Mg4Nb2O9:Cr3+: broadband near-infrared luminescence

The exploration of broadband Near infrared (NIR) phosphors is one of the research priorities to enable the advancement of NIR devices. In this work, Mg4Nb2O9:Cr3+ phosphors were prepared by high-temperature solid-state reaction method. Mg4Nb2O9:Cr3+ can emit ultra-broadband NIR centred at 880 nm with an extremely full width at half maximum (FWHM) of near 200 nm, and NIR phosphor-converted LED (pc-LED) has been fabricated by combining a 450 nm blue LED with MNO:0.5%Cr to demonstrate its application prospect in night vision.


Introduction
NIR spectroscopy has applications in many frontier fields [1] , including spectral detection, biosensing, night vision surveillance, and plant illumination.In recent years, there has been a shift towards miniaturization and integration of NIR equipment [2] .It appears that NIR pc-LEDs may be the only suitable NIR light source for meeting the present demands [3] .That is to say, NIR pc-LEDs demonstrate excellent suitability as light sources for compact NIR devices, thereby offering promising research and application opportunities [4] .
Up to now, Cr 3+ -doped broadband NIR phosphors gradually demonstrate outstanding performance [5] .Therefore, it makes sense to explore Cr 3+ doped phosphor which could help to achieve NIR emission with larger FWHM [6] .Generally, the selection of a host suitable for Cr 3+ doping typically considers the presence of distorted octahedra in the host materials, equivalent states, and similar ionic radii between the central metal ion and Cr 3+ [7] .
However, specific ions such as Ta 5+ or Nb 5+ can create a six-coordination environment with an ionic radius (r = 0.64 Å) that is relatively similar to that of Cr 3+ (r = 0.615 Å), despite possessing a different charge.Therefore, there seems to be some controversy regarding whether Cr 3+ can replace Ta 5+ or Nb 5+ for tantalate/niobate.For instance, evidence suggests that Cr 3+ should replace Nb 5+ in LiNbO3 [8] .However, for Mg4Ta2O9, it has been reported that Cr 3+ only substitutes for Mg 2+ and not for Ta 5+ [9] .Mg4Nb2O9 and Mg4Ta2O9 have exactly the same crystal structure, and the ionic radii of Nb 5+ and Ta 5+ are completely equal.Theoretically, Cr 3+ is expected to merely occupy the crystallographic site of Mg 2+ in Mg4Nb2O9, just as what happened to Cr 3+ in Mg4Ta2O9 [9] .But G. Blasse thought that for Mg4Nb2O9, Cr 3+ should replace both Mg 2+ and Nb 5+ (2Mg 2+ + Nb 5+ →3Cr 3+ ) [10] .It is only very regrettable that a detailed discussion of the luminescent properties of Mg4Nb2O9 is not given.G. Blasse also reported the NIR luminescence of Cr 3+ doped Ba3SrM2O9 (M=Nb, Ta), and it was shown that [Cr/NbO6] and [Cr/TaO6] complexes were responsible for the spectra observed [11] .Apparently, there is still no consensus as to whether Cr 3+ will replace Nb 5+ in Mg4Nb2O9.
In this work, a series of Mg4Nb2O9:x%Cr 3+ (MNO:x%Cr, x=0-5) phosphors were prepared by hightemperature solid-state reaction method.The photoluminescence (PL), photoluminescence excitation (PLE), decay, thermal quenching properties, and luminescence mechanism of MNO:Cr were studied in detail, respectively.We basically realize that in Mg4Nb2O9, Cr 3+ should replace both Mg 2+ and Nb 5+ as G. Blasse thinks [10] .Finally, NIR pc-LED has been fabricated by combining a 450 nm blue LED with MNO:0.5%Crphosphor to demonstrate its application prospect in night vision.

Experimental Section
2.1.Synthesis of MNO:x%Cr 3+ phosphors MNO:x%Cr 3+ (x=0-5) were synthesized by a conventional high-temperature solid-state reaction method.The raw materials, 4MgCO3•Mg (OH)2•5H2O (AR), Cr2O3 (AR) and Nb2O5 (4N), were weighed according to a specific stoichiometry and mixed with moderate alcohol in an agate mortar.The mixture was then thoroughly ground for 30 minutes before waiting for the alcohol to evaporate and dry.Afterward, the mixture was sintered in a muffle furnace at 1475°C for 4 hours.Finally, the sintered product was cooled to room temperature before ground into powder for characterization.

Characterization
The phase purity of samples was characterized by X-ray diffraction (XRD) of Bruker AXS Advance diffractometer with Kαradiation of a Cu target (=1.5406Å).Surface morphology and element distribution of the sample were analyzed using a scanning electron microscope (SEM, Hitachi, SU-70) and an energy dispersive spectrometer (EDS).The PL, PLE, and decay curves of the MNO:x%Cr 3+ were all collected using an FLS-980 fluorescence spectrophotometer (Edinburgh Instruments).Diffuse reflection spectra (DRS) were measured by UV-3600 PLUS (Shimadzu) using BaCO3 as a calibration.

Optical Properties of MNO:x%Cr
The PL and PLE, as well as the DRS of MNO:0.5%Cr, are shown in Fig. 2 (a).The two intense excitation bands monitored at 880 nm observed in the blue (488 nm) and red (685 nm) spectral range, which correspond to the 4 A2→ 4 T1( 4 F) and 4 A2→ 4 T2( 4 F) spin-allowed transitions of Cr 3+ ions, respectively.The strong excitation peak in the UV region corresponds to the 4 A2→ 4 T1 ( 4 P) transition.Additionally, the DRS absorption peaks coincide well with these excitation peaks.The range of PL covers 700 ~ 1300 nm with 187 nm FWHM, and the main peak is located at 880 nm, which corresponds to the 4 T2→ 4 A2 transition of Cr 3+ .Evidently, MNO:Cr is a broad-spectrum NIR phosphor.
The impact of various Cr 3+ concentrations on the luminescence of MNO:x%Cr is depicted in Fig. 1  (b).As the doping concentration increases (x=0-5), the emission peak steadily shifts towards the red, approximately 40 nm.The corresponding FWHM also increases from 187 nm to 208 nm, which is typically ascribed to the reabsorption between activators.Obviously, an excitation intensity of MNO:0.5%Cr is the optimal, beyond which concentration quenching occurs, resulting in a gradual decrease in emission intensity.
The DRS of MNO:x%Cr (x=0-5) is illustrated in Fig. 2 (d).The absorption peaks become stronger as Cr doping increases, but there is no significant shift in the peak position, indicates that the amount of Cr 3+ doping has almost no effect on the crystal field splitting in the 3d orbitals.Based on the Kubelka-Munk equation [12] , the value of Eg calculated for MNO is 4.95 eV, signifying its suitability as a luminescent matrix.
In the octahedral coordination of Cr 3+ ions, the Tanabe-Sugano (T-S) diagram can be used to describe the crystal field splitting [13] .The crystal field parameters Dq/B can be approximately calculated as 2.47, indicating that the Cr 3+ was situated in a medium crystal environment, which corresponds to the broadband emission.
As it is presumed that the luminescence was caused by the interaction of several lattice sites, the emission spectrum of MNO:Cr was fitted with a Gaussian splitting of the peaks, and the results overlap well match with it.The splitting of the peaks revealed three different fitted peaks, indicating the possibility of three different luminescence centers in MNO:Cr.To test this conjecture, the decay curves were tested at different Cr 3+ doping concentrations.Upon fitting the curves with different exponents, it became evident that a single exponential fit was not satisfactory and that the decay could be well described by a double or multiple exponential fit.This indicates the existence of at least two luminescence centers.Nonetheless, considering the charge balance, our preferred explanation is that Cr 3+ substitutes both Mg 2+ and Nb 5+ , which is consistent with Blasse's viewpoint.

Application
A NIR pc-LED device that combines a 450 nm LED with MNO:0.5%Cr was fabricated to demonstrate the suitability of the MNO:Cr (Fig. 1 (a)).The NIR light power density of the device is 0.695 W/cm 2 , and the input power is 1.0 W at a drive current of 200 mA (5 V).Recording objects in visible light using a standard camera as shown in Fig. 3 (b).In the case of Power-off, turning on the NIR pc-LED and using the same camera do not show any image (Fig. 3 (c)).In contrast, the NIR camera can capture grayscale image when NIR pc-LED light source is illuminated as shown in Fig. 3 (d).These results confirm the significant potential of MNO:Cr for NIR pc-LED applications.

Conclusion
In this work, a broadband NIR phosphor Mg4Nb2O9:Cr 3+ was successfully synthesized.Under blue light excitation, the main peak of the emission spectrum of the Mg4Nb2O9:Cr 3+ is located at 880 nm, and the FWHM is nearly 200 nm.The luminescence mechanism of Cr 3+ in Mg4Nb2O9 is most likely to be that Cr 3+ replaces Mg 2+ and Nb 5+ at the same time.These results may provide some useful ideas for discovering more Cr 3+ -substituted Nb 5+ -niobate NIR phosphors.

3. 1 .
Pure phase structure analysis The crystal structure of Mg4Nb2O9 is depicted in Fig. 1 (a) as a corundum-type structure classified under space group Pc1.This structure is composed of three different types of octahedra, including two different [MgO6] octahedra and one [NbO6] octahedra, where the [NbO6] octahedra share faces and edges in various niobium-containing compounds.XRD patterns in Fig. 1 (b) confirm the successful synthesis of the Mg4Nb2O9 phase.Trace amounts of the Mg5Nb4O15 are present at low doping concentrations of Cr 3+, but its presence is found to have essentially no effect on the luminescence in subsequent studies.In Fig.1 (c), the element mapping images show that Mg, Nb, O and Cr are uniformly distributed throughout the scanned area.In addition, the presence of Cr signal peaks is further observed in the EDS spectrum.

Figure 3 .
Figure 3. (a) PL of NIR pc-LED device.(b-d) Photographs under natural light and NIR pc-LED (power off and power on) captured by the corresponding visible camera and an NIR camera.