ABSTRACT
The only known bulk ambient pressure nickel nitride phase is hexagonal Ni(3)N (space group P6(3)22). Multianvil synthesis experiments at 20 GPa and 2000 K using nickel (Ni) and sodium azide (NaN(3)) starting materials, and ex situ analysis using transmission electron microscopy and scanning electron microscopy measurements show that this phase can be recovered at ambient pressure (space group P6(3)22, a = 4.62 Å, c = 4.30 Å, Z = 2). Formation of this phase is correlated with the repulsive interactions between closely spaced nitrogen ions and with the extent of thermal stability of nickel nitride at ambient and at high densities. These two factors are also important in relating the high temperature and pressure behaviour of nickel nitride to those of several other interstitial nitrides recovered from similar pressures after heating. Further, we report formation of a sodium rhenium nitride phase by reaction of the azide with the rhenium capsule in which the reactants were contained.
ABSTRACT
In three different experiments up to 100 gigapascals and 3000 kelvin, (Mg,Fe)SiO3-perovskite, the major component of the lower mantle, remained stable and did not decompose to its component oxides (Mg, Fe)O and SiO2. Perovskite was formed from these oxides when heated in a diamond anvil cell at pressures up to 100 gigapascals. Both MgSiO3 crystals and glasses heated to 3000 kelvin at 75 gigapascals also formed perovskite as a single phase, as evident from Raman spectra. Moreover, fluorescence measurements on chromium-doped samples synthesized at these conditions gave no indication of the presence of MgO.
