ULTRAHIGH PRESSURE CHEMISTRY OF NITROGEN: THE FORMATION OF METAL-INORGANIC FRAMEWORKS WITH POLYMERIC NITROGEN LINKERS AT MEGABAR CONDITIONS

27HIGHLIGHTS 2020

Dinitrogen molecule N2 is one of the most stable molecules at ambient conditions due to the high energy of the triple nitrogen nitrogen bond. The reactivity of nitrogen can be significantly enhanced by the application of high pressure. The reactions between 5d transition metals Hf, W, Os and nitrogen N2 at pressures exceeding 1 million atmospheres counterintuitively lead to a series of complex framework compounds with large pores, which are filled by molecular nitrogen.

Nitrogen-rich compounds are among the best high energy density materials due to the remarkable difference in the average bond energy between the single N N bond (160 kJ/mol), the double N=N bond (418 kJ/mol), and the triple N≡N bond (945 kJ/mol). Because of the same reason, dinitrogen N≡N is a very stable molecule, which is exceptionally unreactive at standard conditions. Application of pressure is one of the effective ways to increase the chemical potential of nitrogen and thereby to gain access to a plethora of energetic nitrogen-rich phases [1,2].

The focus of this study was the synthesis of novel transition metal polynitrides. Tiny pieces of metal foils (Hf, W, Os) were placed in diamond-

Fig. 15: Crystal structures of metal-inorganic frameworks Os5N28·3N2, WN8·N2, ReN8·N2, Hf4N20·N2 at ~100 GPa. Grey spheres represent metal atoms. Blue spheres show nitrogen atoms belonging to the framework,

red spheres show nitrogen atoms belonging to the N2 molecules embedded into the frameworks.

Fig. 16: a) X-ray diffraction imaging map of the Os-N sample. b) Calculated electron density map showing the equivalent electron density distribution between the chain nitrogen atoms in WN8·N2. c) Scheme of the coordination of one polymeric nitrogen chain by metal atoms.