RED LUMINESCENT MATERIAL AND CONVERSION LED

Abstract

The present disclosure provides a phosphor having an empirical formula Li.sub.2SiF.sub.6:Mn.sup.4+, a conversion LED including the phosphor, and a method of making the phosphor by solid-state synthesis.

Claims

1. A phosphor having an empirical formula Li.sub.2SiF.sub.6:Mn.sup.4+.

2. The phosphor as claimed in claim 1, wherein the phosphor crystallizes in a trigonal crystal system.

3. The phosphor as claimed in claim 2, wherein the phosphor crystallizes in a P321 space group.

4. A process for preparing a phosphor having an empirical formula Li.sub.2SiF.sub.6:Mn.sup.4+ by a solid-state synthesis.

5. The process as claimed in claim 4, wherein the solid-state synthesis is performed under elevated pressure and at elevated temperature.

6. The process as claimed claim 4, wherein the solid-state synthesis is performed under an elevated pressure of 25 kbar to 85 kbar and within a temperature range between 500° C. and 1000° C.

7. The process as claimed in claim 4, wherein Li.sub.2SiF.sub.6 and A.sub.2MnF.sub.6, wherein A=Li, Na, K, Rb or Cs, are used as reactants.

8. The process as claimed in claim 7, wherein a molar ratio of Li.sub.2SiF.sub.6 to A.sub.2MnF.sub.6 is between 1:0.2 and 1:0.001.

9. A conversion light emitting diode (LED) comprising a phosphor having an empirical formula Li.sub.2SiF.sub.6:Mn.sup.4+.

10. The conversion LED as claimed in claim 9, comprising: a semiconductor layer sequence configured to emit electromagnetic primary radiation; and a conversion element comprising the phosphor, wherein the conversion element at least partly converts the electromagnetic primary radiation to electromagnetic secondary radiation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In the following, the phosphor and the lighting device described herein are explained in more detail in conjunction with non-limiting aspects and the associated figures.

[0051] FIG. 1A shows the unit cell of cubic K.sub.2SiF.sub.6:Mn.sup.4+ (space group no. 225; Fm-3m).

[0052] FIG. 1B shows the unit cell of the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+.

[0053] FIG. 2 shows an emission spectrum of the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+.

[0054] FIG. 3 shows a PXRD comparison (Mo-kα.sub.1 radiation) of Li.sub.2SiF.sub.6:Mn.sup.4+ with a simulation of Li.sub.2SiF.sub.6.

[0055] FIG. 4 shows an emission spectrum of the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+ compared to K.sub.2SiF.sub.6:Mn.sup.4+ and Cs.sub.2MnF.sub.6.

[0056] FIG. 5 shows the luminous efficacy of radiation of Li.sub.2SiF.sub.6:Mn.sup.4+ compared to K.sub.2SiF.sub.6:Mn.sup.4+.

[0057] FIG. 6 shows absorption spectra and emission spectra of two comparative examples.

[0058] The figures and the proportions of the elements depicted in the figures relative to each other are not to be considered as true to scale. Rather, individual elements may be displayed in an exaggeratedly large format for better presentation and/or comprehensibility.

DETAILED DESCRIPTION

[0059] FIG. 1A shows the unit cell of the crystal structure of K.sub.2SiF.sub.6:Mn.sup.4+, which crystallizes in the cubic Fm-3m space group. The K atoms are shown as unfilled ellipsoids, the F atoms as filled circles, and SiF.sub.6 octahedra in hatched form with Si in the center and F at the vertices. Si has been partly replaced by Mn (not shown). K.sub.2SiF.sub.6:Mn.sup.4+ crystallizes in the K.sub.2PtCl.sub.6 type in the Fm-3m space group (no. 225). The unit cell shows a cubic metric with a lattice parameter a=8.134(1) Å.

[0060] FIG. 1B shows the unit cell of the crystal structure of Li.sub.2SiF.sub.6:Mn.sup.4+. The Li atoms are shown as unfilled ellipsoids, the F atoms as filled circles, and SiF.sub.6 octahedra in hatched form with Si in the center and F at the vertices. Si has been partly replaced by Mn (not shown), such that Mn.sup.4+ is octahedrally surrounded by F atoms. Compared to K.sub.2SiF.sub.6:Mn.sup.4+, Li.sub.2SiF.sub.6:Mn.sup.4+ surprisingly crystallizes in the Na.sub.2SiF.sub.6 type in the P321 space group (no. 150); the unit cell shows a trigonal metric with lattice parameters a=8.2190(1) Å and c=4.5580(1) Å.

[0061] A comparison of FIGS. 1A and 1B shows clearly that the crystal structures differ significantly from one another; for example, the SiF.sub.6 octahedra in the cubic K.sub.2SiF.sub.6:Mn.sup.4+ are uniformly oriented, whereas these assume different spatial orientations in Li.sub.2SiF.sub.6:Mn.sup.4+.

[0062] FIG. 2 shows the emission spectrum of a single grain of the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+ on excitation with blue laser light (λ.sub.exc=450 nm).

[0063] FIG. 3 shows a comparison of x-ray diffractograms (PXRD) (Mo-Kα.sub.1 radiation). What is shown is the measured x-ray diffractogram of the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+ compared to a simulation of Li.sub.2SiF.sub.6 based on data from literature (discussed in “Pressure-supported crystal growth and single-crystal structure determination of Li.sub.2SiF6,” Zeitschrift für Kristallographie 2014, E. Hinteregger et al.). Good agreement is apparent, and so these studies by means of x-ray powder methods show that the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+ was preparable in good quality.

[0064] FIG. 4 shows an emission spectrum of the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+ compared to that of K.sub.2SiF.sub.6:Mn.sup.4+ and of Cs.sub.2MnF.sub.6. The phosphors were excited with blue laser light λ.sub.exc=450 nm.

[0065] Just like K.sub.2SiF.sub.6:Mn.sup.4+, Cs.sub.2MnF.sub.6 crystallizes in the K.sub.2PtCl.sub.6 type. This relationship is likewise apparent from the emission spectra. Thus, the two compounds in the K.sub.2PtCl.sub.6 type, i.e. K.sub.2SiF.sub.6:Mn.sup.4+ and Cs.sub.2MnF.sub.6, show a high level of matches in the number and shape of the individual peaks, but differ from the emission of the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+. For example, the peak at about 618 nm for Li.sub.2SiF.sub.6:Mn.sup.4+ is absent in the case of the two other phosphors K.sub.2SiF.sub.6:Mn.sup.4+ and Cs.sub.2MnF.sub.6. Since the eye sensitivity curve has a large (negative) slope in the region of the here present emission maxima of the three phosphors, even small shifts in the emission band (CIE color coordinates x and y) result in distinctly different spectral efficiency, as shown in the table below and FIG. 5.

TABLE-US-00001 x; y coordinates in the CIE LER / lm rel. λ.sub.dom* / nm λ.sub.max / nm color space W.sub.opt.sup.−1 LER / % Li.sub.2SiF.sub.6: Mn.sup.4+ 618 630 0.688; 0.312 218 107 K.sub.2SiF.sub.6: Mn.sup.4+ 621 631 0.693; 0.307 204 100 Cs.sub.2MnF.sub.6 622 634 0.696; 0.304 187  92 *dominant wavelength

[0066] The dominant wavelength is a way of describing non-spectral (polychromatic) light mixtures in terms of spectral (monochromatic) light that creates a similar perception of 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 in such a way 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.

[0067] The optical data in the table show that the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+ has the greatest luminous efficacy of radiation compared to K.sub.2SiF.sub.6:Mn.sup.4+ and Cs.sub.2MnF.sub.6:Mn.sup.4+.

[0068] The comparison of relative luminous efficacy of radiation between Li.sub.2SiF.sub.6:Mn.sup.4+ and K.sub.2SiF.sub.6:Mn.sup.4+ is shown in the form of a graph in FIG. 5.

[0069] FIG. 6 shows absorption spectra and emission spectra of a comparative example VB2 and K.sub.2SiF.sub.6:Mn.sup.4+. The data for K.sub.2SiF.sub.6:Mn.sup.4+ correspond to those from literature (discussed in “Mn.sup.4+-Activated Red Photoluminescence in K.sub.2SiF.sub.6 Phosphor,” Journal of the Electrochemical Society 2008, T. Takahashi et al.).

[0070] Two identical experiments (precipitation reaction in 60% HF) were conducted, with successful preparation of K.sub.2SiF.sub.6:Mn.sup.4+ (comparative example 1 (VB1)) in one case and, in the other case, in which the only change was the replacement of the K source (K.sub.2CO.sub.3) by the corresponding Li source (Li.sub.2CO.sub.3), no formation of the doped target compound Li.sub.2SiF.sub.6:Mn.sup.4+ (comparative example 2 (VB2)). After dissolution of the SiO.sub.2 in hydrofluoric acid, the respective carbonate was added until the solution was saturated (cf. table below).

TABLE-US-00002 TABLE Reactants for synthesis of VB1 (K.sub.2SiF.sub.6: Mn.sup.4+) and VB2. Reaction in each case in 60% HF and with use of 35% H.sub.2O.sub.2 for reduction of the KMnO.sub.4. The reason for the distinctly different molar amount for Li.sub.2CO.sub.3 is the reduced solubility in aqueous HF. VB1 VB2 SiO.sub.2 / mmol 4.446 4.348 KMnO.sub.4 (dopant, excess) / mmol 7.7208 7.1121 K.sub.2CO.sub.3 / mmol 14.411 Li.sub.2CO.sub.3 / mmol 4.614

[0071] Since Li.sub.2CO.sub.3 (and LiF) is much more sparingly soluble in aqueous HF than K.sub.2CO.sub.3 (and KF), the free Mn.sup.4+ ion may not be stabilized in the solution since there are in fact no free Li ions available for complexation (experiment VB2). Instead, as well as LiF (main phase), the compound K.sub.2SiF.sub.6 is formed from the fractions of KMnO.sub.4 that were used for doping. It is not ultimately possible to clarify the whereabouts of the Mn, but the absorption and emission measurements in FIG. 6 for VB2 show clearly that no Mn.sup.4+-doped phosphor was obtained, and not even in traces since neither emission nor absorption has been recorded for VB2. It was thus possible to show that the Li.sub.2SiF.sub.6:Mn.sup.4+ phosphor may not be synthesized via the known route of synthesis of K.sub.2SiF.sub.6:Mn.sup.4+. In other words, the phosphor Li.sub.2SiF.sub.6:Mn.sup.4+ is not formed from a precipitation reaction in aqueous hydrofluoric acid (HF) using the reactants Li.sub.2CO.sub.2, SiO.sub.2 and a manganese source.

[0072] The product obtained from VB2, as shown in FIG. 6, does not show any absorption or any emission.

[0073] The working examples described in conjunction with the figures and features thereof may also be combined with one another in further working examples, even if such combinations are not shown explicitly in the figures. In addition, the working examples described in conjunction with the figures may have additional or alternative features according to the description in the general part.

LIST OF REFERENCE NUMERALS

[0074] LED light-emitting diode [0075] CRI color rendering index [0076] LER luminous efficacy of radiation [0077] rel. LER relative luminous efficacy of radiation [0078] CCT correlated color temperature [0079] FWHM spectral width of emission, full width at half maximum [0080] ppm parts per million [0081] VB comparative example [0082] rI, Ir relative intensity [0083] mol % mole percent [0084] nm nanometers [0085] ° C. degrees Celsius [0086] λ.sub.exc excitation wavelength [0087] λ.sub.max emission maximum [0088] λ.sub.dom dominant wavelength