solid accessory. PL decay curves, internal and external quantum efficiencies (IQE and EQE), and temperature-dependent PL spectra were recorded using an Edinburgh FLS-1000 spectrometer equipped with a 450 W continuous-wave Xe lamp and a heating system. In situ high-pressure spectra of CMGA_yO:Mn⁴⁺ (y = 0 and 1.5) were recorded using a home-designed spectrometer (Ideaoptics, Shanghai, China), and the 375 nm laser source was generated from the PicoQuant LDH diode head. An asymmetrical diamond anvil cell (DAC) was employed to generate high pressure. Pre-compressed phosphors and ruby balls were loaded into the chamber. Silicone oil was employed as the pressure medium for all high-pressure measurements, and the pressure was calibrated according to the fluorescence peak at 694.2 nm of the ruby balls, excited by the 405 nm incident light. The ultraviolet-visible (UV-vis) light diffuse reflectance spectra for CMGO:xMn⁴⁺ (x = 0.02 and 0.10) were measured at room temperature using a Shimadzu UV-3600 spectrometer operating over a wavelength from 200 to 800 nm. The fine powders were spread on compressed BaSO₄ as a 100% reflectance standard. The 310 nm UV-chip was purchased from Anhui UV-Chips semiconductor technology Co. Ltd. The EQE values of the deep-red pc-LED under various currents were measured using an HAAS 2000 photoelectric measuring system (EVERFINE, China).

3. Results and discussion

3.1 Crystal structure and phase identification

Ca₇Mg₂Ga₆O₁₈ (CMGO) crystallizes in a cubic structure described by the space group F432.³⁶ Mg²⁺ and Ga³⁺ ions occupy five crystallographically independent M-sites (M1–M5). The tetrahedrally coordinated M1–M3 sites are co-occupied by Mg²⁺ and Ga³⁺ cations, while the remaining 4-fold coordinated M4 and 6-fold coordinated M5 (Ga5) sites are occupied exclusively by Ga³⁺ ions. As shown in Fig. 1, the structural framework of CMGO is built by MO₄-tetrahedra through corner-sharing, leaving the large voids distributed over the body and edge centres of the unit cell. These structural cavities are stuffed by the [Ca₁₄GaO₆] cluster, which is composed of six Ca1 cations and a perovskite-like [CaGaO₃] unit with a regular Ga5O₆ octahedron (Fig. 1b and c). The isolated Ga5O₆ octahedrons in the CMGO matrix are well separated from each other with a minimum distance of ∼10.7 Å. Such a structural feature of CMGO coincides with our target design of high-performance Mn⁴⁺-activated oxide phosphors, which thus promoted us to investigate the photoluminescence of CMGO:Mn⁴⁺.

Fig. 1 (a) The crystal structure of CMGO viewed along the a-axis. (b) The surrounding environment of the Ga5O₆ octahedron. (c) The coordination environment of the Ga5 ion.

PXRD patterns of CMGA_yO:Mn⁴⁺ and CMGO:xMn⁴⁺ phosphors are shown in Fig. 2a and S1,† respectively. PXRD patterns of these phosphors show similar profiles and are consistent with that of the pristine CMGO host, indicating all these materials are phase-pure and isomorphous. The lattice parameters of CMGO:xMn⁴⁺ are independent of chemical compositions (Fig. S2†), suggesting that the incorporation of Mn⁴⁺ has a negligible impact on the crystal structure. In contrast, the equivalent substitution of Ga³⁺ ions (0.62 Å in 6-fold coordination) with smaller Al³⁺ (0.535 Å in 6-fold coordination) ions in CMGA_yO:Mn⁴⁺ led to progressive peak shifts toward high-angle regions (Fig. 2a).³⁷ This observation is in line with the linear decreases of lattice parameters as a function of the Al³⁺-content (y) (Fig. 2b), firmly corroborating that Al³⁺ ions were successfully incorporated into the host lattice. We should note that the solution range of Al³⁺ in CMGA_yO:Mn⁴⁺ is limited to y ≤ 1.5 because a higher Al³⁺-dopping content led to the formation of the impurity phase Ca₃Al₂O₆ (Fig. S3†). To further shed light on the Al³⁺-substitution-induced structural modifications, Rietveld refinements were performed on CMGA_yO:Mn⁴⁺ (y ≤ 1.5). Since Mg²⁺ and Al³⁺ cations possess identical X-ray scattering abilities, they were regarded as the same cation during the refinement process. The PXRD data for CMGA_yO:Mn⁴⁺ were perfectly fitted using CMGO as the initial structure model. The final Rietveld refinement patterns are shown in Fig. 2c and d and S4.† As shown in Fig. S5,† all the refined average M–O bond lengths in CMGA_yO:Mn⁴⁺ show apparent contraction trends, involving a noticeable contraction of the M3–O bond length and slight contractions of the remaining M–O bond lengths, indicating that Al³⁺ ions tend to reside on the tetrahedrally coordinated M3-site.

Fig. 2 (a) PXRD patterns for CMGA_yO:Mn⁴⁺. (b) Plots of lattice parameters and cell volumes as a function of the Al³⁺-content (y) in CMGA_yO:Mn⁴⁺. Rietveld refinement plots of PXRD data for CMGO:0.02Mn⁴⁺ (c) and CMGA_1.5O:Mn⁴⁺ (d). The blue circles and red and black solid lines represent the observed, calculated, and differences between the former two, respectively. The expected Bragg positions are given as green bars at the bottom of the patterns.

3.2 Photoluminescence properties of CMGO:xMn⁴⁺

PLE spectra (λ_em = 721 nm) of CMGO:xMn⁴⁺ phosphors comprise two absorption bands spanning from 250 to 550 nm (Fig. 3a and S6†), signifying that these phosphors can be excited by both ultraviolet and blue LED chips. These two absorption bands are consistent with the broad absorption bands centred at 305 and 465 nm in the UV-vis light reflectance spectra of CMGO:xMn⁴⁺ (x = 0.02 and 0.10) (Fig. S7†). The PLE spectrum of CMGO:0.02Mn⁴⁺ can be well deconvolved into four Gaussian peaks centred at ∼280 (∼35 715 cm⁻¹), 303 (∼33 003 cm⁻¹), 338 (∼29 586 cm⁻¹), and 464 nm (∼21 552 cm⁻¹), corresponding to the contributions of O²⁻-to-Mn⁴⁺ charge transfer (CT) and ⁴A_2g → ⁴T_1g, ⁴A_2g → ²T_2g, and ⁴A_2g → ⁴T_2g transitions of Mn⁴⁺ ions, respectively.^11,38

Fig. 3 (a) The excitation spectrum of CMGO:0.01Mn⁴⁺. (b) The PL spectra for CMGO:xMn⁴⁺. (c) The evolution of integrated PL intensities of CMGO:xMn⁴⁺ as a function of the Mn⁴⁺-content (x). The inset shows the linear fitting of the lg(I/x) − lg(x) curve. (d) The Tanabe–Sugano energy level diagram of Mn⁴⁺ (d³) in the octahedral crystal field of the CMGO host.

Upon excitation at 291 nm, CMGO:xMn⁴⁺ phosphors emit bright red light in the wavelength range of 660–780 nm, which is ascribed to the spin-forbidden ²E_g → ⁴A_2g transition of the Mn⁴⁺ ion. As illustrated in Fig. 3b and c, the emission intensity first increases along with the doping content (x) of Mn⁴⁺ ions and reaches a maximum at x = 0.02 and then declines gradually due to the onset of concentration quenching. Importantly, although Mn⁴⁺-activators in CMGO:Mn⁴⁺ experience a similar surrounding environment to those in perovskites (Fig. 1b), the optimal doping content (x = 0.02) is much higher than those of the Mn⁴⁺-activated perovskite-type phosphors (see Table 1). It is vitally important for the application of Mn⁴⁺-doped materials in w-LEDs and pc-LEDs, as high Mn⁴⁺ doping concentrations are required for sufficient absorption of the blue LED light in parity-forbidden Mn⁴⁺ d–d transitions. Together with the crystal structure features, we can speculate that the structural confinement of Mn⁴⁺ activators at isolated octahedral sites can effectively inhibit the energy migration amongst the Mn⁴⁺ ions, which is critical for the achievement of high-concentration Mn⁴⁺ doping. Impressively, the IQE and EQE values for CMGO:0.02Mn⁴⁺ were estimated to be 85.2% and 66.1%, respectively (Fig. S8†). These values are superior to most of the Mn⁴⁺-activated oxide phosphors tabulated in Table 1.

Table 1 Comparison of photoluminescence of some excellent Mn⁴⁺-activated far-red phosphors

Host matrix	Mn^4+-content	λ em (nm)	IQE (%)	EQE (%)	Intensity (%, 150 °C)	Ref.
CaGdAlO4	0.002	713	61	—	—	47
CaNa0.5La0.5MgWO6	0.009	700	94	82	—	31
Ca3La2W2O12	0.016	711	48	—	29	48
Ca3Al4ZnO10	0.016	714	60	—	51	39
NaMgLaTeO6	0.02	703	57.4	—	75	32
Gd2ZnTiO6	0.002	705	65.6	39.7	27	30
La2LiSbO6	0.003	712	92	—	58	49
La1−xLuxAlO3	0.001	729	86	62.1	<20	50
CMGO	0.02	721	85.2	66.1	99	This work
CMGA1.5O	0.02	721	90.2	75.9	87.9	This work

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Intriguingly PL spectra of CMGO:xMn⁴⁺ comprise a periodic peak series with an energy interval of ∼25 meV, which differs dramatically from most of the Mn⁴⁺-activated oxide phosphors. Such a spectroscopic feature was previously observed for a limited number of Mn⁴⁺-activated phosphors, including Ca₃ZnAl₄O₁₀:Mn⁴⁺,³⁹ Ca₁₄Zn₆(Al/Ga)₁₀O₃₅:Mn⁴⁺,⁴⁰ La₂LiTaO₆:Mn⁴⁺,⁴¹ and La₂MgTiO₆:Mn⁴⁺.⁴² To shed light on this spectroscopic feature, low-temperature PL spectra for CMGO:0.02Mn⁴⁺ were collected in the temperature range of 10–250 K. As seen in Fig. S9,† the intensity of the peak sideband at the high energy side (<700 nm) decreases upon cooling and disappears completely at 10 K. In contrast, the sideband at the low energy side (>700 nm) increases sharply with decreasing temperature. These spectroscopic features of Mn⁴⁺-emitters suggest that the emission peak at ∼700 nm is ascribed to the zero phonon line (ZPL) of the spin-forbidden ²E_g → ⁴A_2g transition, the low energy (>700 nm) and high energy (<700 nm) sidebands are assigned to be the phonon-associated Stokes and anti-Stokes shift emission peaks, respectively.

The well-known Tanabe–Sugano energy level diagram was further illustrated to understand the deep-red emissions for Mn⁴⁺-emitters in CMGO in-depth. As shown in Fig. 3d, the ZPL excitation and emission energies are related to the crystal field strength D_q and Racah parameters B and C. Based on the ZPL energy levels of ⁴T_2g, ⁴T_1g, and ²E_g states, the values of D_q, B, and C can be estimated using the following equations:^8,38

10D_q = E(⁴T_2g → ⁴A_2g) (1)

D_q/B = 15(δ − 8)/(δ² − 10δ) (2)

The values of the D_q, B, and C were calculated as 2155, 842, and 2561 cm⁻¹, respectively. Then the D_q/B value was calculated as 2.56, corresponding to a ²E_g/B (E/B) value of 21.7 (see Fig. 3d). However, such a crystal field strength would lead to a ZPL emission energy of 18 270 cm⁻¹ (547 nm) for the ²E_g → ⁴A_2g transition according to the expression E(²E_g → ⁴A_2g)_ZPL = 21.7B, contradicting the detected ZPL emission energy of 14 245 cm⁻¹ (702 nm). Such a large discrepancy has been frequently encountered for Mn⁴⁺-activated phosphors and stems from the use of the peak energies of PL and PLE bands rather than their ZPL energies in the calculations of eqn (1)–(4). Additionally, the ZPL energies of PLE states are barely determined accurately, particularly for E(⁴T_1g)_ZPL, which is the critical issue that leads to unreliable Racah parameters.

Recently, Adachi proposed a revised model to estimate more accurate values of the Racah parameters based on the standard crystal-field theory.^43–45 The most important assumption of the revised model is that the Racah parameters exhibit a quantitative relationship of C = 4.7B.^43–45 In combination with eqn (1) and (4), the values of B and C parameters were revised as 657 and 3088 cm⁻¹, respectively. The D_q/B value was then calculated as 3.28, implying Mn⁴⁺ experiences a strong crystal field strength. Furthermore, the expected ZPL emission wavelength was further derived to be 701 nm (14 257 cm⁻¹), which matches well with the measured ZPL emission wavelength (702 nm) of CMGO:xMn⁴⁺. Therefore, the revised Racah and crystal field parameters are self-affirming, strongly confirming the validation of these parameters.

It is known that the emission of the Mn⁴⁺ ion is in fact determined by the strength of the Mn⁴⁺-ligand covalent bonding, namely the nephelauxetic effect (β₁), which can be quantified according to the following equation:⁴⁶

Where B₀ (1160 cm⁻¹) and C₀ (4303 cm⁻¹) are Racah parameters for a free Mn⁴⁺ ion. A smaller β₁ value indicates a stronger covalent Mn⁴⁺-ligand bonding, thus resulting in a longer emission wavelength. Herein, the value of β₁ was calculated to be 0.91, indicating a strong covalent Mn⁴⁺–O²⁻ bonding in CMGO. Quantitatively, for most Mn⁴⁺-activated oxide phosphors, the energy of the ²E_g → ⁴A_2g transition can be further predicted using the β₁-related linear equation:⁴⁶

E(²E_g → ⁴A_2g) = −880.49 + 16 261.92β₁ ± σ (6)

where σ represents the root-mean-square deviation with a value of 332 cm⁻¹. According to the β₁ value, the emission energy of E(²E_g → ⁴A_2g) for CMGO:Mn⁴⁺ is expected in the range of 13 586–14 250 cm⁻¹. According to the PL spectra of CMGO:Mn⁴⁺, the value of E(²E_g → ⁴A_2g) is calculated as 14 225 cm⁻¹, within the predicted energy range, thereby explaining the deep-red emission of the Mn⁴⁺ ion in CMGO:Mn⁴⁺.

3.3 Al-alloying-induced significantly improved photoluminescence properties

Chemical substitution is widely utilized to manipulate the local structure of Mn⁴⁺-activators, so as to the realize efficient red-emission. Our attempt of equivalent Al³⁺-to-Ga³⁺ substitution in CMGA_yO:Mn⁴⁺ was applied to improve the photoluminescence performance. As shown in Fig. 4a, PL intensities were enhanced expectedly by incorporating Al³⁺ ions into CMGA_yO:Mn⁴⁺. Quantum efficiency measurements further revealed that both the IQE and EQE values were improved remarkably from 85.2 and 66.1% for y = 0 to 90.2 and 75.9% for y = 1.5 (Fig. 4b and S8†), reaching up to the top values of previously reported Mn⁴⁺-activated oxide phosphors. As seen in Fig. 4c, the decay curves for CMGA_yO:Mn⁴⁺ monitored at 721 nm were perfectly fitted with a single exponent function given below:where I(t) and I₀ are the intensities at time t and 0, respectively, A is a constant, and τ is the decay time. These results demonstrate that there exists only one type of Mn⁴⁺-emitting centre in CMGA_yO:Mn⁴⁺, which is consistent with the structural feature of the host matrix. The lifetimes were calculated to be 2.94, 2.93, 2.95, 3.03, and 3.09 ms for y = 0, 0.25, 0.5, 1, and 1.5 in CMGA_yO:Mn⁴⁺, respectively. These values are comparable to previously reported millisecond scale lifetimes for Mn⁴⁺-activated deep-red phosphors.^51,52 Clearly, the lifetimes of Mn⁴⁺ ions were prolonged by Al-alloying. According to the results of our structure analysis (Fig. S5†), Al³⁺-alloying led to a more compact MnO₆-octahedron, which thus facilitated the mix of the odd parity ⁴T_2u and ⁴T_1u states with the even parity ²E_g state, thereby breaking the selection rule of the spin- and parity-forbidden ²E_g → ⁴A_2g transitions of Mn⁴⁺ ions.⁵³ Additionally, the lattice contraction also consolidates the structural rigidity and thus suppresses nonradiative transition. As a result, highly efficient emission was achieved by substituting Ga³⁺ ions with smaller Al³⁺ ions in CMGA_yO:Mn⁴⁺.

Fig. 4 PL spectra (a), internal quantum efficiencies (b), and time-resolved decay curves (c) of CMGA_yO:Mn⁴⁺.

3.4 Temperature dependence of the Mn⁴⁺ luminescence

Besides QE, the thermal stability is another critical factor that determines the potential application of a phosphor, because the working temperature of typical LEDs can reach 150 °C (423 K). This stimulated us to further investigate the temperature-dependent PL properties of CMGO:Mn⁴⁺ and CMGA_yO:Mn⁴⁺ phosphors. As shown in Fig. 5a, the ZPL and Stokes vibronic emissions of CMGO:0.02Mn⁴⁺ decline gradually upon heating, whereas the anti-Stokes vibronic emission increases along with the temperature. As a result, the integrated PL intensity first increases and reaches a maximum value at 333 K and then declines slowly. As for CMGO:0.05Mn⁴⁺ (Fig. S10†), all the emission peaks increase along with temperature and reach a maximum at 373 K. Then, decreases were observed for the ZPL and Stokes vibronic emissions, while the anti-Stokes emission remains an increasing trend. Consequently, CMGO:0.05Mn⁴⁺ exhibits a wider negative TQ temperature range (298–473 K) than that of CMGO:0.02Mn⁴⁺ (298–333 K). Impressively, as we can see in Fig. 5b, the PL intensities for CMGO:xMn⁴⁺ with x = 0.02 and 0.05 at 423 K were still retained 99% and 103% of the initial intensities at 298 and 283 K, respectively, signifying their superior thermal stability to the Mn⁴⁺-activated red phosphors reported so far. Moreover, the emission-peak position of CMGO:xMn⁴⁺ is independent of the temperature, indicating outstanding color stabilities of these phosphors at high temperatures.

Fig. 5 Temperature-dependent PL spectra for CMGO:0.02Mn⁴⁺ (a) and CMGA_0.5O:Mn⁴⁺ (c). The evolutions of integrated PL intensity of CMGO:xMn⁴⁺ with x = 0.02 and 0.05 (b) and CMGA_yO:Mn⁴⁺ with y = 0.5 and 1.5 (d) versus temperature (T). The solid lines in (b) and (d) were obtained by fitting the I_PL − T curves according to eqn (8). The inset of (d) shows the mechanism of TQ at high temperatures.

The effect of Al³⁺-alloying on the PL thermal stability of CMGA_yO:Mn⁴⁺ (y = 0.5 and 1.5) were also investigated and their temperature-dependent PL spectra are depicted in Fig. 5c and S11.† The evolution of PL spectra for CMGA_0.5O:Mn⁴⁺versus temperature is similar to that of CMGO:Mn⁴⁺. However, the decline trends of the ZPL and Stokes emissions of CMGA_0.5O:Mn⁴⁺ were much slower, while the enhancement in anti-Stokes emissions was more significant upon heating. As a result, an abnormal negative TQ behaviour over the measured temperature range was observed for CMGA_0.5O:Mn⁴⁺ (Fig. 5c) and the integrated PL intensity was enhanced to 104.2% at 473 K. In contrast to CMGA_0.5O:Mn⁴⁺, sharp reductions in PL intensities of ZPL and Stokes emissions were observed for CMGA_1.5O:Mn⁴⁺ at elevated temperatures. As a result, a normal TQ behaviour above room temperature was observed (Fig. S11†). However, CMGA_1.5O:Mn⁴⁺ still exhibited a superior thermal stability with 87.9% with the PL intensity being retained at 423 K.

Till now, negative TQ has been observed for many Mn⁴⁺-activated fluoride and oxyfluoride phosphors.⁵⁴ In contrast, a negative TQ was relatively rarely observed for Mn⁴⁺-doped oxide phosphors and only a few examples have been documented as far as we know, such as Sr₂Ca_0.9La_0.1WO₆:Mn⁴⁺⁵⁵ and Ba₂CaWO₆:Mn⁴⁺.⁵⁶ Negative TQ observed for rare-earth-,^57,58 Bi³⁺-,⁵⁹ and Mn²⁺-activated^60,61 phosphors are usually attributed to the energy compensation stemming from energy transfer from the lattice defects to the emitting centres at elevated temperatures. However, recent studies on Mn⁴⁺-doped fluorides have unravelled that the negative TQ is caused by an intrinsic effect, i.e. the electron–phonon interaction, rather than any extrinsic effects.⁶² Specifically, the electric dipole (parity) forbidden Mn^{4+ 4}A_2g → ⁴T_2g can gain intensity at high temperatures via coupling with local vibrations of the Mn⁴⁺-octahedron,⁶³ which is the origin of negative TQ for Mn⁴⁺ emitters. According to Adachi, the temperature-dependent PL intensity can be quantitatively accounted for by the expression given below^63,64

where the term on the right side contains ν_s reflects the effective electron–phonon interactions arising from both the ⁴T_2g and ²E_g states, k_B is the Boltzmann constant. ΔE₁ (j = 1) and ΔE₂ (j = 1) represent the thermal quenching energy barriers at lower and higher temperature ranges, respectively. As shown in Fig. 5b and d, the experimental data were successively described by eqn (8). The hν_s values for CMGO:0.02Mn⁴⁺, CMGO:0.05Mn⁴⁺, CMA_0.5GO:Mn⁴⁺, and CMA_1.5GO:Mn⁴⁺ were estimated as 73.5, 73.8, 73.8, and 73.6 eV, respectively, suggesting similar strengths of electron–phonon interactions in these phosphors due to the structural confinement effect. As shown in Table S1,† our fitting results deciphered that the negative TQ was dedicated by the a₁ value, which reflects the nonradiative combination centre density. A smaller a₁ value would thus lead to a stronger negative TQ, which matches well our experimental and fitting results (Fig. 5 and Table S1†). Despite the ΔE₁ value being of the same order of magnitude as hν_s, however, ΔE₁ is related not to the electron–phonon interaction, but to the bulk or surface defects that act as nonradiative combination centres. ΔE₂ is much larger than ΔE₁ because ΔE₂ represents the TQ energy barrier of the nonradiative relaxation process via the crossing point of ⁴T_2g and ⁴A_2g parabolas (B → C → D → A), as illustrated in the inset of Fig. 5d. The ΔE₂ values of CMGO:0.02Mn⁴⁺, CMGO:0.05Mn⁴⁺, and CMA_1.5GO:Mn⁴⁺ were fitted to be 0.66, 0.63, and 0.59 eV, respectively. Such large ΔE₂ values successfully explain the superior thermal stabilities of these phosphors. We should also note that the ΔE₂ value of CMA_0.5GO:Mn⁴⁺ is inestimable because no TQ was detected at high temperatures.

3.5 Electroluminescence performance of the prepared LED devices

As discussed above, the excellent photoluminesce of CMGA_yO:Mn⁴⁺ suggests their potential applications in red pc-LEDs. CMGA_yO:Mn⁴⁺ (y = 0, 0.5, and 1.5) phosphors were thus combined with 310 nm chips to prepare the red LEDs to further evaluate their available applications. The electroluminescence (EL) spectra of the prepared red LED devices are shown in Fig. 6a. The photographs of these pc-LEDs are given in the insets of Fig. 6a, where the bright red emission light can be observed by the naked eye. Fig. 6b shows the CIE chromaticity coordinates of (0.7334, 0.2665) for the EL spectra of the fabricated red LEDs. To further demonstrate the potential applications, the electroluminescence spectra of the representative pc-LED device prepared by CMGA_1.5O:Mn⁴⁺ were recorded under various currents (140–300 mA). As shown in Fig. S12a,† the deep-red emission band increases along with the driven current and no saturation can be observed. The corresponding EQE value first increases along with the driven current and reaches a maxim value approaching 0.8% at 190 mA and then decreases thereafter (Fig. S12b†). The low EQE value could be further improved by optimizing the deep-red pc-LED packing procedure, which is worth exploring.

Fig. 6 (a) EL spectra of the red LED devices driven by 310 nm chips under a bias voltage of 6 V. (b) The CIE colour coordinates of electroluminescence spectra of the red LEDs.

3.6 In situ high-pressure photoluminescence of CMGA_yO:Mn⁴⁺ (y = 0 and 1.5)

In situ pressure-dependent PL spectra of CMGO:0.02Mn⁴⁺ and CMGA_1.5O:Mn⁴⁺ were recorded over the pressure range of 0.5 to 20.2 GPa under a constant excitation wavelength of 375 nm. Upon compression, a sharp decline in the emission intensity can be observed for both CMGO:0.02Mn⁴⁺ and CMGA_1.5O:Mn⁴⁺ when the pressure is lower than 5 GPa (Fig. 7 and S13†), beyond which the emission intensity decreases slowly. When the external pressure excess is 15 GPa, the deep-red emission of CMGA_1.5O:Mn⁴⁺ almost completely disappeared (Fig. S13†), whereas the deep-red emission still could be detected for CMGO:0.02Mn⁴⁺ up to 20.2 GPa. This phenomenon might be ascribed to the pressure quenching effect because the cell volume of CMGA_1.5O:Mn⁴⁺ is much smaller than that of CMGO:0.02Mn⁴⁺.

Fig. 7 (a) The emission spectra of CMGO:0.02Mn⁴⁺ under an excitation wavelength of 375 nm and different pressures. (b) The plots of λ_max as a function of pressure.

The Mn–O covalent bonding can be consolidated upon compression, thereby leading to a shift of the deep-red emission band toward a longer wavelength (Fig. 7 and S13†). As for CMGA_1.5O:Mn⁴⁺, a red-shift of 12 nm can be obtained from 0.5 to 15 GPa with the peak shape remaining unchanged. For CMGO:0.02Mn⁴⁺, a redshift of 12 nm was observed from 0.5 to 5 GPa with the emission band being dominated by the stokes sideband (insets of Fig. 7), while the ZPL emission starts to dictate the emission band and shows a 16 nm red-shift from 6.7 to 20.2 Gpa, signifying a plausible structure phase transition at around 6 Gpa. Furthermore, in situ high-pressure PXRD should be conducted to verify this possible structure transition. This spectroscopic phenomenon reflects that Al³⁺-alloying can consolidate the structural stiffness, which is consistent with the photoluminescence properties of CMGA_yO:Mn⁴⁺. Upon decompression, both the emission peak position and shape can recover to the ambient state. Such a pressure-sensitive red-shift can be utilized to gauge pressure for high-pressure technology. Consequently, we plotted the wavelength at the maximal intensity (λ_max) as a function of pressure (P). As shown in Fig. 7b, the λ_max − P curves show simple linear correlations in two distinct pressure regions. Linear fittings to the λ_max − P curves resulted in pressure-sensing coefficients of 2.8(2) and 1.16(4) nm GPa⁻¹ in the relatively low (0.5–5 GPa) and high (6.7–20.2 GPa) pressure-regions, respectively. We should emphasise that these pressure-sensing coefficients are much larger than those of LiCaY₅(BO₃)₆:Ce³⁺ and ruby (∼0.35 nm GPa⁻¹, Al₂O₃:Cr³⁺),^65,66 signifying great potential as a pressure sensor.

4 Conclusion

In summary, a series of highly efficient and thermally stable far-red emission (λ_em = 721 nm) phosphors were rationally designed by structural confinement of Mn⁴⁺ ions at the octahedrally coordinated sites of the oxide host matrix CMGO. Excellent quantum efficiencies (IQE = 85.2%, EQE = 66.1%) were obtained expectedly for the CMGO:0.02Mn⁴⁺. Al³⁺-to-Ga³⁺ substitution in CMGA_yO:Mn⁴⁺ further improved the IQE and EQE values to 90.2 and 75.9%, respectively, due to the local structural modification-induced suppression of nonradiative transitions and breaking of the selection rule of the spin- and parity-forbidden ²E_g → ⁴A_2g transitions of Mn⁴⁺ ions. This local structure modification could also facilitate the energy capture of ²E_g → ⁴A_2g transitions by coupling with local MnO₆-octahedral vibrations, which led to an abnormal negative thermal quenching behaviour for CMGA_0.5O:Mn⁴⁺. Additionally, in situ high-pressure photoluminescence measurements revealed that the deep-red emission of CMGO:0.02Mn⁴⁺ can maintain up to a pressure as high as 20.2 GPa. More importantly, the linear positive correlations between the maximal emission wavelengths and applied pressures in two distinct pressure regions can be utilized to pressure gauge. These newly discovered phosphors exhibit superior photoluminescence to most of the previously reported Mn⁴⁺-activated oxide phosphors, signifying their great potential application as far-red emission phosphors.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (22271030, 22171032) and the Natural Science Foundation of Chongqing (cstc2021jcyj-msxmX0971). We thank Ting Wen at the Center for High Pressure Science and Technology Advanced Research and Yonggang Wang in Peking Univeristy for in situ high-pressure photoluminescence data collection.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00252g

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