Nitrogen usually consists of molecules where two atoms are strongly triple-bonded. Here, we report on an allotropic form of nitrogen where all atoms are connected with single covalent bonds, similar to carbon atoms in diamond. The compound was synthesized directly from molecular nitrogen at temperatures above 2,000 K and pressures above 110 GPa using a laser-heated diamond cell. From X-ray and Raman scattering we have identified this as the long-sought-after polymeric nitrogen with the theoretically predicted cubic gauche structure (cg-N). This cubic phase has not been observed previously in any element. The phase is a stiff substance with bulk modulus >or=300 GPa, characteristic of strong covalent solids. The polymeric nitrogen is metastable, and contrasts with previously reported amorphous non-molecular nitrogen, which is most likely a mixture of small clusters of non-molecular phases. The cg-N represents a new class of single-bonded nitrogen materials with unique properties such as energy capacity: more than five times that of the most powerfully energetic materials.

Single-bonded cubic form of nitrogen

THE temperature distribution in the Earth's core places important constraints on the Earth's internal heat budget and on models of the geodynamo. The solid inner core crystallizes from a liquid outer core, consisting mainly of iron alloyed with a lighter element, at a depth of about 5,100 km (corresponding to a pressure of about 3.3 Mbar). Thus, the most reliable means of determining the temperature gradient in the core is to estimate the melting temperature of iron and iron-rich compounds at the pressure of the inner core boundary. Current estimates range from about 4,000 to 8,000 K; but these estimates, obtained from shock compression1–3, theory (discussed in ref. 4) and extrapolation of static pressure data2,3,5, are poorly constrained. Here I present melting-point measurements on iron and iron–oxygen compounds at static pressures of up to Mbar. Extrapolation of these results to 3.3 Mbar yields a temperature at the inner-core boundary of 4,850±200 K. A weak change in optical absorption observed above 2,000 K may correspond to the solid–solid phase transition found in shock experiments at 2 Mbar (ref. 1).

Temperatures in the Earth's core from melting-point measurements of iron at high static pressures

Silicon nitride (Si3N4) is used in a variety of important technological applications The high fracture toughness, hardness and wear resistance of Si3N4-based ceramics are exploited in cutting tools and anti-friction bearings1; in electronic applications, Si3N4 is used as an insulating, masking and passivating material2 Two polymorphs of silicon nitride are known, both of hexagonal structure: α- and β-Si3N4 Here we report the synthesis of a third polymorph of silicon nitride, which has a cubic spinel structure This new phase, c-Si3N4, is formed at pressures above 15 GPa and temperatures exceeding 2,000 K, yet persists metastably in air at ambient pressure to at least 700 K First-principles calculations of the properties of this phase suggest that the hardness of c-Si3N4 should be comparable to that of the hardest known oxide (stishovite3, a high-pressure phase of SiO2), and significantly greater than the hardness of the two hexagonal polymorphs

Synthesis of cubic silicon nitride

Phase transformations and their bearing on the constitution and dynamics of the mantle

Earth’s core is structured in a solid inner core, mainly composed of iron, and a liquid outer core. The temperature at the inner core boundary is expected to be close to the melting point of iron at 330 gigapascal (GPa). Despite intensive experimental and theoretical efforts, there is little consensus on the melting behavior of iron at these extreme pressures and temperatures. We present static laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast x-ray diffraction as a primary melting diagnostic. When extrapolating to higher pressures, we conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 kelvin. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the mantle.

https://science.sciencemag.org/content/sci/340/6131/464.full.pdf

Melting of Iron at Earth’s Inner Core Boundary Based on Fast X-ray Diffraction

The melting curve of iron to 1.2 Mbar was remeasured in a laser-heated diamond anvil cell. A defocused 35 W Nd∶YAG laser beam provided low-temperature gradients which were measured within an area of 2 μm in diameter with a chromatically corrected optical geometry. Argon to 350 kbar and A12O3 to 1.2 Mbar were used as pressure media. Melting was detected by in situ measurements of optical reflectivity and electrical resistivity and by visual observation of the surface texture. At 1.2 Mbar, iron melts at 3000 ± 100 K. The same laser-heating technique was used to measure volumes of fcc-(γ)-iron to 168 kbar and 2000 K using synchrotron radiation, leading to a decrease in the thermal expansion coefficient of γ-iron represented by (∂ In α/∂ In V)T = 6.5±0.5. The α-e transition of iron has been studied by X ray and by the change in resistivity of a fine wire using a variety of pressure media. The hysteresis of the transition changes systematically with the shear strength of the pressure medium with an equilibrium transition pressure at 130±10 kbar.

N. von Bargen

Papers

Melting, thermal expansion, and phase transitions of iron at high pressures