With high energy conversion efficiency, low energy consumption, long lifetime, and environment-friendly, white light-emitting diodes (WLEDs) have stimulated the development of solid-state lighting technology.^[1] In most commercial WLED devices with a typical internal structure, including blue GaN chip and yellow emitting YAG:Ce³⁺ phosphor, the low red emission intensity of YAG:Ce³⁺ phosphors gives rise to the poor color rendering index (CRI) and higher correlated color temperature. The one possible solution was found to adopt trichromatic phosphors combining the near-UV (NUV) LED chips.^{[2, 3]} In addition, the high emission efficiency and thermally stable red-emitting phosphors are necessary to lower the thermal quenching at the high operating temperature (up to 425  K) of chips.^[4]

Currently, the selected red-emitting phosphors for WLEDs are focused on nitrides- and sulfide-based compounds with activated centers, such as rare-earth ions^{[2– 6]} and transition metal ions.^{[7– 9]} Eu²⁺ -doped nitride phosphors with high color rendering indexes are regarded as good candidate red-emitting phosphors for WLEDs under UV or blue chip excitation.^{[2– 4]} However, synthesis conditions are required, including high reaction temperature and the high pressure N₂ gas. Although the higher emission efficiency from sulfide red-emitting phosphor, such as (Ca_{1 − x}, Sr_x)S:Eu²⁺ , ^[5] has been achieved, ^[6] the poor thermal stability and sensitivity to moisture may restrict the stability and lifetime of the device. Mn⁴⁺ -activated dialkali hexafluorometallate hosts, such as BaSiF₆, ^[7] ZnSiF₆, ^[8] and K₂SiF₆, ^[9] are found to match well with the emission wavelength of InGaN chip and can achieve highly efficient red luminescence.

It is generally considered that energy level structure and electronic transition probability of luminescence centers are highly interrelated with the crystal field strength of the ligands.^[10] Bachmannet et al.^[2] found a red-shift in the Eu²⁺ emission band when replacing part of Sr²⁺ ions by Ca²⁺ ions in the host of SrSi₂O₂N₂. They suggested that an increased crystal field splitting lowered the energy difference between 5d– 4f state of Eu²⁺ . Dai et al.^[10] investigated the PL spectra of Eu²⁺ -doped KSrPO₄ when substituting part of K⁺ cations by Na⁺ cations in the host. A red-shift in 430-nm band was attributed to the shift of Eu²⁺ 5d energy level which was influenced by contraction and distortion of the unit cell. As the feature of the outside 3d electrons, ^[11] the energy level of transition metal ions are much affected by the crystal field strength of the matrix.^[12] For Mn⁴⁺ -activated Ln₂Sn₂O₇ (Ln = Lu³⁺ , Y³⁺ , and Gd³⁺ ), the increased Mn⁴⁺ – O^{2 −} bond length induced the decrease in crystal field strength and energy level difference between ⁴A₂ and ⁴T₂.^[13]

In consideration of the formation of stable polyhedron structure, Al³⁺ co-doping has a significant role in crystal field strength and luminescence properties.^{[14– 16]} The increase of Al³⁺ addition in ZnGa₂O₄:Cr³⁺ induced a blue-shift of the excitation spectra due to the increase of crystal field strength.^[17] The deep red luminescence of the Mn⁴⁺ ion  in 3.5MgO· 0.5MgF₂· GeO₂, which was derived by replacing part of MgO in 4MgO· GeO₂ with MgF₂, had given rise to an increasing luminescence intensity.^{[18, 19]} In addition, the two broad excitation bands in a range of 300  nm– 480  nm make it suitable for UV and blue LED chips. In this paper, 3.5MgO· 0.5MgF₂· (1− 2x)GeO₂· xAl₂O₃:0.01Mn⁴⁺ (0 ≤ x ≤ 0.15  mol) phosphors are synthesized by the high temperature solid-state reaction method. The influence of foreign Al³⁺ ions on energy levels of the Mn⁴⁺ ions and thermal properties of phosphors are also discussed.

Powder samples were prepared by the high temperature solid-state reaction of mixtures with appropriate  molar ratios for 3.5MgO· 0.5MgF₂· (1− 2x)GeO₂· xAl₂O₃:0.01Mn⁴⁺ (0 ≤ x ≤ 0.15  mol) with starting materials consisting of MgO, MgF₂ of 99.9% purity, GeO₂, Al₂O₃ of 99.999% purity, and MnO₂ of 99.99% purity. The mixture was thoroughly mixed and transferred to a corundum crucible, which was heated at 1100  ° C for 6  h under atmosphere. After cooling to room temperature, the as-prepared samples were completely ground.

The crystal structures of powder samples were characterized by using a Rigaku 2500/PC x-ray diffractometer (XRD) with Cu Kα radiation source (0.1542  nm) at 40  kV and 150  mA. The scanning electron microscope (SEM) images and energy dispersive spectroscopy (EDS) spectra were obtained by field emission scanning electron microscopy (FESEM, HITACHI SU8010). Photoluminescence (PL) and excitation (PLE) measurements were performed by Jobin Yvon FL3 fluorescence spectrophotometer with a 450-W xenon lamp as an excitation source. Temperature-dependent PL spectra were measured at temperatures ranging from 300  K to 540  K by the same spectrophotometer with home-made heating equipment. Raman spectra were recorded using a RENISHAW InVia Raman microscope and an argon ion laser operating at 785  nm. All of the measurements were carried out in ambient atmosphere.

The XRD patterns for phosphor samples are revealed in Fig.  1. The samples each have an Mg₂₈Ge₁₀O₄₈-like structure that belongs to orthorhombic symmetry with space  group Pbam and cell parameters a = 14.343  Å , b = 10.196  Å , and c = 5.908  Å , which were reported by Von Dreele et al.^[20] and Bless et al.^[21] No diffraction peak shift is observed in any of the samples with different doping amounts of Al³⁺ . Considering the ion valence state and electro-negativity, Ge⁴⁺ could be substituted by Al³⁺ ions to keep the electric neutrality by substituting F ⁻ into O^{2 −} sites.^{[21, 22]} For similar effective ionic radii of Ge⁴⁺ (r = 0.39  Å , CN = 4) and Al³⁺ (r = 0.39  Å , CN = 4), Al³⁺ substituted in host lattice can be determined as no detectable change in cell volume. The SEM images and EDS spectra for the as-prepared phosphors at Al₂O₃ concentrations of 0, 0.05, 0.1, and 0.15  mol are shown in Fig.  2. Similar particle morphologies of polyhedra and sizes are observed for all of the samples. The atomic ratios for all cations in samples, which are summarized in Table  1, are consistent with the  molar ratios of raw materials.

Fig.  1.

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	Fig.  1. The XRD patterns for the 3.5MgO· 0.5MgF2· (1− 2x)GeO2· xAl2O3:0.01Mn4+ (0 ≤ x ≤ 0.15  mol) phosphors.

Fig.  2.

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	Fig.  2. The SEM images and EDS spectra of the samples at Al2O3 content values of 0, 0.05, 0.1, and 0.15  mol.

Table 1.

Table 1.

Table 1. Atomic ratios of all elements doped with different amounts of Al2O3. Al2O3 content/mol O/at.% F/at.% Mg/at.% Mn/at.% Ge/at.% Al/at.% 0 47.816 8.68 34.711 0.082 8.705 0.006 0.05 47.632 8.752 34.812 0.087 7.846 0.871 0.1 47.398 8.774 34.998 0.084 6.988 1.758 0.15 47.174 8.765 35.101 0.082 6.198 2.68

Table 1. Atomic ratios of all elements doped with different amounts of Al2O3.

Fig.  3.

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	Fig.  3. (a) Normalized PLE spectra of the samples with Al2O3 content values of 0, 0.05, 0.1, and 0.15  mol for the 657-nm emission of Mn4+ . The inset shows the variation of peak position for wavelength band 300  nm– 380  nm with Al3+ concentration. (b) The energy level diagram for the d3 configuration.

The PLE spectra of 3.5MgO· 0.5MgF₂· (1− 2x)GeO₂· xAl₂O₃:0.01Mn⁴⁺ (x = 0, 0.05, 0.1, and 0.15  mol) are shown in Fig.  3(a). The two broad wavelength ranges of 300  nm– 380  nm and 380  nm– 480  nm correspond to the spin-allowed ⁴A₂→ ⁴T₁ and ⁴A₂ → ⁴T₂ transitions of Mn⁴⁺ ions, respectively.^{[18, 19]} The higher-energy band peak shows the blue-shift from 334  nm to 320  nm with the increase of Al³⁺ concentration, whereas the lower-energy excitation band has no obvious change. As described by Von Dreele et al.^[20] and Bless et al., ^[21] Mg²⁺ ions occupied the center of the octahedral coordination sites by six oxygen atoms with two different Mg²⁺ – O^{2 −} bond distances. And Mn⁴⁺ ions were suggested to occupy Mg²⁺ sites of the center octahedral symmetry.^[22] The bond distance calculated by Vienna ab-initio simulation package (VASP) is 2.164  Å for and 2.091  Å for without Al₂O₃ incorporated, which are slightly larger than 2.158  Å and 2.086  Å for the Al₂O₃-doped structure, respectively. According to the Tanabe– Sugano energy diagram of d³ coordination in Fig.  3(b), the excitation energy between the ground state ⁴A₂ and the excited state ⁴T₁/⁴T₂ is strongly related to the crystal field strength, and their relationship can be expressed as^{[23, 24]}

where D_q is the crystal field strength for octahedral symmetry, R is the distance between the central ion and its ligand, Z is the charge or valence of the anion, e is the charge of the electron, and r is the radius of the d-wave function. The increased crystal field strength from shortened Mn⁴⁺ – O^{2 −} bond distance in the six-fold coordination brings about the shifts of ⁴T₁ and ⁴T₂ energy levels of Mn⁴⁺ , resulting in the observed blue-shift of ⁴A₂ − ⁴T₁ excitation band.

Fig.  4.

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	Fig.  4. The PL spectra of the 3.5MgO· 0.5MgF2· (1− 2x)GeO2· xAl2O3:0.01Mn4+ phosphors with Al2O3 concentrations of x = 0, 0.05, 0.1, and 0.15  mol.

Figure  4 shows the PL spectra of 3.5MgO· 0.5MgF₂· (1− 2x)GeO₂· xAl₂O₃:0.01Mn⁴⁺ phosphors with different Al₂O₃ concentrations of 0, 0.05, 0.1, and 0.15  mol. There are five sharp red emission peaks located at 623, 630, 639, 649, and 657  nm, respectively, which is ascribed to the different vibrational modes of ²E → ⁴A₂ of Mn⁴⁺ ions. The two dominant peaks around 649  nm and 657  nm originate from Stokes vibrational band by the coupling of the asymmetric Mn₄ + − O^{2 −} deformation and stretching modes.^{[19, 22]} Since the ²E → ⁴A₂ transition is spin- and parity-forbidden, the two weak emission bands from the anti-Stokes can be observed at around 623  nm and 630  nm. And the peak at 639  nm, which seems to be weakest, is from the zero-phonon transition.^{[19, 22]} Owing to their weak dependence on crystal field strength, the emission peaks for ²E energy level are almost unchanged.

A main obstacle in developing  practical WLED technology is the thermal quenching of phosphors; i.e., the luminescent efficiency and color rendering index deteriorate with an increase of the operating temperature. Figure  5 shows the PL emission spectra of 3.5MgO· 0.5MgF₂· (1 − 2x)GeO₂· xAl₂O₃:0.01Mn⁴⁺ (x = 0, 0.05, 0.1, and 0.15  mol) phosphors under 400-nm excitation in a temperature range from 300  K to 540  K. Their main emission peak positions are kept at 657  nm for all samples, whereas the full width at half maximum (FWHM) is obviously broadened with the increase of the measurement temperature as shown in Fig.  6(a). This phenomenon is associated with the enhanced electron– phonon interaction by the increased population density of phonons at high temperature.^[25] The normalized emission intensities under different measurement temperatures are displayed in Fig.  6(b).

Fig.  5.

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	Fig.  5. Temperature-dependent PL spectra of phosphors with Al2O3 concentration x = 0, 0.05, 0.1, and 0.15  mol under the excitation of 400  nm.

Fig.  6.

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	Fig.  6. Temperature-dependence of (a) FWHMs and (b) normalized emission intensities of the 3.5MgO· 0.5MgF2· (1− 2x)GeO2· xAl2O3:0.01Mn4+ phosphors with x = 0, 0.05, 0.1, and 0.15  mol.

As the temperature increases from room temperature up to 425  K, the emission intensities reach 45.7%, 49.8%, 72.5%, and 52.3% of initial intensity for x = 0, 0.05, 0.1, and 0.15  mol, respectively. The activation energy can be determined with fitting procedure by using the Arrhenius equation, ^{[25, 26]}

where I₀ is the initial intensity, I_T is the intensity at a given

Fig.  7.

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	Fig.  7. Fitted curves for ln[I0/IT)-1] versus 1/kT from Arrhenius equation. The symbols and straight lines represent the experimental data and fitted line under Al2O3 concentrations of 0, 0.05, 0.1, 0.15  mol, respectively.

temperature T, c is a constant for a certain host, Δ E is the activation energy for thermal quenching, and k is the Boltzmann’ s constant. The thermal quenching data under different Al₂O₃ concentrations in Fig.  6(b) are fitted for the curves in Fig.  7. The slope values, which are indicated as Δ E, are 0.23, 0.30, 0.41, and 0.33  eV, respectively.

The nonradiative transition process at high temperature is usually associated with the enhanced electron– phonon interaction due to the increased population density of phonons.^{[25, 26]} The influence of Al³⁺ doping on the phonon energy of 3.5MgO· 0.5MgF₂· GeO₂ is discussed according to Raman spectra in Fig.  8. Except for the band of octahedral MnO₆^[27] and tetrahedral GeO₄ group, ^{[28, 29]} the emerging band at 702  cm^{− 1} for Al₂O₃ doping samples is ascribed to the stretching vibration of tetrahedral AlO₄ groups.^{[30, 31]} It was suggested in Refs.  [23] and [32] that the nonradiative relaxation probability can be described by W_NP = C[n(T) + 1]^Pexp (− α Δ E), in which constant C is dependent on the phonon density of state ρ (ω _P)^[33] and expressed as C∝ ρ (ω )^{5 / 3}. The maximum phonon density of state ρ _m (ω _P) is typically described as the integral area ratio of the band at 827  cm^{− 1}– 867  cm^{− 1}, corresponding to maximum phonon energy, to the whole band of the Raman spectrum. The values of ρ _m (ω _P) are calculated to be 0.1086, 0.1081, 0.0804, and 0.1051 for Al₂O₃ concentration x = 0, 0.05, 0.1, and 0.15  mol, respectively. The minimum value of ρ _m (ω _P) at 0.1-mol Al₂O₃ doping is consistent with the observed weak thermal quenching.

Fig.  8.

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Fig.  8. Raman spectra for Al2O3 content values of 0, 0.05, 0.1, and 0.15  mol at room temperature. The bands denoted as a and b are the asymmetric stretching modes of octahedral MnO6. The band d is due to the stretching vibration of tetrahedral AlO4. The symmetric stretching, antisymmetric stretching, and stretching vibration modes of tetrahedral GeO4 are assigned to c, f, g, e, and h, respectively.

In this work, we synthesize a series of red-emission Mn⁴⁺ -activated 3.5MgO· 0.5MgF₂· (1− 2x)GeO₂· xAl₂O₃:0.01Mn⁴⁺ (0 ≤ x ≤ 0.15  mol) phosphors by high temperature solid-state reaction. The phosphors each exhibit a strong red emission under UV light excitation. The broad excitation band in 300  nm– 380  nm, which originates from the ⁴A₂ → ⁴T₁ transition of Mn⁴⁺ , is observed to be blue-shifted with the increase of addition Al₂O₃. We conclude that the crystal field strength D_q of the octahedral lattice structure could be changed by Al³⁺ additive due to the variation of Mn⁴⁺ – O^{2 −} bond length. Temperature-dependent PL spectra are measured in a temperature range of 300  K– 540  K. The thermal stability is found to be optimal for 0.1  mol of Al³⁺ doping concentration with the activation energy of 0.41  eV. The maximum phonon density of state is used to explain the thermal quenching effect.