Discovery of a Red-Emitting Li₃RbGe₈O₁₈:Mn⁴⁺ Phosphor in the Alkali-Germanate System: Structural Determination and Electronic Calculations

Abstract

A solid-state combinatorial chemistry approach, which used the A-Ge-O (A = Li, K, Rb) system doped with a small amount of Mn as an activator, was adopted in a search for novel red-emitting phosphors. The A site may have been composed of either a single alkali metal ion or of a combination of them. This approach led to the discovery of a novel phosphor in the above system with the chemical formula LiRbGeO:Mn. The crystal structure of this novel phosphor was solved via direct methods, and subsequent Rietveld refinement revealed a trigonal structure in the P3̅1m space group. The discovered phosphor is believed to be novel in the sense that neither the crystal structure nor the chemical formula matches any of the prototype structures available in the crystallographic information database (ICDD or ICSD). The measured photoluminescence intensity that peaked at a wavelength of 667 nm was found to be much higher than the best intensity obtained among all the existing AGeO (A = Li, K, Rb) compounds in the alkali-germanate system. An ab initio calculation based on density function theory (DFT) was conducted to verify the crystal structure model and compare the calculated value of the optical band gap with the experimental results. The optical band gap obtained from diffuse reflectance measurement (5.26 eV) and DFT calculation (4.64 eV) results were in very good agreement. The emission wavelength of this phosphor that exists in the deep red region of the electromagnetic spectrum may be very useful for increasing the color gamut of LED-based display devices such as ultrahigh-definition television (UHDTV) as per the ITU-R BT.2020-2 recommendations and also for down-converter phosphors that are used in solar-cell applications.

Formation mechanism and optimized luminescence of Mn4+‐doped unequal dual‐alkaline hexafluorosilicate Li0.5Na1.5SiF6

“…The excitation and emission spectra, and the decay curves of the red phosphor LNSF:Mn measured at 78 and 298 K are shown in Figure . The two excitation bands peaking at 370 and 470 nm in the excitation spectra monitored at 620 nm are due to the 4 A 2g → 4 T 1g and 4 A 2g → 4 T 2g transitions of Mn 4+ according to the Tanabe‐Sugano diagram for Mn 4+ , which are consistent with the reported results of other Mn 4+ activated fluoride compounds . The maximum excitation band locating in blue region indicates that the red phosphor LNSF:Mn shows great promise in GaN based WLEDs.…”

“…For Mn 4+ ‐doped red phosphors with host lattice of complex oxides, repeated high‐temperature sintering procedures are required in their synthesis process. Furthermore, the red phosphors of Mn 4+ ‐doped complex oxides often show deep red luminescence and their emission spectra are located in near IR region (>650 nm) which are not insensitive to naked eyes …”

“…Furthermore, the red phosphors of Mn 4+ -doped complex oxides often show deep red luminescence and their emission spectra are located in near IR region (>650 nm) which are not insensitive to naked eyes. [12][13][14][15][16][17][18] Mn 4+ -doped fluorides exhibit superior luminescence performance with ideal chromaticity ordinations and ease of operation. A series of red phosphors A 2 XF 6 :Mn 4+ and A 3 YF 6 : Mn 4+ (A is one type of alkaline ions, such as K, Na, Rb, or Cs; X is Si, Ge, or Zr; Y is Al or Ga) have been obtained by simple synthetic processes at room temperature.…”

Rare‐earth‐free Li5La3Ta2O12:Mn4+ deep‐red‐emitting phosphor: Synthesis and photoluminescence properties

“…A series of LLTO:Mn 4+ phosphors are prepared in air by a conventional high-temperature solid-state reaction method. 12 :xMn 4+ (LLTO:xMn 4+ ) (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol%), put into an agate mortar and thoroughly ground for 20 minutes in order to form homogeneous mixture. The mixtures are then filled into alumina crucibles and firstly sintered at 700°C for 4 hours in air.…”

“…As one of non-rare-earth activators for red-emitting phosphors, Mn 4+ ion with 3d 3 electron configuration is attracting considerable interest because it can show broad absorption band in the range from 220 to 570 nm and exhibit red or deep red emission in the region from 600 to 790 nm due to the 2 E→ 4 A 2 transition. 12,13 Mn 4+ ion can usually be stable | 5911 CAO et Al. in octahedral environments and substitute for the Al 3+ , Sc 3+ , Ga 3+ , Ti 4+ , Si 4+ , Ge 4+ , Zr 4+ , Sn 4+ , Ta 5+ , Nb 5+ , Sb 5+ , Te 6+ , Mo 6+ , and W 6+ sites in octahedral cell and form the octahedral center in host lattice. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] The luminescence properties of Mn 4+ -doped phosphors can be affected by the co-valency of the "Mn 4+ -Ligand" bonding and the host crystal field environment.…”