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.

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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