HP-Ca 2 Si 5 N 8 -A New High-Pressure Nitridosilicate: Synthesis, Structure, Luminescence, and DFT Calculations

Eu- and Tb-adsorbed Si3N4 and Ge3N4: tuning the colours with one luminescent host

Phosphor-converted white light emitting diodes (pc-LEDs) are efficient light sources for applications in lighting and electronic devices. Nitrides, with their wide-ranging applicability due to their intriguing structural diversity, and their auspicious chemical and physical properties, represent an essential component in industrial and materials applications. Here, we present the successful adsorption of Eu and Tb at the grain boundaries of bulk β-Si3N4 and β-Ge3N4 by a successful combustion synthesis. The adsorption of europium and terbium, and the synergic combination of both, resulted in intriguing luminescence properties of all compounds (red, green, orange and yellow). In particular, the fact that one host can deliver different colours renders Eu,Tb-β-M3N4 (M = Si, Ge) a prospective chief component for future light emitting diodes (LEDs). For the elucidation of the electronic properties and structure of β-Si3N4 and β-Ge3N4, Mott-Schottky (MS) measurements and density functional theory (DFT) computations were conducted for the bare and RE adsorbed samples.

Synthesis, structure and properties of a calcium oxonitridosilicate phosphor showing green or red luminescence upon doping with Eu2+ or Ce3

The phase Li_xCa_16-xSi₁₇N_32-xO_2+x was synthesized by solid-state metathesis at 1050 °C in weld-sealed niobium ampoules. The crystal structure was solved from powder X-ray diffraction data in the space group F4[combining macron]3m (a = 14.7391(1) Å, Z = 4); it is closely related to the previously reported structure of Ca₁₆Si₁₇N₃₄. The structure of Li_xCa_16-xSi₁₇N_32-xO_2+x is based on a complex network of [SiN₄] tetrahedra with interstitial voids filled by Ca ions, partially substituted by Li. This Li substitution is charge-compensated by replacement of N by O in the structure. Theoretical calculations confirm that the unit cell volume decreases with increasing stoichiometric factor x. Europium doped samples of this compound show intense Eu²⁺ emission at 536 nm on excitation with near-UV or blue radiation at 350-480 nm. Ce³⁺ doping yields orange to red emission with two broad emission bands at 600 and 665 nm.

A density functional study of the high-pressure chemistry of MSiN(2)(M = Be, Mg, Ca): prediction of high-pressure phases and examination of pressure-induced decomposition

Normal pressure modifications and tentative high-pressure phases of the nitridosilicates MSiN(2) with M =  Be, Mg, or Ca have been thoroughly studied by density functional methods. At ambient pressure, BeSiN(2) and MgSiN(2) exhibit an ordered wurtzite variant derived from idealized filled β-cristobalite by a C1-type distortion. At ambient pressure, the structure of CaSiN(2) can also be derived from idealized filled β-cristobalite by a different type of distortion (D1-type). Energy-volume calculations for all three compounds reveal transition into an NaCl superstructure under pressure, affording sixfold coordination for Si. At 76 GPa BeSiN(2) forms an LiFeO(2)-type structure, corresponding to the stable ambient-pressure modification of LiFeO(2), while MgSiN(2) and CaSiN(2) adopt an LiFeO(2)-type structure, corresponding to a metastable modification (24 and 60 GPa, respectively). For both BeSiN(2) and CaSiN(2) intermediate phases appear (for BeSiN(2) a chalcopyrite-type structure and for CaSiN(2) a CaGeN(2)-type structure). These two tetragonal intermediate structures are closely related, differing mainly in their c/a ratio. As a consequence, chalcopyrite-type structures exhibit tetrahedral coordination for both cations (M and Si), whereas in CaGeN(2)-type structures one cation is tetrahedrally (Si) and one bisdisphenoidally (M) coordinated. Both structure types, chalcopyrite and CaGeN(2), can also be derived from idealized filled β-cristobalite through a B1-type distortion. The group-subgroup relation of the BeSiN(2)/MgSiN(2), the CaSiN(2), the chalcopyrite, the CaGeN(2) and the idealized filled β-cristobalite structure is discussed and the displacive phase transformation pathways are illustrated. The zero-pressure bulk moduli were calculated for all phases and have been found to be comparable to compounds such as α- Si(3)N(4), CaIrO(3) and Al(4)C(3). Furthermore, the thermodynamic stability of BeSiN(2), MgSiN(2) and CaSiN(2) against phase agglomerates of the binary nitrides M(3)N(2) and Si(3)N(4) under pressure are examined.