Elisabeta Horvath-Bordon

Alkalicyamelurates, M3[C6N7O3].xH2O, M = Li, Na, K, Rb, Cs: UV-luminescent and thermally very stable ionic tri-s-triazine derivatives.

Cyamelurates are salts of cyameluric acid, a derivative of tri-s-triazine (1,3,4,6,7,9-hexaazacyclo[3.3.3]azine or s-heptazine). These compounds are thermally very stable and possess interesting structural and optical properties. Only very few tri-s-triazine derivatives have been reported in the literature. The water-soluble alkali cyamelurates were extensively characterized using NMR, FTIR, Raman, UV, luminescence spectroscopy and elemental analysis. In addition, the single crystal X-ray structure analyses of the four hydrates of lithium, sodium, potassium and rubidium cyamelurates (Li(3)[C(6)N(7)O(3)].6H(2)O; Na(3)[C(6)N(7)O(3)].4.5H(2)O; K(3)[C(6)N(7)O(3)].3H(2)O; Rb(3)[C(6)N(7)O(3)].3H(2)O) are presented. Thermogravimetric analysis shows that the dehydrated salts start to decompose at temperatures above 500 degrees C. The thermal stability does not depend on the cations which is in contrast to the analogous s-triazine salts, i.e. the alkali cyanurates M(3)[C(3)N(3)O(3)]. The photoluminescence spectra indicate a very strong solid state UV-emission with maxima between 280 and 400 nm.

Molecular Chemical Concepts for the Synthesis of Novel Ceramics

There is presently much effort in basic science and applied research to work on-novel ceramics with properties far beyond those of the existing materials. Aim and scope of the research in this field is to develop materials with superior thermomechanical, physical and chemical properties. In particular, the synthesis of carbide- and nitride-based ceramic,devices from molecular compounds such as inorganic polymers has attracted increasing attention for the production of dense and porous ceramic composites, fibers or coatings. Another important novel development is the synthesis of hybrid materials using sol-gel techniques. In our studies hybrid organic-inorganic xerogels; of general formula (NCN)(1.5)Si-(CH2)(n)-Si(NCN)(1.5) and (NCN)(1.5)Si-CH2-C6H4-CH2-Si(NCN)(1.5) were prepared. These polymeric carbodiimide-gels represent a novel approach to the field of non-oxidic hybrid materials. All the products were characterized by nitrogen adsorption (BET), SEM, TEM and FTIR spectroscopy. Moreover, novel ceramic compositions such as binary carbon nitrides can be synthesized by the polymer-to-ceramic transformation route. The sp(3)-hybridised carbon nitrides are expected to exhibit ultra high hardness. We prepared ON precursor compounds based on the tri-s-triazine unit. The molecular, oligomeric and polymeric compounds exhibit not only high thermal stability, but also interesting optical properties such as strong photoluminescence. Preliminary experiments indicate that the precursors are promising candidates for high pressure bulk synthesis of ceramic CNx materials.

High-pressure high-temperature synthesis of novel binary and ternary nitride phases of group 4 and 14 elements

Our recent experiments on high-pressure high-temperature synthesis of novel ternary nitrides of group 4 and 14 elements are presented. Dense carbon nitride imide, C2N2(NH), was synthesized for the first time in a laser heated diamond anvil cell (LH-DAC) at pressures above 27 GPa and temperatures around 2000 K. Based on results of the electron diffraction-, EELS-and SIMS-measurements combined with theoretical calculations the structure of this new C-N-H phase was suggested to be of the defect-wurtzite type. Farther, macroscopic amounts of a new oxynitride of zirconium having cubic Th3P4-type structure, c-Zr2.86(N0.88O0.12)4, were synthesized at high pressures and temperatures using a multi-anvil apparatus. Earlier this structure was observed for binary nitrides of zirconium(IV) and hafnium(IV) synthesized in microscopic amounts in a LH-DAC. The lattice parameter of c-Zr2.86(N0.88O0.12)4 was found to be a0 = 6.7549(1) A which is slightly larger than that of c-Zr3N4. Isotropic bulk and shear moduli of c-Zr2.86(N0.88O0.12)4 of B0 = 219 GPa and G0 = 96 GPa, respectively, were determined from the compression and nanoindentation measurements. The Vickers microhardness, HV(1), of the porous (about 30 vol. %) sample of c-Zr2.86(N0.88O0.12)4 was measured to be 12 GPa, similar to that of single crystal δ-ZrN.

Single-crystals of a new carbon nitride phase with all-sp3 carbon

We prepared a carbon nitride compound by treating the molecular precursor C2N4H4in a laser-heated diamond-anvil-cell (LH-DAC) at pressures exceeding 27 GPa and temperatures above 1700°C. After quenching we recovered single crystals of the new material with diameters of about 1 μm. Using several experimental characterization techniques in conjunction with computational methods we determined composition, hybridization of constituting atoms, and the crystal structure. The new carbon nitride imide, C2N2(NH), adopts a defect-wurtzite structure and is isotypic to Si2N2(NH).