Understanding the origin and mechanisms of luminescence is a crucial point when it comes to the development of new phosphors with targeted luminescence properties. Herein, a new phosphor belonging to the substance class of alkali metal lithosilicates with the generalized sum formula Cs4−x−y−zRbxNayLiz[Li3SiO4]4:Eu2+ is reported. Single crystals of the cyan‐emitting UCr4C4‐type phosphor show a peculiar double‐band luminescence with one ultranarrow emission band at 473 nm and a narrow emission band at 531 nm under excitation with UV light (λexc=408 nm). Regarding occupation of the channels by the light metal ions, investigations of single‐crystal XRD data led to the assumption that domain formation with distinct lithium‐ and sodium‐filled channels occurs. Depending on which of these channels hosts the activator ion Eu2+, a green or blue emission results. The herein‐presented results shed new light on the luminescence process in the well‐studied UCr4C4‐type alkali metal lithosilicate phosphors.

Eu2+‐activated phosphors of the UCr4C4 structure type have emerged as promising new (ultra‐)narrow band phosphors. Especially the UCr4C4‐type alkali lithosilicates have caused a sensation in the field of phosphor research in the recent years due to their consistent (ultra)narrow‐band luminescence in the cyan and green range that can pave the way to novel backlighting LEDs with high color gamut. The compound “K2Na2[Li3SiO4]4:Eu2+” stands out with a strikingly narrow emission in the cyan range (<0.2 eV or <30 nm full width at half maximum in the energy or wavelength scale, respectively) but the position of the Eu2+ ions within this structure has been so far controversial. High‐resolution photoluminescence spectroscopy at 10 K reveals a complex emission spectrum with seven emission bands in the violet to green range. Combination with single‐crystal X‐ray diffraction (SCXRD) demonstrates that the cation channels in the structure also contain Li+ ions. The presence of emission bands in the violet and blue spectral range reveals that the Eu2+ ions also occupy the available K sites in the structure independently confirmed by advanced ligand field calculations. This study demonstrates that a combination of SCXRD, photoluminescence spectroscopy, and theory can help elucidate advanced structural problems.

Eu 2+ , light-emitting diodes, narrow-band red emitter, Sr[Li 2 Al 2 O 2 N 2 ] (SALON), UCr 4 C 4 -type phosphors

In order to expand the field of alkali lithosilicates, a new representative of the substance class with a previously unknown structure type was found based on solid‐state synthesis. The novel compound with the sum formula Rb[Li5Si2O7] crystallizes in the orthorhombic space group Pbcm (no. 57) with a=7.6269(3), b=9.5415(4), and c=9.4095(3) Å by means of single‐crystal X‐ray diffraction. The structure consists of a highly condensed lithosilicate framework, built up of corner‐ and edge‐linked [LiO4]‐tetrahedra and [Si2O7]‐units, and the rubidium ions aligned in channels. Suitable crystals of the material were obtained using sealed tantalum ampoules as reaction tube at a temperature of 750 °C. The new compound was further characterized via powder diffraction, Rietveld analysis, and EDX measurements. At first glance, Eu2+‐doped Rb[Li5Si2O7] reveals an intense green luminescence. In‐depth crystal analysis shows that a core‐shell formation is present even for apparently high quality single‐crystals. As a minority phase, the known green phosphor RbLi[Li3SiO4]2:Eu2+ is the origin of the luminescence, representing a tiny core inside of the particles surrounded by a large matrix of transparent Rb[Li5Si2O7] dominating the single‐crystal diffraction pattern.

During the investigation of a solid‐solution series between the alkali lithosilicate Na[Li3SiO4] and the hypothetical nitridolithoaluminate Eu[LiAl3N4] with the generalized sum formula Na1–xEux[Li3–2xSi1‐xAl3xO4–4xN4x], a hitherto unknown oxo‐nitride has been synthesized. The new material with the composition Na1–xEux[Li3–2xSi1‐xAl3xO4–4xN4x] (x = 0.22) crystallizes in the tetragonal space group I4/m (no. 87) with a = 7.704(2) and c = 3.1584(6) Å. To the best of our knowledge this is the first synthesis of a litho‐alumo‐oxo‐nitridosilicate containing either alkali‐, or rare‐earth metals. Upon excitation of this novel compound with near UV to blue light, a broadband amber emission with a maximum at 612 nm and a full width at half maximum of 2835 cm‐1 (≈ 105 nm) is observed. Furthermore, single‐crystals of the solid‐solution series could be synthesized on the oxygen‐rich side between 0 < x < 0.2. The thereby received phosphors crystallize in the Na[Li3SiO4]‐structure type and their emission maxima can be shifted in the spectral range from 469 nm up to 588 nm.