Silicates are one of the most important classes of compounds on this planet, and more than 1000 silicates have been identified in the mineral kingdom. Additionally, several hundreds of artificial silicates have been synthesized. The substitution of oxygen by nitrogen leads to the structurally diverse and manifold class of nitridosilicates. Silicon nitride, one of the most important non-oxidic ceramic materials, is the binary parent compound of nitridosilicates, and it symbolizes the inherent material properties of these refractory compounds. However, prior to the last decades, a broad systematic investigation of nitridosilicates had not been accomplished. In the meantime, these and related compounds have reached a remarkable level of industrial application. This review illustrates recent progress in synthesis and structure-property relationships and also applications of nitridosilicates, oxonitridosilicates, and related SiAlONs.

The highly efficient nitridosilicate phosphors M 2 Si 5 N 8 (M ) Sr, Ba, Eu) for phosphor-converted pcLEDs were synthesized at low temperatures using a novel precursor route involving metal amides M(NH 2 ) 2 . These precursors have been synthesized by dissolution of the respective metals in supercritical ammonia at 150°C and 300 bar. The thermal behavior and decomposition process of the amides were investigated with temperature programmed powder X-ray diffractometry and thermoanalytical measurements (DTA/ TG). These investigations rendered the amides as suitable intermediates for reaction with silicon diimide (Si(NH) 2 ). Thus, the desired nitridosilicate phosphors were obtained at relatively low temperatures around 1150-1400°C which is approximately 300°C lower compared to common synthetic approaches starting from metals or oxides. The influence of the thermal treatment on the phosphor morphology has been studied extensively. The accessibility of spherical phosphor particles represents another striking feature of this route since it improves light extraction from the crystallites due to decreasing light guiding and decreasing re-absorption inside the phosphor particle.

Highly efficient red-emitting nitridosilicate phosphors Sr 2 Si 5 N 8 :Eu 2+ and Ba 1.5 Sr 0.5 Si 5 N 8 :Eu 2+ (doping level 1 %) applicable to phosphor converted pc-LEDs were synthesized in nanocrystalline form at low temperatures employing a novel single-source precursor approach. Synthesis starts from nanocrystalline silicon and uses mixed metal amides M(NH 2 ) 2 with M ) Sr, Ba, Eu as reactive intermediates. In a second approach, a single-source precursor mixture obtained from a one-pot reaction of the corresponding elements (Sr/Ba, Eu, Si) was obtained in supercritical ammonia. Thermoanalytical in situ investigations gain a deeper insight into the degradation mechanism of the mixed metal amide precursors and revealed the onset for the formation of the 2-5-8 phosphor materials at temperatures slightly above 900°C. Formation of the products is complete below 1400°C. Under these conditions, the nitridosilicate phosphors form spherically shaped particles with crystallites of 200 nm in size. Spherical particles are desirable for phosphor application because light extraction may be improved by decreased light trapping and re-absorption losses. As a major advantage of the one-pot precursor approach, the exact Sr/Ba content in the solid solution series Sr 2-x Ba x Si 2 N 8 :Eu 2+ and the doping concentration of Eu 2+ can easily be controlled in a wide range by the relative amount of the elemental starting materials (Sr, Ba, Eu, Si). Simultaneously, thorough mixing of these elements down to an atomic level (Sr, Ba, Eu) or at least at nanoscopic dimensions (silicon) is achieved by the solution approach. As a consequence, no milling and pre-reaction steps are necessary which might give rise to contamination. Advantageously, this approach can easily be extended to large-scale processes by simultaneously preserving complete mixing. Furthermore, the influence of the starting materials (single-source precursor, nanocrystalline silicon) and the reaction conditions on the crystal shape and finally on the luminescence properties of the products was investigated. The obtained nanophosphors exhibit luminescence properties comparable to coarsely crystalline nitridosilicate phosphor powders prepared by conventional high-temperature processing.

By studying the thermal condensation of melamine, we have identified three solid molecular adducts consisting of melamine C(3)N(3)(NH(2))(3) and melem C(6)N(7)(NH(2))(3) in differing molar ratios. We solved the crystal structure of 2 C(3)N(3)(NH(2))(3)C(6)N(7)(NH(2))(3) (1; C2/c; a=21.526(4), b=12.595(3), c=6.8483(14) A; beta=94.80(3) degrees ; Z=4; V=1850.2(7) A(3)), C(3)N(3)(NH(2))(3)C(6)N(7)(NH(2))(3) (2; Pcca; a=7.3280(2), b=7.4842(2), c=24.9167(8) A; Z=4; V=1366.54(7) A(3)), and C(3)N(3)(NH(2))(3)3 C(6)N(7)(NH(2))(3) (3; C2/c; a=14.370(3), b=25.809(5), c=8.1560(16) A; beta=94.62(3) degrees ; Z=4; V=3015.0(10) A(3)) by using single-crystal XRD. All syntheses were carried out in sealed glass ampoules starting from melamine. By variation of the reaction conditions in terms of temperature, pressure, and the presence of ammonia-binding metals (europium) we gained a detailed insight into the occurrence of the three adduct phases during the thermal condensation process of melamine leading to melem. A rational bulk synthesis allowed us to realize adduct phases as well as phase separation into melamine and melem under equilibrium conditions. A solid-state NMR spectroscopic investigation of adduct 1 was conducted.

Amorphous silicon bis(carbodiimide) “Si(CN2)2” has been identified as a reactive precursor for the synthesis of nitridosilicates that is specifically useful in the temperature region below 1000 °C. In this context the applicability and reactivity of amorphous “Si(CN2)2” towards Li3N in comparison to silicon diimide Si(NH)2 has been studied using high‐temperature in situ powder diffraction and DSC measurements. During the current investigation single crystals of Li2SiN2 have been obtained and the crystal structure of this Li+ ion conductor has been determined and refined: [Pbca, no. 61, a = 9.907(2), b = 9.907(2), c = 15.014(3) Å, Z = 32, R1 = 0.038, 1460 data, 142 parameters]. In the solid, Li2SiN2 consists of two interpenetrating cristobalite type nets which are made up from hetero‐adamantane‐like [Si4N6]N4/2 groups. The eight crystallographically independent Li+ sites are ordered at room temp. and exhibit coordination numbers 3 to 5. The 7Li and 29Si solid‐state MAS NMR spectra of Li2SiN2 are reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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Abstract:The mixed valence europium nitridosilicate Eu 2 SiN 3 has been synthesized at 900°C in welded tantalum ampules starting from europium and silicon diimide Si(NH) 2 in a lithium flux. The structure of the black material has been determined by single-crystal X-ray diffraction analysis (Cmca (no. 64), a ) 542.3(11) pm, b ) 1061.0(2) pm, c ) 1162.9(2) pm, Z ) 8, 767 independent reflections, 37 parameters, R1 ) 0.017, wR2 ) 0.032). Eu 2 SiN 3 is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN 4 tetrahedra running parallel [100] with a maximum stretching factor f s ) 1.0. The compound is isostructural with Ca 2 PN 3 and Rb 2 TiO 3 , and it represents the first example of a nonbranched chain silicate in the class of nitridosilicates. There are two crystallographically distinct europium sites (at two different Wyckoff positions 8f) being occupied with Eu 2+ and Eu 3+ , respectively. 151 Eu Mö ssbauer spectroscopy of Eu 2 SiN 3 differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu 2+ and Eu 3+ signals over the whole temperature range. The Eu 2+ site shows magnetic hyperfine field splitting at 4.2 K. Magnetic susceptibility measurements exhibit Curie-Weiss behavior above 24 K with an effective magnetic moment of 7.5 µ B /f.u. and a small contribution of Eu 3+ , in accordance with Eu 2+ and Eu 3+ in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at T C ) 24 K. DFT calculations of Eu 2 SiN 3 reveal a band gap of ∼0.2 eV, which is in agreement with the black color of the compound. Both DFT calculations and lattice energetic calculations (MAPLE) corroborate the assignment of two crystallographically independent Eu sites to Eu 2+ and Eu 3+ .

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