Alkali Lithosilicates: Renaissance of a Reputable Substance Class with Surprising Luminescence Properties.

A hitherto unknown synthetic access to alkali lithosilicates, a substance class first described by Hoppe in the 1980s, is reported. With the synthesis and characterization of NaK7 [Li3 SiO4 ]8 , a new representative has been discovered, expanding the family of known alkali lithosilicates. Astonishingly, NaK7 [Li3 SiO4 ]8 and the already established alkali lithosilicates Na[Li3 SiO4 ] as well as K[Li3 SiO4 ] display unforeseen luminescence properties, when doped with Eu2+ . Na[Li3 SiO4 ]:Eu2+ exhibits an ultra-narrow blue, K[Li3 SiO4 ]:Eu2+ a broadband, and NaK7 [Li3 SiO4 ]8 :Eu2+ a yellow-green double emission upon excitation with near-UV to blue light. Consequently, all of the investigated substances of this class of compounds are highly interesting phosphors for application in phosphor converted LEDs.

Over the last couple of years, the hunt for new alternative light sources to replace incandescent light bulbs has resulted in the creation of countless phosphors for the use in phosphor converted light‐emitting diodes (pc‐LEDs).1, 2, 3, 4 In these pc‐LEDs, a short wavelength emitting light source (UV to blue LED) is combined with additional converting phosphors, which emit light of longer wavelengths, to compose the desired color.5, 6, 7, 8 While narrowband blue emitting phosphors like AELi2[Be4O6]:Eu2+ (AE=Sr, Ba) with a small Stokes shift are of essential importance for UV‐pumped RGB pc‐LEDs,9 narrowband red emitting phosphors are needed in In1−GaN based blue pumping pc‐LEDs. During the search for adequate phosphors, many examples regarding red phosphors showed that especially ternary and multinary alkaline earth nitrides represent suitable host structures for doping with Eu2+.10, 11, 12, 13 Therefore, scientific research mainly focused on these systems. As a result, phosphors like Ca[LiAl3N4]:Eu2+,14 Sr[Mg3SiN4]:Eu2+,15 M[Mg2Al2N4]:Eu2+ (M=Ca, Sr, Ba, Eu),16 Sr[LiAl3N4]:Eu2+,17 and Sr4[LiAl11N14]:Eu2+  18 were found, which have been proven to exhibit surprising narrowband red emissions.

Interestingly, Ca[LiAl3N4] and Sr[Mg3SiN4] both crystallize in the Na[Li3SiO4] structure type,14, 15 and Sr[LiAl3N4] in the Cs[Na3PbO4] structure type.17, 19 The latter structure type is also shared by K[Li3GeO4] and K[Li3SiO4].20 This finding is highly uncommon owing to the fact that the above mentioned pure nitrogen compounds, represented by three‐dimensional tetrahedra networks, only rarely possess pure oxide species as isotypic counterparts. Ca[LiAl3N4]–Na[Li3SiO4] and Sr[LiAl3N4]–K[Li3SiO4] are two examples of these rare cases. Having the major structural similarities between these compounds in mind, it is more than astonishing that only little is known about mixed phases in this field.4

Based on these considerations, the question arose if there are alkali metal silicates or lithosilicates, which could be suitable for modern phosphor materials. To answer this question, several alkali metal silicates such as Na[Li3SiO4] and K[Li3SiO4], originally synthesized by Hoppe in the 80s and 90s, have been revisited.20, 21 Owing to the fact that this substance‐class only hosts monovalent cations, it was widely believed that their host structures would be unsuitable for doping with Eu2+.

In the following, we report on the three lithosilicate phosphors Na[Li3SiO4]:Eu2+, K[Li3SiO4]:Eu2+, and NaK7[Li3SiO4]8:Eu2+ (new structure type), which are the first examples of this substance class. Surprisingly, these novel phosphors exhibit extremely diverse luminescence properties, despite their structural similarities, while maintaining a relatively high quantum efficiency (QE) and low thermal quenching (TQ). Their emissions range from narrow blue to a broadband warm white, which we explain in terms of structure‐property relationships. In particular, NaK7[Li3SiO4]8 doped with Eu2+ shows a combination of narrowband green and broadband red emission upon excitation with blue light, making it a promising compound for application in pc‐LEDs.1

The original syntheses of Hoppe et al. started from the pure alkali oxides in closed Ni‐ampules. Herein, we report the syntheses of Na[Li3SiO4], K[Li3SiO4], and the novel compound NaK7[Li3SiO4]8 by solid‐state synthesis from the easy accessible alkali carbonates and SiO2 in an open system (see the Experimental Section). The crystal structures of Na[Li3SiO4] and K[Li3SiO4] were confirmed by powder diffraction. The structure of NaK7[Li3SiO4]8 was elucidated by single‐crystal X‐ray diffraction analysis (see below).

Na[Li3SiO4] crystallizes in the tetragonal space group I41/a (no. 88) with a=1078.4(1) and c=1263.3(1) pm. The structure consists of endless chains of Na‐polyhedra (coordination number (CN)=7+1) along [001], interconnected via LiO4 and SiO4 tetrahedra, which for their part form the three‐dimensional anionic framework. K[Li3SiO4] on the other hand crystallizes in the triclinic space group P (no. 2) with a=574.4, b=734.8, c=971.02 pm, α=83.5, β=76.6, and γ=79.9°. Two crystallographically distinguishable potassium sites form endless chains along [111] and, like in Na[Li3SiO4], LiO4 and SiO4 tetrahedra form the anionic framework. Although Na[Li3SiO4] and K[Li3SiO4] do not share the same crystal structure, they show significant structural similarities.20, 21 The new compound NaK7[Li3SiO4]8 crystallizes in a novel structure type in the tetragonal space group I41/a (no. 88) with a=1555.57(8) and c=1274.71(7) pm.22 The compound shows a close structural relationship to the aforementioned compounds Na[Li3SiO4] and Ca[LiAl3N4]. In analogy, it can be described as an ordered variant of the UCr4C4‐structure type.23

The structure of NaK7[Li3SiO4]8 consists of a highly condensed network built up from SiO4 tetrahedra and distorted LiO4 tetrahedra. These tetrahedra form vierer ring24 channels alongside [001] by corner‐sharing. Each oxygen atom centers four different tetrahedra (3 LiO4 and 1 SiO4) leading to a degree of condensation of κ=1 (atomic ratio (Li,Si)/O). The mean Li−O (Ø=201.7(4) pm) and Si−O (Ø=163.9(2) pm) bond lengths are in good agreement with the sum of the ionic radii.25 The unit cell consists of two types of channels designated as CH1 and CH2 (Figure 1 a,b), which are distinguishable by their central cations (see below). Within the unit cell, every CH1 is surrounded by four CH2 and vice versa. All channels are interconnected by joint edges and vertices. The channels CH1 and CH2 assemble the anionic framework building up a third, empty channel (center of Figure 1 a). The first type of channel CH1 has a 41 screw axis in its center. This screw axis is not aligned with the K1 position, leading to a slight offset of the central atom. Furthermore, each vierer ring in this channel consists of one SiO4 and three LiO4 tetrahedra. Owing to the 41 screw axis, the SiO4 tetrahedra form a helix‐like structure along [001] (Figure 1 b/top). Therefore, six LiO4 and two SiO4 tetrahedra surround the K1 position. This asymmetric surrounding results in a variance of the K−O bond lengths from 262.5(2) to 287.6(2) pm.

Figure 1

Crystal structure of NaK7[Li3SiO4]8. Figure 1 a shows the arrangement of the vierer ring channels along [00 ], whereby the K1‐strands (CH 1/Figure 1 b, top) are represented by the gray vierer ring channels and the Na‐K‐strands (CH 2/Figure 1 b, bottom) by the orange ones. Both strands are viewed in the [ 00] direction. Figure 1 c shows the complete structure alongside [ 00] with the LiO4 tetrahedra (blue), SiO4 tetrahedra (red), K atoms (yellow), and Na atoms (green).

The second type of channel CH2 contains three crystallographically distinguishable sites: K2, K3, and the Na1 site. In comparison to CH1, the channel symmetry is different. All of the cation sites are aligned along a inversion axis with the cation K3 on the inversion center. There is an obvious preference from the SiO4 tetrahedra towards the sodium site, which is surrounded by four LiO4 and four SiO4 tetrahedra. This can be attributed to the smaller ionic radius of Na+. To compensate the size difference between a potassium and a sodium cation, the smaller SiO4 tetrahedra are located near the sodium site. For the same reason, the K2 site shows an asymmetry in its surrounding. The K+ ion is surrounded by two SiO4 and six LiO4 tetrahedra, dislocated from its central position away from the sodium cation and the two SiO4 tetrahedra. The K3 site is the only position solemnly surrounded by LiO4 tetrahedra (Figure 1 b, bottom).

In summary, the structure contains four sites (Na1, K1, K2, and K3) suitable for hosting an Eu2+ cation, which could serve as an activator ion for the luminescence. On all possible sites, Eu2+ would substitute a monovalent cation. The hollow space in the empty channels is too small to house Eu2+. Further details on the crystallographic data are given in the Supporting Information.

The investigations of the luminescence properties were carried out on powder samples for Na[Li3SiO4]:Eu2+ and K[Li3SiO4]:Eu2+, while for the novel compound NaK7[Li3SiO4]8:Eu2+ single‐crystals, with the cell parameters received from the structural refinement, were measured, to ensure that the measured properties originate exclusively from the determined structure. As excitation sources, near‐UV light (400 nm) for powders or blue light (460 nm) for single‐crystals were used. Na[Li3SiO4]:Eu2+ exhibits a narrow blue luminescence (λ max=469 nm/FWHM=32 nm). Compared to the known narrowband blue emitters SrSi6N8:Eu2+ (λ max=450 nm/FWHM=44 nm)26 and Sr0.25Ba0.75Si2O2N2:Eu2+ (λ max=472 nm/FWHM=37 nm),27, 28 the novel phosphor shows a narrower emission profile. The recently discovered material AELi2[Be4O6]:Eu2+ (AE=Sr, Ba) with λ max=454–456 nm exhibits an even smaller FWHM value of only about 25 nm.9 K[Li3SiO4]:Eu2+ shows a broadband, near‐warm‐white emission (Figure 2). Further information on the execution of the measurements, and the luminescence properties (thermal quenching, quantum efficiency and excitation spectra) can be found in the Supporting Information.

Figure 2

Powder emission spectra of Na[Li3SiO4]:Eu2+ (black) and K[Li3SiO4]:Eu2+ (gray) by excitation with near‐UV light (400 nm).

The substance NaK7[Li3SiO4]8:Eu2+ shows a bright‐green emission with a unique spectrum upon excitation at 460 nm (Figure 3). This spectrum consists of two different emission bands, the first peaking at 515 nm with an exceptionally small full‐width at half‐maximum (FWHM) of 49 nm, whereas the second exhibits a broad peak with a maximum at 598 nm and a FWHM of about 138 nm. Even though these are the first unoptimized samples of the new phosphors, they already show a QE of about 60 % and a favorable thermal quenching (TQ) behavior (Supporting Information, Figure S3).

Figure 3

Single‐crystal emission spectrum of NaK7[Li3SiO4]8:Eu2+ (solid line, λ exc.=460 nm) in combination with two Gaussian curves (gray) to describe the total emission (calculation, black circles) as a two peak band.

To understand the various luminescence properties, a closer look at the coordination of the possible Eu2+ sites is needed. Compared to the nitrogen‐coordinated Eu2+ sites in the nitrides Ca[LiAl3N4]:Eu2+ and Sr[LiAl3N4]:Eu2+, the surroundings of the possible doping sites in the three structures presented here show great similarities. In the first coordination sphere of the lithosilicates, these sites exhibit an eight‐fold coordination regarding O, with a varying degree of local symmetry. In the case of Na[Li3SiO4], there is only one Na position (coordination number (CN): 7+1) suitable for doping, whereas in K[Li3SiO4] there are two highly asymmetric coordinated K sites (both with a CN of 8), which could host Eu2+. An eight‐fold coordination can also be found for the four possible doping sites (Na1, K1, K2, and K3) in the compound NaK7[Li3SiO4]8, where the atoms Na1 and K3 have a symmetric cuboid coordination, while the atoms K1 and K2 exhibit a slightly more asymmetric coordination sphere. In total, we assume that the first coordination sphere of all potential doping sites is too similar to explain the highly diverse luminescence properties observed experimentally. Obviously, the influence of the extended coordination sphere on the doping sites has a significant effect on the luminescence properties. The extended coordination sphere of the alkali metal cations is quite complex, including silicon and lithium cations (Supporting Information, Figures S1 and S2). In a first approach, the coordination polyhedra towards the silicon cations are considered. In comparison to lithium, the Si4+ cations have a higher partial charge exhibiting a substantially better and more rigid positioning in the anionic framework. This presumably leads to a higher interaction with the central cation position. As can be seen from the Supporting Information, Figures S1 and S2, the coordination towards the lithium cations is much more complex and because of their higher mobility they are left out of consideration. The Na‐site in Na[Li3SiO4]:Eu2+ shows a tetrahedral coordination by Si4+, leading to the before mentioned ultra‐narrow blue emission band. In contrast, the second coordination sphere in K[Li3SiO4]:Eu2+, where both K‐sites show a five‐fold coordination by Si4+, yields a double banded spectrum with its main emission being an orange broadband one. In comparison with these two given phosphors, the luminescence properties of NaK7[Li3SiO4]8:Eu2+ lie somewhere in between those two phases. In the following, we give an explanation on how these properties are linked to the new structure type, and we propose that in this particular case the Eu2+‐luminescence is mainly influenced by the extended coordination sphere.

When compared to Na[Li3SiO4]:Eu2+, it can be assumed that the Na‐site in NaK7[Li3SiO4]8:Eu2+ is unsuitable for an occupation with Eu2+. A narrowband blue emission would be expected due to the tetrahedral coordination by Si4+ in the extended coordination sphere. The differences in the first and extended coordination sphere are not sufficiently significant to explain an emission shift from narrow blue to narrow green. Eu2+‐substitution on the K1‐site seems to be the most likely cause of the broadband orange emission. By comparison of the coordination of this site with the two K‐sites in K[Li3SiO4]:Eu2+, it is clear that they are quite similar. In NaK7[Li3SiO4]8:Eu2+, K1 shows an asymmetric cuboid coordination towards O, as well as a 4+1 extended coordination sphere (Si‐cations) resulting in an asymmetric trigonal bipyramid (Figure 4). Therefore, it is reasonable to assign this site to be responsible for the broadband emission of NaK7[Li3SiO4]8:Eu2+.

Figure 4

View of the extended coordination sphere of the monovalent cations regarding the silicon atoms (red) in NaK7[Li3SiO4]8:Eu2+ with a view along [ 00].

Consequently, the narrowband green emission of NaK7[Li3SiO4]8:Eu2+ should derive from one of the two remaining sites, the K2 or the K3 position. Both sites exhibit a cuboid (K3) or a slightly deformed cuboid coordination (K2) towards oxygen. The bond lengths lie between 272.0(2) and 289.8(2) pm, which is an adequate space for an Eu2+ cation. In both cases, the extended coordination sphere (Si cations) is not comparable to the coordination of the cation sites in Na[Li3SiO4] or K[Li3SiO4]. K2 shows a trigonal prismatic and K3 a square planar surrounding by Si4+ (Figure 4). Therefore, both sites could act as the doping site, but it is reasonable to assume that only one of those is favored, otherwise a broader or even split emission profile should be observed. Unfortunately, an exact localization of the luminescence center is not possible.

In all of the here presented novel phosphors, Eu2+ replaces monovalent cations. To ensure charge neutrality, vacancies on the highly mobile Li sites, resulting in a local variation of the Li/Si distribution in the network, or vacancies on the monovalent cation sites are needed. Because of the low degree of substitution, these effects withdraw themselves from detection via single‐crystal X‐ray diffraction and most other analytical methods.

In conclusion, a new, previously overlooked member of the family of alkali lithosilicates, namely NaK7[Li3SiO4]8, is introduced. The compound shows a novel structure type with a highly condensed network of LiO4 and SiO4 tetrahedra containing monovalent cations positioned in vierer ring channels along [001]. Na[Li3SiO4]:Eu2+, K[Li3SiO4]:Eu2+, and the new compound NaK7[Li3SiO4]8:Eu2+ represent the first three members of a novel phosphor‐substance‐class, where purely monovalent cations could be partially substituted with Eu2+, resulting in unexpected luminescence properties not foreseen for this substance class. Na[Li3SiO4]:Eu2+ shows a ultra‐narrow blue emission λ max at 469 nm with a FWHM of 32 nm. According to LED simulations, K[Li3SiO4]:Eu2+ and NaK7[Li3SiO4]8:Eu2+ both could be used in single phosphor white pc‐LEDs. K[Li3SiO4]:Eu2+ in combination with a near‐UV LED yields a warm white emission with a color point close to the Planckian locus at 2700 K and high color rendering index (CRI>80, R9>0). NaK7[Li3SiO4]8:Eu2+ on the other hand could result in a cold‐white single phosphor pc‐LED with a CCT higher than 8000 K and high color rendering index (CRI>80, R9>0).

Furthermore, by comparing the properties of the novel phosphor NaK7[Li3SiO4]8:Eu2+ to the luminescence of simpler structures like Na[Li3SiO4]:Eu2+ or K[Li3SiO4]:Eu2+, structure property relations regarding the luminescence could be given. Consequently, by revisiting an old substance class with new or improved methods, it could be shown that alkali lithosilicates hold the potential for application in pc‐LEDs. Owing to the straightforward and comparably inexpensive synthesis method, these new materials could even break the supremacy of alkaline‐earth based pure nitrides and oxo‐nitrides as the go‐to materials for phosphors.

Experimental Section

The different compounds were synthesized by means of a conventional solid‐state reaction in a tube‐furnace at 1000 °C. As starting materials various mixtures of Na2CO3, K2CO3, Li2CO3, SiO2, and 2 mol % Eu2O3 as a doping agent were used. The reduction from Eu3+ to Eu2+ was facilitated by a constant flow of forming‐gas (92.5/7.5) during the experiment. The obtained lithosilicate samples can be handled at ambient air. They are stable over weeks in the laboratory and can be heated to >200 °C without decomposition (Supporting Information, Figure S6). Furthermore, they are suitable for mixtures with silicone as a matrix material.

A more detailed description of the synthesis and further experimental details are provided in the Supporting Information.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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