Designing a super broadband near infrared material Mg₃Y₂Ge₃O₁₂:Cr³⁺ using cation inversion for future light sources

Abstract

A series of broad emission band near infrared materials Mg₃Y₂Ge₃O₁₂:Cr³⁺ (650–1200 nm) was prepared based on cation inversion. For trivalent chromium ions (Cr³⁺), garnet structural components can provide conditions for the occurrence of cation inversion. With an increase in Cr³⁺ concentration, the Mg²⁺ and Ge⁴⁺ cations are inverted to ensure valence equilibrium, which was explained by recording the low temperature spectrum of the structure and carrying out structural refinement. As a result, this structure provides a new luminescent center [GeO₆] for Cr³⁺, leading to a secondary enhancement in emission intensity. The wavelength of the main peak was found to move from 771 to 811 nm, and the full width at half maximum (FWHM) was broadened from 180 to 226 nm. The lattice occupation, luminescence mechanism and the reasons behind the red-shift and broadening of the spectra were studied in detail. By analyzing the crystallinity and particle size distributions of the samples, as well as the Cr³⁺ ion energy level shift, it was determined that cation inversion is an effective method that can be used to tune the luminescence performance. Meanwhile, a super broad near infrared light emitting diode (LED) with a FWHM of 260 nm was obtained by combining a GaN chip with MYG:0.40Cr³⁺.

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1 Introduction

To date, phosphor-converted light-emitting diodes (LEDs) have played an important role in the field of luminescent applications because of their energy-saving, environmental protection, and other important characteristics.^1–3 However, the application value of broad band near-infrared (NIR) light is becoming ever more important, especially in food detection, biological detection, fingerprint identification and other fields.^4–6 Therefore, research on NIR LEDs has attracted increasingly more attention. The most widely used method for producing NIR LED devices is to combine a NIR phosphor and GaN blue LED chip.^7,8

NIR phosphors have been studied extensively. For example, Kolesnikov et al. reported NIR phosphors with YVO₄ as a matrix and Nd³⁺ as an activator. YVO₄:Nd³⁺ exhibits two emission bands at around 808 and 880 nm when excited by light at 532 nm. Since these bands are thermally coupled over a wide temperature range, YVO₄:Nd³⁺ can be used as a ratiometric luminescence thermal sensor. Singh et al. reported that ZnMgAl₁₀O₁₇:Er³⁺ emits NIR light in the ranging of 1450–1650 nm.⁹ At the same time, the introduction of Yb³⁺ ions was shown to improve the luminescence intensity of Er³⁺ by energy transfer, which does not affect the occupation of Er³⁺. Since the luminescence of rare earth ions originates from transitions within the 4f energy level, the emission of the ions is hardly affected by the change in the surrounding crystal field environment, due to the shielding effect of the 5s and 5p electron layers. Transition metal ions are a good choice as NIR activators because their emission spectra can be well adjusted by changing the crystal field environment. For example, the novel NIR luminescent material Li₂ZnGe₃O₈:Mn²⁺ was prepared by our research group, with an emission spectrum ranging from 650 to 900 nm.¹⁰ However, the emission of Mn²⁺ ions is closer to the red light region, such as for Ca_{9−x−y−z}Mg_xSr_yBa_zCe(PO₄)₇:Eu²⁺, Mn²⁺,¹¹ and MgGeO₃:Mn²⁺.¹² The Cr³⁺ ion is also an excellent NIR activator, which shows different luminescence properties in different crystal field environments. For example, in a strong crystal field environment, the emission of Cr³⁺ originates from the spin forbidden transition ²E(²G) → ⁴A₂(⁴F), it displays a sharp band spectrum (the peak is near 700 nm) and is slightly affected by the change in crystal field strength, according to its Tanabe–Sugano (3d³) diagram. Deng et al. prepared LiGa₅O₈:Cr³⁺ with enhanced persistent luminescence that has a narrow peak located at 700 nm.¹³ Cr³⁺ and Sn⁴⁺ ions prefer to occupy octahedral sites. At the same time, the introduction of Sn⁴⁺ can effectively improve afterglow performance. In a weak crystal field, the emission of Cr³⁺ originates from the spin-allowed transition ⁴T₂ → ⁴A₂, showing a broadband emission (probably in the range of 600–1000 nm) that is easily affected by the change in the crystal field environment.^14,15 For example, the NIR phosphor Ca₂MgWO₆:Cr³⁺ was prepared by Meng et al., which exhibited a broadband peak at 803 nm when excited at 371 nm.¹⁶ In Ca₂MgWO₆, Cr³⁺ ions substitute Mg²⁺ sites and are located in a weak crystal field. Shao et al. reported the NIR material ScBO₃:Cr³⁺ and obtained a NIR LED.⁸ In the host, the Cr³⁺ ions occupied the Sc³⁺ sites with relatively low crystal field strength and showed a broadband emission peak at 800 nm.

A garnet has a cubic structure with Ia3̄d symmetry, which can be described in terms of a 160 atom body-centered cubic unit cell.¹⁷ Its chemical formula is generally A₃B₂C₃O₁₂, in which the atoms A, B, and C occupy the lattice sites 24c, 16a, and 24d, respectively.^18,19 The coordination numbers of atoms A, B and C are 8, 6, 4, respectively, and each tetrahedron is connected to four octahedra, providing multiple sites for active ions to occupy.²⁰ Since the 1970s, Cr³⁺-doped garnet phosphors have been one of the most famous types of NIR phosphors, for example, YAG:Cr³⁺, which is a typical NIR material.²¹ In addition, Kiss and Duncan further studied the non-radiative energy transfer from Cr³⁺ to Nd³⁺ in YAG.^22,23 Another characteristic of garnets is the cationic inversion that occurs between the tetrahedra and octahedra, which can be used to improve the luminescence performance of an activator, such as improving luminescence efficiency, broadening excitation or emission spectra.^24–26 In order to improve the probability of wide-spectrum NIR emission, an inverse-garnet structure can be used as a host material. In a normal garnet, all three polyhedra are regular polyhedra. In order to increase the probability of wide-spectrum emission, the inverse-garnet structure is a good choice because its polyhedra are distorted. Therefore, in this work, Mg₃Y₂Ge₃O₁₂ with an inverse-garnet structure was chosen as the matrix and Cr³⁺ ions were selected as an activator to achieve broadband NIR emission. A series of NIR materials were prepared that can cover from 700 to 1200 nm. The full width at half maxima (FWHM) reached 226 nm and the emission intensity was enhanced due to cationic inversion. A NIR LED was prepared by combining MYG:0.40Cr³⁺ with a GaN chip.

2 Experimental

2.1 Materials and synthesis

A series of Mg₃Y₂Ge₃O₁₂:xCr³⁺ materials were synthesized via a high-temperature solid-state method using MgO (99.99%), Y₂O₃ (99.99%), GeO₂ (99.99%) and Cr₂O₃ (99.99%) as raw materials. The raw materials were weighed using electronic scales with a 0.0001 g accuracy. Then, the raw materials were thoroughly mixed and ground in an agate mortar for 30 min. The mixed materials were transferred into alumina crucibles and sintered at 1350 °C for 6 h in a high temperature furnace. Finally, the synthesized samples were cooled to room temperature and ground to a powder. The chemical reaction formula is as follows:

3MgO + (1 − x/2) Y₂O₃ + 3GeO₂ + x/2Cr₂O₃ → Mg₃Y_2−xGe₃O₁₂:xCr³⁺

2.2 Characterization

The phase structures of the as-prepared samples were analyzed using X-ray diffraction (XRD), which was carefully performed using a D8A25 Focus diffractometer (Bruker) measuring at 40 kV and 40 mA and recording patterns in the range of 2θ = 10–80° at a scan rate of 0.05° per second, with a step size of 0.01°. The steady-state photoluminescence spectra of the phosphors were measured using an FL3 (Edinburgh Instruments) fluorescence spectrometer. Furthermore, diffuse reflection spectra of the phosphors were measured using a Hitachi U-4100 machine with a scanning wavelength range of 200–800 nm. The thermoluminescence spectra of the samples were recorded using a FJ-427A1 TL dosimeter with a fixed heating rate of 1 °C s⁻¹ within the range of 25–300 °C. Low temperature photoluminescence (PL) spectra were measured from 10–300 K by a spectrometer equipped with a CCS-150 JANIS temperature control device. An electron paramagnetic resonance (EPR) spectrum was measured using a JES FA300 spectrometer, which can detect that the minimum absolute rotation number is less than 1 × 10⁹ spins per G.

3 Results and discussion

3.1 Phase characterization and crystal analysis

Fig. 1 shows the crystal structure of Mg₃Y₂Ge₃O₁₂ (MYG), which is cubic and in the space group Ia3̄d. The cell features three types of polyhedra, a [GeO₄] tetrahedron, a [Mg2O₆] octahedron and a twisted [Mg1/Y1O₈] dodecahedron, making it an inverse garnet structure. The views of the [111] and [010] directions are shown in Fig. 1(a). In Fig. 1(b), the XRD sample data (MYG, MYG:0.03Cr³⁺ and MYG:0.30Cr³⁺) are compared with the standard card of MYG (ICSD #280049). There are no impurity peaks that can be observed and the XRD patterns match well, which indicates that all of the samples are pure. With an increase in Cr³⁺ concentration, there is no shifting of the peaks, thus the doping of Cr³⁺ does not change the crystal structure. The X-ray photoelectron (XPS) spectrum of MYG:0.03Cr³⁺ in Fig. S1† shows that the valence state of Cr did not change. In addition, the Rietveld refinement of MYG was carried out using a general structure analysis system (GSAS) with the initial structure model of Mg₃Y₂Ge₃O₁₂ (ICSD #280049) crystallized in a cubic system, which is shown in Fig. 1(c). The values of the refinement parameters, χ² (=1.428), R_wp (=11.58%), and R_p (=8.59%), are satisfactory, which also suggests that MYG was well synthesized. The other results of the refinement are presented in Table S1.†

Fig. 1 (a) Crystal structure of Mg₃Y₂Ge₃O₁₂; (b) XRD patterns of MYG:xCr³⁺ (x = 0, 0.03, 0.30) and the standard Mg₃Y₂Ge₃O₁₂ (ICSD #280049) crystal data; (c) X-ray Rietveld refinements for MYG.

The EPR spectrum of MYG was measured to elucidate its structural characteristics at room temperature, as shown in Fig. 2(a). The EPR spectrum exhibits an intense resonance signal centered at g ∼ 2.00, which indicates that there are typical oxygen vacancies in the host.^27,28 Generally, an oxygen vacancy can capture one electron, forming a sequential resonance source with S = 1/2, which results in the appearance of a stable EPR signal at around g ∼ 2.00. That is to say, the oxygen vacancies are formed in the process of MYG crystal growth. Furthermore, the thermoluminescence (TL) glow curve of MYG was measured in the range of 300–600 K after 10 minutes of ultraviolet lamp irradiation (254 nm), as shown in Fig. 2(b). There is a peak located at 395 K in the spectrum, which can be attributed to oxygen vacancies. For the host, the trap depth (E) can be obtained using the peak shape method, as shown in the following equation:^29,30

ω = δ + τ (1)

where k is the Boltzmann constant (1.38 × 10⁻²³ J K⁻¹), T is the temperature, (395 K for the samples in this work), ω is the FWHM of the peak, τ is the value of the partial FWHM at low temperature, and δ is the value of the partial FWHM at high temperature. According to eqn (1), the trap depth for the host was calculated to be 0.41 eV. The generation of oxygen defects can be attributed to the synthetic means, as shown in Fig. 2(c), due to burning at high temperatures in the air.

Fig. 2 (a) EPR spectrum of MYG at room temperature; (b) TL spectra of MYG; (c) the process of the generation of oxygen defects.

3.2 Properties of low concentration doping MYG:xCr³⁺(x = 0.005–0.07)

Fig. 3(a) shows the photoluminescence excitation (PLE) spectrum of MYG:0.03Cr³⁺ and the photoluminescence spectra (PL) of MYG:xCr³⁺ (x = 0.005, 0.01, 0.03, 0.05, 0.07). The PLE spectrum features three peaks ranging from 250 to 750 nm, which are at 250 nm (⁴A₂ → ⁴T₁(⁴P)), 436 nm (⁴A₂ → ⁴T₁(⁴F)) and 624 nm (⁴A₂ → ⁴T₂(⁴F)).^31,32 The phosphors can be excited best under 436 nm irradiation. At the same time, the emission of the GaN chips at 455 nm can also be well absorbed, the absorption intensity at which is about 92% of that at 436 nm, which indicates that the MYG:xCr³⁺ samples are promising phosphors. The emission spectra ranging from 650 to 1200 nm were obtained under 436 nm irradiation, which originates from the spin-allowed transition ⁴T₂ → ⁴A₂ of Cr³⁺.^33,34 To better observe the changes in the emission spectra with an increase in Cr³⁺ concentration, the normalization diagram is shown in Fig. 3(b). It can be obviously seen that the emission spectra red-shift and that the FWHM broadens, with more detailed spectral change data given in Fig. 3(c). The peak shifts from 771 to 783 nm with increasing Cr³⁺ concentration. At the same time, the FWHM of Cr³⁺ emission is broadened from 180 to 189 nm. When the concentration of Cr³⁺ is up to 0.03, the emission intensity decreases gradually because of concentration quenching.

Fig. 3 (a) Excitation spectra of MYG:0.03Cr³⁺ and emission spectra of MYG:xCr³⁺(x = 0.005–0.07); (b) normalized emission spectra of MYG:xCr³⁺; (c) the trend in the emission intensity, wavelength and FWHM of MYG:xCr³⁺with increasing x.

Generally, the broadband emission of Cr³⁺ is generated in intermediate or weak crystal field environments. The order of crystal field strength is tetrahedral > octahedral > dodecahedral, therefore, it is hard for Cr³⁺ to enter into a tetrahedral lattice with its strong crystal field, but it tends to enter octahedral [Mg2O₆] and dodecahedral [Mg1/Y1O₈] lattices.^5,35 To gain further knowledge about the occupation of Cr³⁺ in MYG and structural information on MYG:xCr³⁺, the XRD Rietveld refinements of MYG:xCr³⁺ (x = 0.01, 0.03, 0.05, 0.07) were carried out using the GSAS program, the results of which are shown in Fig. 4(a). A model of MYG (ICSD #280049) was used as the initial structure to refine the samples in the 2θ range of 10–80°. The parameters of the refinement results (R_p, R_wp and χ²) were found to be in line with the standard, which indicates that the Rietveld refinement results are reliable. The atomic positions are shown in Table S2.†

Fig. 4 (a) Rietveld refinement results of MYG:0.01Cr³⁺, 0.03Cr³⁺, 0.05Cr³⁺ and 0.07Cr³⁺; (b) the trend in the cell parameters a/b/c and cell volume V with an increase in Cr³⁺; (c) the occupancy of Cr³⁺ ions in MYG.

The cell parameters a, b and c, as well as the cell volume, V, of the samples are shown in Fig. 4(b). The values of a, b, c and V gradually decrease, which can be attributed to the radius of Cr³⁺ (0.0615 nm) being smaller than those of Mg²⁺ (0.072 nm) and Y³⁺ (0.1019 nm). In Fig. 4(c), the occupying information of the Cr³⁺ ions at different Cr³⁺ concentrations is shown. It can be seen that the Cr³⁺ ions are only doped into the [Mg2O₆] site and [Mg1/Y1O₈] site, as we expected. Therefore, there are three luminance centers of Cr³⁺, which was also proven by the low temperature spectrum of MYG:0.03Cr³⁺ shown in Fig. S2.† Fig. 5(a) shows the PL spectrum of MYG:0.03Cr³⁺, which was divided into three sub-peaks with peaks at 748 nm (peak 1), 806 nm (peak 2) and 899 nm (peak 3) using a Gaussian multiple peak fitting technique. At the same time, the corresponding excitation spectra of MYG:0.03Cr³⁺, monitoring the three sub-peaks, were measured and are shown in Fig. S3.† Taking the energy of the ⁴A₂ state to be equal to zero, the value of the crystal field strength, D_q, and the Racah parameter, B, can be calculated using the following eqn (2) and (3):^36–38

E(⁴T₂) = 10D_q

Fig. 5 (a) The PL and the three sub-peaks of MYG:0.03Cr³⁺; (b) fluorescence decay curves of Cr³⁺ emission in MYG:xCr³⁺ phosphors monitored at 748, 806 and 899 nm.

The relative energy between the ⁴T₁(⁴F) and ⁴T₂(⁴F) levels is represented by the symbol ΔE. Therefore, the Racah parameter, B, can be calculated by simplifying eqn (2), in the following way:

According to eqn (2) and (3), the values of D_q and D_q/B are 1594, 1574, 1557 and 2.38, 2.24, 2.13, indicating that the three sub-peaks arise from different luminescence centers. The value of D_q is inversely proportional to R⁵.³⁹ Therefore, the bond lengths corresponding to peaks 1, 2 and 3 increase gradually. The average bond lengths of Mg2–O and Mg1/Y1–O are 2.065 and 2.390 Å, respectively. Because the radius of Mg1 (0.089 nm) is smaller than that of Y1 (0.1019 nm), the average bond length of Y1–O is longer than that of Mg1–O. The emission peaks of Cr_Mg2, Cr_Mg1 and Cr_Y1 correspond to peaks 1, 2 and 3, respectively. Fig. 5(b) shows the fluorescence decay curves of Cr³⁺ emission in the MYG:xCr³⁺ phosphors monitored at 748, 806 and 899 nm, which were fitted using a double exponential equation:⁴⁰

I(t) = I₀ + A₁ e^−t/τ₁ + A₂ e^−t/τ₂ (4)

where I(t) is the emission intensity at time t, A₁ and A₂ are constants, and τ₁ and τ₂ the lifetimes for rapid and slow decay, respectively. The decay process in the sample can be characterized using the average lifetime τ*, which can be obtained from eqn (5) as follows:

τ* = (A₁τ₁² + A₂τ₂²) (5)

The lifetimes of the peaks located at 748, 806 and 899 nm are 25.3, 21.1 and 6.3 μs, respectively. Besides this, according to Fig. 4(c), with an increase in the Cr³⁺ concentration, the Cr³⁺ content in [Mg1/Y1O₈] (Cr_Mg1/Y1) increases, but first increases and then decreases in [Mg2O₆] (Cr_Mg2), which is the same change trend as for the emission intensity, as shown in Fig. 3(c). It can be inferred that in the range of 0 < x < 0.1, the luminous intensity of Cr³⁺ in [Mg2O₆] determines the overall luminous intensity.

Because there are defects in the host, as seen from Fig. 2(b), the effect of defects on the luminescence intensity should be considered. Therefore, the TL spectra of the samples were measured and are shown in Fig. S4.† However, the TL peak was observed to gradually disappear, which can be attributed to the unequal substitution of Cr³⁺. The substitution process of Cr³⁺ in the host can be represented by the two equations:

Cr³⁺ → Y³⁺ + Cr_Y (7)

When Mg²⁺ is substituted by Cr³⁺, a Mg²⁺ cation vacancy occurs due to different valence states. However, when Y³⁺ is substituted by Cr³⁺, there are no cation vacancies. Because of the existence of oxygen vacancies, in MYG, the make up for the defects with an increase in the Cr³⁺ concentration, which results in the disappearance of the TL peak. In addition, the refined data shown Fig. 4(a) also indicate that the crystallinity improves.

The reflection spectra of MYG:xCr³⁺ (x = 0.01, 0.03, 0.05, 0.07) were measured and are shown in Fig. 6(b). The two absorption peaks arising from the ⁴T₁(⁴F) and ⁴T₂(⁴F) levels shift to a shorter wavelength with an increase in Cr³⁺ concentration. Using eqn (2) and (3), the values of D_q, B and D_q/B were obtained and are displayed in Table 1. It can be seen from Table 1 that the D_q/B values decrease with an increase in Cr³⁺ concentration. Together with the Tanabe–Sugano (3d³) diagram shown in Fig. 6(a), the energy difference between the energy levels ⁴T₂ and ⁴A₂ decreases as the value of D_q/B decreases, which leads to a red-shift in the emission spectra.

Fig. 6 (a) Tanabe–Sugano (3d³) diagram; (b) reflection spectra of MYG:xCr³⁺ (x = 0.01, 0.03, 0.05, 0.07).

Table 1 The calculation results of eqn (2) and (3)

MYG:xCr^3+	Dq	B	Dq/B
x = 0.01	1587.30	648.25	2.45
x = 0.03	1589.83	651.24	2.44
x = 0.05	1594.90	671.25	2.38
x = 0.07	1600.00	677.59	2.36

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The spectral broadening is related to the degree of energy splitting. An enhancement in the energy level splitting leads to spectral broadening, whereas in contrast, a reduction in the energy level splitting leads to spectral narrowing. The splitting degree of the energy level is positively correlated with the crystal field strength, D_q. According to Table 1, the value of D_q increases with an increase in the Cr³⁺ concentration, which leads to an enhanced splitting of the ⁴T₂ energy level and spectral broadening.

3.3 Properties of high concentration doping in MYG:xCr³⁺(x = 0.1–0.4)

It is well known that energy transfer leads to energy loss, hence we tried three other possible ways to improve the luminous properties of Cr³⁺. First, for a garnet structure, the inversion of cation sites may occur, which will result in the formation of new lattices to improve the luminescence properties.³⁵ The second is to construct appropriate defects, which can increase the channel of energy acquisition of the Cr³⁺ ion.^41–43 Third, the upper level ⁴T₂ can obtain more electrons via overlapping of the energy levels ⁴T₂ and ²E.⁴⁴ The ²E level can act as the energy storage level, when the energy levels overlap, the energy transfer from the ²E to ⁴T₂ level occurs. However, through the above analysis, the ⁴T₂ energy level gradually moves down and the defects disappear with an increase in Cr³⁺ concentration. Thus, the first way is a good choice. Because the selection of host components needs to consider the valence relationship between Cr³⁺ and the cationic states in the host, Cr³⁺ not only acts as an activator, but also regulates the composition of the host, which can avoid the introduction of new ions to destroy the structure. We speculate that the structure of the inverse garnet is further disordered due to an increase in the concentration of exogenous ions, resulting in a new arrangement of cations, the process of which is shown in Fig. 7. The cation inversion between a tetrahedron and octahedron occurs and the occupancy of Cr³⁺ in the octahedron is in agreement with the following equation:

Fig. 7 Model diagram of when the concentration of Cr³⁺ continues to increase.

At the same time, a new luminescence center, is formed, which enhances the emission intensity and broadens the emission spectra. As the concentration of Cr³⁺ continues to increase, the value of D_q/B continues to decrease, resulting in the continued red shift of the spectrum. Therefore, it is necessary to further increase the concentration of Cr³⁺ ions to obtain NIR emission materials with a wider FWHM that are closer to long wave.

The PL spectra of MYG:xCr³⁺(x = 0.1, 0.2, 0.3, 0.35, 0.4) are shown in Fig. 8(a). The emission intensities of the samples are enhanced again, the spectra keep red shifting, and the FWHM continues to broaden. In order to show the spectral changes more clearly, the normalized spectra and the PL spectral change curves are shown in Fig. 8(b) and (c), respectively. It is obvious that the emission wavelength moves from 789 to 811 nm, and the FWHM extends 204 nm to 226 nm. The emission intensity first increases and then decreases, and the intensity is strongest at x = 0.30, which is about 1.5 times the emission intensity exhibited by MYG:0.03Cr³⁺. According to the experimental design, with an increase in the Cr³⁺ ion concentration, a new luminescence center Cr_Ge should arise. The low temperature spectrum of MYG:0.30Cr³⁺ at 10 K was measured and is shown in Fig. S5.†

Fig. 8 (a) Emission spectra of MYG:xCr³⁺(x = 0.1–0.4); (b) normalized emission spectra of MYG:xCr³⁺ (x = 0.1–0.4); (c) the trend in the emission intensity, wavelength and FWHM of MYG:xCr³⁺ (x = 0.005–0.4).

It can be seen that a new peak (peak 4) appears, which means that a new luminescence center is formed with an increase in Cr³⁺ concentration. The formation of the new luminescence center is due to Ge⁴⁺, with a smaller radius, entering the octahedron, i.e., cation inversion. In order to explore the change in sample properties in more detail, the XRD data of the samples were refined and the results are shown in Fig. 9(a). The refinement parameters (R_p, R_wp and χ²) all meet the requirements. The atomic positions are shown in Table S3.†

Fig. 9 (a) Rietveld refinement results of MYG:0.10Cr³⁺, 0.20Cr³⁺, 0.30Cr³⁺ and 0.40Cr³⁺; (b) the change in the cell parameters (a/b/c); (c) the occupancy of Cr³⁺ ions in MYG:xCr³⁺(x = 0.10, 0.20, 0.30, 0.40); (d) the changes in the [Mg2O₆], [(Mg1/Y1)O₈] and [GeO₄] volumes.

The important parameters of the refinement results are shown in Fig. 9(b)–(d), such as a/b/c, V, the occupancy of Cr³⁺ and the volume changes of the tetrahedron (V-[GeO₄]), octahedron (V-[Mg2O₆]), and dodecahedron (V-[Mg1/Y1O₈]). From Fig. 9(b), it can be seen that the cell parameters (a/b/c and V) decrease, as observed in the samples with low Cr³⁺concentration. According to Fig. 9(c), the Cr³⁺ ions still substitute for the [Mg2O₆] and [Mg1/Y1O₈] sites. However, the concentration of Cr_Mg2 increases despite a slight decrease at x > 0.3, which is important for enhancing the luminescence intensity. The increase in Cr_Mg2 indicates that a new luminescence center arises in [Mg2O₆], corresponding to peak 4 shown in Fig. S5.†

The source of the new luminescence center can be well observed by analyzing the polyhedral volume in the host shown in Fig. 9(d). Since the Cr³⁺ is in the positions of [Mg2O₆] and [Mg1/Y1O₈], and the radius of Cr³⁺ (0.0615 nm) is less than those of Mg²⁺ (0.072 nm, N = 6), (0.089 nm, N = 8) and Y³⁺ (0.1019 nm, N = 8), V-[Mg2O₆] and V-[Mg1/Y1O₈] will be decreased. As for [GeO₄], there are two reasons for the change in volume. One reason is that the [GeO₄] volume increases due to the tugging action of [Mg1/Y1O₈] and [Mg2O₆]. Another reason is the occurrence of cation inversion between Mg2 (0.057 nm, N = 4), (0.072 nm, N = 6) and Ge (0.039 nm, N = 4), (0.053 nm, N = 6). When x > 0.30, due to the decrease in the degree of cation inversion, the volume of [GeO₄] decreases. Therefore, it can be inferred that with an increase in Cr³⁺ concentration (x < 0.30), the degree of cation inversion increases, which leads to an increase in the concentration of Cr³⁺ in the octahedron, which enhances the emission intensity. However, when x > 0.30, the degree of cation inversion begins to decrease and the emission intensity decreases.

In addition, the reflection spectra of MYG:xCr³⁺ (x = 0.10, 0.20, 0.30, 0.40) were measured and are shown in Fig. 10(a). Using eqn (2) and (3), the values of D_q, B and D_q/B were calculated and the results are shown in Fig. 10(b), where it can be seen that the value of D_q/B gradually decreases. Therefore, together with the information shown in Fig. 6(a), it was determined that the ⁴T₂ level is close to the ⁴A₂ level, which causes a red shift in the spectrum. The value of D_q gradually increases, which leads to spectral broadening. With an increase in Cr³⁺ concentration, the change in the crystal field environment is in line with the experimental design and requirements.

Fig. 10 (a) Reflection spectra of MYG:xCr³⁺ (x = 0.10, 0.20, 0.30, 0.40); (b) the curves of the changes in D_q, B and D_q/B.

3.4 Preparation and performance tests on a NIR LED

In order to realize its practical application, a NIR LED was prepared by combining MYG:0.40Cr³⁺ and a GaN blue chip (455 nm). The emission spectrum of the LED was measured under 3 V voltage and 30 mA current, which is shown in Fig. 11. It can be seen that the broad emission band ranges from 650 to 1200 nm, and the FWHM is 260 nm. The photoelectric efficiency and optical power are about 6.6% and 5.9 mW, respectively.

Fig. 11 Spectrum of the NIR LED measured under 3 V and 30 mA.

4 Conclusions

Broadband NIR Mg₃Y₂Ge₃O₁₂:Cr³⁺ materials were obtained using a high-temperature solid-state method. With an increase in Cr³⁺ concentration, the emission peak of the materials shift from 771 to 811 nm and the FWHM is broadened from 180 to 226 nm, which can be attributed to the change in the crystal field strength, D_q and D_q/B. when x > 0.1, cation inversion between Mg2 and Ge occurs to ensure valence equilibrium, which results in the generation of a new luminescence center (Cr_Ge). The formation of cation inversion leads to an increase in Cr³⁺ concentration in the octahedra and also results in an increase in emission intensity. A super broadband (260 nm) NIR LED was obtained by combining a GaN chip with MYG:0.40Cr³⁺. This work provides a useful method for achieving a Cr³⁺-doped gallate composition.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (No. 51672066, 51902080), the Funds for Distinguished Young Scientists of Hebei Province, China (No. A2018201101), the Natural Science Foundation of Hebei Province, China (No. E2019201223), and the personnel training project of Hebei Province, China (No. A201902005).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra01742f

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