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

Abstract 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[Li3SiO4]8, a new representative has been discovered, expanding the family of known alkali lithosilicates. Astonishingly, NaK7[Li3SiO4]8 and the already established alkali lithosilicates Na[Li3SiO4] as well as K[Li3SiO4] display unforeseen luminescence properties, when doped with Eu2+. Na[Li3SiO4]:Eu2+ exhibits an ultra‐narrow blue, K[Li3SiO4]:Eu2+ a broadband, and NaK7[Li3SiO4]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.


Experimental Procedures Synthesis
According to Hoppe [1] , powders and single-crystals of NaLi3SiO4 and KLi3SiO4 can only be obtained via a stoichiometric hightemperature reaction of the respective oxides in well-shut Ni-ampoules. As our experiments showed, the same results can be obtained via conventional solid-state reactions.
Undoped samples of all compounds could also be obtained by repeating the same synthesis in Pt-crucibles in a muffle furnace.

Single-crystal structural analysis
Small single-crystals of the phase NaK7(Li3SiO4)8 were selected from the sample using a polarization microscope. A D8 Quest Kappa diffractometer (Bruker, USA) equipped with a Photon 100 detector using monochromatic MoKα1 radiation (λ = 71.07 pm) generated by a microfocus X-ray tube (Incoatec, Germany) was used to collect the single-crystal intensity data at lower temperatures (-80 °C). SAINT [2] and SADABS [3] were applied for data processing and multiscan absorption correction. The structure was solved with SHELXS [4] (version 2014/1) and refined via SHELXL [5] (version 2013/4) using WINGX [6] (version 2014/1). The full-matrix least squares refinement against F 2 yielded R1 = 0.0320, wR2 = 0.0850, and GOF = 1.187. For detailed information on the refinement parameters and crystallographic data, such as positional parameters, anisotropic displacement parameters, and interatomic distances see Tables 1-4. Further information on the crystal structure investigation can be obtained from the joint CCDC/FIZ Karlsruhe deposition service on quoting the deposition number CCDC-1861384.

Luminescence
Luminescence Spectroscopy: The emission signal of NaK7(Li3SiO4)8:Eu 2+ was measured by exciting crystals showing the cell of NaK7(Li3SiO4)8 with a 460 nm laser (model Sapphire 460/10, 10 mW; COHERENT). The converted light was collected using a multimode optical fiber (QP 600-2-VIS/BX; Ocean Optics) and finally detected in a spectrometer (QE 65000; Ocean Optics). In order to determine the luminescence properties of Na[Li3SiO4]:Eu 2+ and K[Li3SiO4]:Eu 2+ powder samples, a HORIBA Fluoromax 4 spectrophotometer was used. The emission spectrum was measured in the wavelength range between 430 and 780 nm (step size 1 nm) using an excitation wavelength of 400 nm and an integration time of 0.2 seconds per step.

Results and Discussion
Crystal structure details

Charge distribution and Madelungs part of lattice energy calculations
Charge distributions were calculated according to both the bond-length/bond-strength (∑V) [7][8] and the CHARDI (∑Q) [9] concept (Table  S5). Within the limits of these concepts, the calculated charges correlate well with the expected values. The MAPLE value (Madelung part of lattice energy) [10][11][12] Figure S1. Extended coordination spheres of the Na1, K1, K2, and K3 sites towards the silicon (red) and lithium (blue) cations.