Synchrotron x-ray diffraction and Raman scattering data supported by ab initio calculations are reported for the dense tetrahedrally bonded phase ͑C 2 N 3 H͒ with a defective wurtzite ͑dwur͒ structure synthesized by laser heating from dicyandiamide ͑C 2 N 4 H 4 ͒ at high pressure in a diamond anvil cell. This work confirms the structure deduced in previous work from electron diffraction experiments. The phase ͑Cmc2 1 ͒ is recoverable to ambient conditions. The ambient pressure volume ͑V 0 = 137.9 Å 3 ͒ and bulk modulus ͑K 0 = 258Ϯ 21 GPa͒ are in excellent agreement with density functional calculations ͑V 0 = 134.7 Å 3 ; K 0 = 270 GPa͒. The calculated Raman frequencies and pressure shifts are also in good agreement with experiment. Ammonia ͑P2 1 2 1 2 1 ͒ was identified among the reaction products as expected from the synthesis reaction.

Combining metastable precursors with high pressure-temperature treatment is a powerful tool to make nitrogen-rich metal nitrides. Two new dense polymorphs of Ta3N5 have previously been theoretically predicted, with U3Se5 (Pnma) and U3Te5 (Pnma) structure types, and are now shown to exist. Amorphous Ta3N5 from thermal ammonolysis of an amorphous polymeric precursor was laser heated at 22 GPa and examined using synchrotron X-ray diffraction to reveal the emergence of these two novel polymorphs.

Abstract:There is interest in identifying novel materials for use in radioactive waste applications and studying their behavior under high pressure conditions. The mineral zirconolite (CaZrTi 2 O 7 ) exists naturally in trace amounts in diamond-bearing deep-seated metamorphic/igneous environments, and it is also identified as a potential ceramic phase for radionuclide sequestration. However, it has been shown to undergo radiation-induced metamictization resulting in amorphous forms. In this study we probed the high pressure structural properties of this pyrochlore-like structure to study its phase transformations and possible amorphization behavior. Combined synchrotron X-ray diffraction and Raman spectroscopy studies reveal a series of high pressure phase transformations. Starting from the ambient pressure monoclinic structure an intermediate phase with P2 1 /m symmetry is produced above 15.6 GPa via a first order transformation resulting in a wide coexistence range. Upon compression to above 56 GPa a disordered metastable phase III with a cotunnite-related structure appears that is recoverable to ambient conditions. We examine the similarity between the zirconolite behavior and the structural evolution of analogous pyrochlore systems under pressure.

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch-Murnaghan equation of state fit for baddeleyite structured β-TaON indicates a high bulk modulus value K o =3284 GPa with K o '=4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta-(O,N) and Ta-Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnitetype structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO 2 phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus K o ~ 370 GPa, close to the theoretically predicted value.