Rb-based hexafluoride red-emitting phosphors, Rb 2 SiF 6 :Mn 4+ and Rb 2 TiF 6 :Mn 4+ , were synthesized by the coprecipitation method. Optical microscopy observation, X-ray diffraction (XRD) measurement, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and luminescence decay characteristics measurements were used to study the structural and optical properties of the phosphors. The photographs of the bulk samples showed clear crystallographic habits originating from the cubic and trigonal symmetries of the Rb 2 SiF 6 and Rb 2 TiF 6 hosts, respectively, in agreement with the XRD results. The phosphors exhibited an intense narrow-band Mn 4+ ( 2 E g → 4 A 2g ) red emission with internal quantum efficiencies higher than 90% upon blue light excitation. The Franck−Condon analysis of the PLE data yielded the Mn 4+ intra-d-shell transitions to occur at ∼2.47 eV (∼2.34 eV) for the 4 A 2g → 4 T 2g transition and at ∼2.86 eV (∼2.83 eV) for the 4 A 2g → 4 T 1g transition in the Rb 2 SiF 6 :Mn 4+ (Rb 2 TiF 6 :Mn 4+ ) phosphor. Temperature dependence of the PL spectra from T = 20 to 500 K gave the quenching temperature values (T q 's) at which the PL intensity has fallen to half its maximum value to be T q ∼ 490 and ∼450 K for Rb 2 SiF 6 :Mn 4+ . To the best of our knowledge, only one paper has been published on Rb 2 TiF 6 reporting that it crystallizes in the trigonal structure with a = 0.5892 nm and a = 0.4795 nm. Our synthesized phosphors were examined by the X-ray diffraction (XRD) measurement, PL, PLE, and PL decay curve measurements. Temperature dependence of the PL properties was also examined from T = 20 to 500 K in 10-K increments. Efficient sharp red emissions with internal quantum efficiencies larger than 90% were observed from our phosphors that may promise improving a color rendering index of the conventional white-LED lamps.z E-mail: tnakamura@gunma-u.ac.jp; adachi@gunma-u.ac.jp ExperimentalThe rubidium hexafluorometallate phosphors were grown by the coprecipitation method. First, XO 2 (X = Si, Ti) and Rb 2 CO 3 in molar ratio of 1 : 1 were dissolved in an HF solution. Then, KMnO 4 powder was added in the HF solution at 300 K. This mixed solution was left in dark for a few days and resulted in a large single-crystalline phosphor growth (see Fig. 2 below). The resultant bulk single-crystalline phosphors were paper-filtered and dried in room ambient. They were grained in an agate mortar and then used for various measurements.The crystal structures of the synthesized phosphors were examined by performing an XRD analysis using a SmartLab X-ray diffractometer (Rigaku) with Cu Kα radiation at λ = 0.1542 nm. PL measurements were carried out using a single monochromator equipped with a charge-coupled device (Princeton Instruments PIXIS 100) in a CryoMini cryostat (Iwatani Industrial Gases) and a stainless cryostat (Technolo Kogyo) at T = 20 − 500 K in increments of 10 K. A He−Cd laser at λ ex = 325 nm (Kimmon IK3302R-E) was used as the excitation light source.PLE measurements were performed at 300 K...

Rb 2 GeF 6 :Mn 4+ red-emitting phosphors were synthesized by coprecipitation and their structural and optical properties were investigated by laser microscopy observation, X-ray diffraction (XRD) analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and PL decay measurement. Single-crystalline ingots in the form of a hexagonal pyramid were prepared with a basal plane diameter of >2 mm. The XRD analysis suggested that Rb 2 GeF 6 crystallizes in the hexagonal structure (C 4 6v ¼ P6 3 mc) with a = 0.5955 nm and c = 0.9672 nm. The phosphor exhibited the strong Mn 4+ -related zero-phonon line (ZPL) emission peak typically observed in host crystals with piezoelectrically active lattices such as a hexagonal lattice. The quantum efficiencies of the bulk ingot and powdered samples were 87 and 74%, respectively, with nearly the same luminescence decay time of >6 ms. The exact ZPL energies and related crystal-field and Racah parameters were obtained from the PL and PLE spectra by Franck-Condon analysis. Temperature-dependent PL intensities were analyzed from T = 20 to 500 K using a thermal quenching model by considering Bose-Einstein phonon statistics. A comparative discussion on the phosphor properties of Rb 2 GeF 6 :Mn 4+ and Rb 2 MF 6 :Mn 4+ with M = Si and Ti was also given.

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