RED LUMINESCENT MATERIAL AND CONVERSION LED

Abstract

A luminescent material may have an empirical formula A.sub.1-yA′.sub.yLiXF.sub.6:Mn.sup.4+, where: A=Na, K, Rb, Cs, or combinations thereof; A′=Na, K, Rb, Li, Cs, or combinations thereof; X=Si, Ti, Hf, Zr, Sn, Pb, Ge, or combinations thereof; 0≤y<1; and A and A′ are selected differently.

Claims

1. A luminophore having the empirical formula A.sub.1-yA′.sub.yLiXF.sub.6:Mn.sup.4+, wherein: A=Na, K, Rb, Cs, or combinations thereof; A′=Na, K, Rb, Li, Cs, or combinations thereof; X=Si, Hf, Zr, Sn, Pb, Ge, or combinations thereof; 0≤y<1; and A and A′ are different.

2. The luminophore as claimed in claim 1 having the empirical formula A.sub.1-yA′.sub.yLiSiF.sub.6:Mn.sup.4+, wherein: A=Na, K, Rb, Cs, or combinations thereof; A′=Na, K, Rb, Li, Cs, or combinations thereof; 0≤y<1; and A and A′ are different.

3. The luminophore as claimed in claim 1 having the empirical formula ALiSiF.sub.6:Mn.sup.4+, wherein A=Na, K, Rb, Cs, or combinations thereof.

4. The luminophore as claimed in claim 3, wherein A=K, Cs or both.

5. The luminophore as claimed in claim 1 having the empirical formula KLiSiF.sub.6:Mn.sup.4+.

6. The luminophore as claimed in claim 1, wherein the luminophore crystallizes in an orthorhombic crystal system.

7. The luminophore as claimed in claim 1, wherein the luminophore crystallizes in the space group Pbcn.

8. A process for preparing a luminophore having the empirical formula A.sub.1-y A′.sub.yLiXF.sub.6:Mn.sup.4+, wherein: A=Na, K, Rb, Cs, or combinations thereof; A′=Na, K, Rb, Li, Cs, or combinations thereof; X=Si, Hf, Zr, Sn, Pb, Ge, or combinations thereof; 0≤y<1; and A and A′ are different, by a solid-state synthesis.

9. The process as claimed in claim 8, wherein no aqueous HF is employed in the solid-state synthesis.

10. The process as claimed in claim 8, wherein the solid-state synthesis is performed at elevated pressure and elevated temperature.

11. The process as claimed in claim 8, wherein the solid-state synthesis is performed at an elevated pressure of 25 kbar to 85 kbar and in at a temperature ranging from 500° C. to 1000° C.

12. The process as claimed in claim 8 for preparing a luminophore having the empirical formula A.sub.1-yA′.sub.yLiSiF6:Mn.sup.4+, wherein the reactants employed are A.sub.2SiF.sub.6, where A=Na, K, Rb or Cs, A′.sub.2SiF.sub.6, where A′=Na, K, Rb, Li and/or Cs, Li.sub.2SiF.sub.6 and X′.sub.2MnF.sub.6, where X′=Li, Na, K, Rb or Cs, or ALiSiF.sub.6, where A=Na, Rb, K or Cs, and X′.sub.2MnF.sub.6, where X′=Li, Na, K, Rb or Cs.

13. A conversion LED comprising a luminophore as claimed in claim 1.

14. The conversion LED as claimed in claim 13, further comprising: a semiconductor layer sequence adapted to emit electromagnetic primary radiation; and a conversion element comprising the luminophore, wherein the conversion element is configured to at least partly convert the electromagnetic primary radiation to electromagnetic secondary radiation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Further advantageous embodiments and developments are apparent from the working examples described hereinbelow with reference to the figures.

[0084] FIG. 1A shows the unit cell of cubic K.sub.2SiF.sub.6 (space group no. 225; Fm-3m).

[0085] FIG. 1B shows the unit cell of cubic KLiSiF.sub.6.

[0086] FIG. 2 shows an emission spectrum of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+.

[0087] FIG. 3A shows a PXRD comparison (Mo-Kai radiation) of KLiSiF.sub.6 with a simulation of KLiSiF.sub.6 based on single-crystal data.

[0088] FIG. 3B shows a PXRD comparison (Mo-Kai radiation) of KLiSiF.sub.6:Mn.sup.4+ with KLiSiF.sub.6.

[0089] FIG. 3C shows a comparison of PXRD simulations (Mo-Kα.sub.1 radiation) of KLiSiF.sub.6 and Li.sub.2SiF.sub.6 based on single-crystal data.

[0090] FIG. 3D shows a comparison of PXRD simulations (Mo-Kα.sub.1 radiation) of KLiSiF.sub.6 and K.sub.2SiF.sub.6 based on single-crystal data.

[0091] FIG. 4 shows an emission spectrum of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ compared to K.sub.2SiF.sub.6:Mn.sup.4+.

[0092] FIG. 5 shows a luminous efficacy of radiation (LER) of KLiSiF.sub.6:Mn.sup.4+ compared to K.sub.2SiF.sub.6:Mn.sup.4+.

DETAILED DESCRIPTION

[0093] FIG. 1A shows the unit cell of the crystal structure of K.sub.2SiF.sub.6, which crystallizes in the cubic space group Fm-3m. The K atoms are shown as unfilled circles, the F atoms as filled circles, and [SiF.sub.6].sup.2− octahedra with Si in the center and F at the vertices with hatching. In the luminophore K.sub.2SiF.sub.6:Mn.sup.4+ Si has been partly replaced by Mn (not shown, no measurable effect on crystal structure). K.sub.2SiF.sub.6 (with or without Mn.sup.4+) crystallizes in the K.sub.2PtCl.sub.6 type in the space group Fm-3m (no. 225). The unit cell shows a cubic metric with a lattice parameter a=8.134(1) Å.

[0094] FIG. 1B shows the unit cell of the crystal structure of KLiSiF.sub.6. The K atoms are shown as unfilled circles, the F atoms as filled circles, [SiF.sub.6].sup.2− octahedra with Si in the center and F at the vertices with fine hatching and [LiF.sub.6].sup.5− octahedra with Li in the center and F at the vertices with coarse hatching. In the luminophore KLiSiF.sub.6:Mn.sup.4+ Si has been partly replaced by Mn (not shown, no measurable effect on crystal structure) and so Mn.sup.4+ is octahedrally surrounded by F atoms. Compared to K.sub.2SiF.sub.6 (with or without Mn.sup.4+), KLiSiF.sub.6 (with or without Mn.sup.4+) surprisingly crystallizes in the space group Pbcn (no. 60) and the unit cell shows an orthorhombic metric with lattice parameters a=747.50(3) pm, b=1158.58(5) pm and c=979.77(4) pm. The crystal structure, the units and the bonding patterns thereof are similar to what is observed in (NH.sub.4)MnFeF.sub.6.

[0095] The crystallographic data are shown in table 1.

TABLE-US-00001 TABLE 1 Empirical formula KLiSiF6 Crystal system orthorhombic Space group Pbcn (no. 60) a/pm 747.50(3) b/pm 1158.58(5) c/pm 979.77(4) Cell volume/nm.sup.3 0.8485(1) Z 8 Density/g × cm.sup.−3 2.945 T/K 203(2) Diffractometer BRUKER D8 Quest Radiation/Å Mo-Kα (λ = 0.71073) Measured range/°  .sub. 6.5 < 2θ < 75.8 −12 < h < 12 −19 < k < 19 −16 < l < 16 R.sub.1/wR.sub.2 [I ≥ 2σ(I)] 0.0188/0.0420 R.sub.1/wR.sub.2 [all data] 0.0255/0.0438 GooF 1.080

[0096] Comparison of the FIGS. 1A and 1B clearly shows that the crystal structures appreciably differ from one another. Cubic K.sub.2SiF.sub.6 for example comprises only [SiF6].sup.2− octahedra, which are spatially separate from one another, while KLiSiF.sub.6 comprises two different units, [SiF6].sup.2− and [LiF6].sup.5− octahedra, which are additionally bonded to one another. The same differences are thus also present in the crystal structures of K.sub.2SiF.sub.6:Mn.sup.4+ and KLiSiF.sub.6:Mn.sup.4+.

[0097] FIG. 2 shows the emission spectrum of a single-crystal of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ upon excitation with blue laser light (λ.sub.exc=448 nm). Excitation with blue laser light causes KLiSiF.sub.6:Mn.sup.4+ to show (deep) red luminescence with a typical line spectrum for Mn.sup.4+-doped luminophores. An emission maximum of ≈631 nm means that the emission is also in a preferred range for red luminophores.

[0098] Since the electronic transitions for Mn.sup.4+ (d-d transitions) occur in inner, shielded electron shells, the position of the emission bands is not strongly dependent on the environment of the activator in the crystal structure as is the case for Eu.sup.2+-based luminophores. Red emission thus typically results when Mn.sup.4+ is surrounded by six F atoms (in the shape of an octahedron) in the structure (for example replacement of Si.sup.4+ by Mn.sup.4+). However, slight variations in emission are achievable for example by altering the coordination number (CN) of the counterions in the structure. Compounds having night counterions emit at shorter wavelength than their variants with identical molar composition but heavier counterions (Highly Efficient and Stable Narrow-Band Red Phosphor Cs.sub.2SiF.sub.6:Mn.sup.4+ for High-Power Warm White LED Applications, ACS Photonics 2017, E. Song et al.). Cs.sub.2SiF.sub.6:Mn (CsSF:Mn) for example shows an emission maximum at higher wavelengths (λ.sub.max=632 nm). This red shift simultaneously causes reduced efficiency and is therefore undesired for most applications.

[0099] FIG. 3A shows a comparison of powder x-ray diffraction (PXRD) diffractograms (Mo-Kai radiation). Shown here is the measured x-ray diffraction diffractogram of the undoped precursor KLiSiF.sub.6 of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ compared to a simulation of KLiSiF.sub.6 based on single-crystal data (see table 1). Good agreement is apparent and these analyses by x-ray powder methods therefore show that KLiSiF.sub.6 was prepared in good quality.

[0100] FIG. 3B shows a comparison of x-ray diffraction (PXRD) diffractograms (Mo—Kα.sub.1 radiation). Shown here are the measured x-ray diffraction diffractograms of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ (top) compared to the measured x-ray diffraction diffractograms of the reactant KLiSiF.sub.6 before ball-milling (bottom). Due to the small amounts of Mn.sup.4+ no difference between undoped and doped form is visible in PXRD. Good agreement is apparent and shows that the crystal structure remains unchanged even after ball-milling of KLiSiF.sub.6 with K.sub.2MnF.sub.6. It can be concluded from FIGS. 3A and 3B that the inventive luminophore can be prepared in good quality.

[0101] FIG. 3C shows a comparison of x-ray diffraction (PXRD) diffractograms (Mo-Kα.sub.1 radiation). Shown here is an x-ray diffraction diffractogram of KLiSiF.sub.6 simulated from single-crystal data compared to an x-ray diffraction diffractogram of Li.sub.2SiF.sub.6 simulated from single-crystal data.

[0102] FIG. 3D shows a comparison of x-ray diffraction (PXRD) diffractograms (Mo-Kα.sub.1 radiation). Shown here is an x-ray diffraction diffractogram of KLiSiF.sub.6 simulated from single-crystal data compared to an x-ray diffraction diffractogram of K.sub.2SiF.sub.6 simulated from single-crystal data.

[0103] As is apparent from FIGS. 3C and 3D the x-ray powder diffractogram of KLiSiF.sub.6 differs markedly from those of Li.sub.2SiF.sub.6 and K.sub.2SiF.sub.6 and accordingly the x-ray powder diffractogram of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ also differs markedly from those of K.sub.2SiF.sub.6:Mn.sup.4+ and Li.sub.2SiF.sub.6:Mn.sup.4+.

[0104] FIG. 4 shows an emission spectrum of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ compared to that of K.sub.2SiF.sub.6:Mn.sup.4+. The luminophores were excited with blue laser light (λ.sub.exc=448 nm).

[0105] The emission of K.sub.2SiF.sub.6:Mn.sup.4+ differs from the emission of the inventive luminophore KLiSiF.sub.6:Mn.sup.4+. For example the peak at about 622 nm of KLiSiF.sub.6:Mn.sup.4+ is missing in the case of the luminophore K.sub.2SiF.sub.6:Mn.sup.4+. The emission at about 622 nm corresponds to the so-called “zero phonon line”, i.e. a transition which occurs without involvement of phonons. In cubic K.sub.2SiF.sub.6:Mn.sup.4+, symmetry dictates that due to the perfectly octahedral Mn environments the corresponding transition is not allowed/possible. By contrast in the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ the loss of symmetry going to orthorhombic crystal system breaks the perfect octahedral symmetry, thus resulting in a clear intensity of the peak at 622 nm. Since the eye sensitivity curve has a large (negative) gradient in the region of the emission maximum present here (λ.sub.max˜631 nm), even small additional signals on the short-wave side of the emission maximum result in a marked difference in luminous efficacy of radiation (LER) as shown in table 2 and FIG. 5.

TABLE-US-00002 TABLE 2 Optical data for K.sub.2SiF.sub.6:Mn.sup.4+ (comparative example) and KLiSiF.sub.6:Mn.sup.4+. x, y coordinates Λ.sub.dom */ λ.sub.max/ in CIE-x-y LER/lm rel. nm nm color space W.sub.opt.sup.−1 LER/% KLiSiF.sub.6:Mn.sup.4+ 620 631 0.692 (1); 208 102 0.308 (1) K.sub.2SiF.sub.6:Mn.sup.4+ 621 631 0.693 (1); 204 100 0.307 (1) * dominant wavelength

[0106] The dominant wavelength is a means of describing non-spectral (polychromatic) light mixtures in terms of spectral (monochromatic) light which produces a perceived similar hue. In the CIE color space, the line that connects a point for a particular color and the point CIE-x=0.333, CIE-y=0.333 can be extrapolated such that it meets the outline of the space at two points. The point of intersection closer to said color represents the dominant wavelength of the color as the wavelength of the pure spectral color at this point of intersection. The dominant wavelength is thus the wavelength that is perceived by the human eye.

[0107] The optical data of table 2 show that the inventive luminophore KLiSiF.sub.6:Mn.sup.4+ exhibits a greater luminous efficacy of radiation (LER) compared to K.sub.2SiF.sub.6:Mn.sup.4+.

[0108] The comparison of the relative luminous efficacy of radiation (LER) between KLiSiF.sub.6:Mn.sup.4+ and K.sub.2SiF.sub.6:Mn.sup.4+ is represented graphically in FIG. 5.

[0109] The working examples described in connection with the figures and the features thereof may also be combined with one another according to further working examples even if such combinations are not explicitly shown in the figures. Furthermore, the working examples described in connection with the figures may comprise additional or alternative features according to the description in the general part.

LIST OF REFERENCE NUMERALS

[0110] LED light emitting diode [0111] CRI color rendering index [0112] LER luminous efficacy of radiation [0113] rel. LER relative luminous efficacy of radiation [0114] CCT correlated color temperature [0115] FWHM spectral width of emission, full width at half maximum [0116] ppm parts per million [0117] Ir relative intensity [0118] mol % mole percent [0119] nm nanometers [0120] ° C. degrees Celsius [0121] λ.sub.exc excitation wavelength [0122] λ.sub.max emission wavelength [0123] λ.sub.dom dominant wavelength [0124] PXRD powder x-ray diffraction diffractogram