Photochemistry within Compressed Sodium Azide

Synthesis of high nitrogen containing materials has been the subject of research interest for use as alternative clean sources of fuel and explosives. Here we present experimental evidence for the photochemical synthesis of new energetic materials from sodium azide (NaN₃) at 4.8–8.1 GPa. We show that excitation into the conduction band generates color centers within the compressed α-NaN₃ phase lattice with minimal or no molecular N₂ evolution. Photochemical changes to the sample were monitored by X-ray diffraction (XRD), infrared (IR) absorption, and Raman spectroscopy. These high pressure products were found to be stable upon decompression at 300 K down to 1.6 GPa, although it is suspected that the material can be recoverable to ambient pressure with cold decompression

Synthesis of Ultra-incompressible sp³‑Hybridized Carbon Nitride with 1:1 Stoichiometry

The search of compounds with C_<i>x</i>N_<i>y</i> composition holds great promise for creating materials which would rival diamond in hardness due to the very strong covalent C–N bond. Early theoretical and experimental works on C_<i>x</i>N_<i>y</i> compounds were based on the hypothetical structural similarity of predicted C₃N₄ phases with known binary A₃B₄ structural types; however, the synthesis of C₃N₄ other than g-C₃N₄ remains elusive. Here, we explore an “elemental synthesis” at high pressures and temperatures in which the compositional limitations due to the use of precursors in the early works are substantially lifted. Using in situ synchrotron X-ray diffraction and Raman spectroscopy, we demonstrate the synthesis of a highly incompressible <i>Pnnm</i> CN compound (<i>x</i> = <i>y</i> = 1) with sp³-hybridized carbon above 55 GPa and 7000 K. This result is supported by first-principles evolutionary search, which finds that CN is the most stable compound above 14 GPa. On pressure release below 6 GPa, the synthesized CN compound amorphizes, maintaining its 1:1 stoichiometry as confirmed by energy-dispersive X-ray spectroscopy. This work underscores the importance of understanding the novel high-pressure chemistry laws that promote extended 3D C-N structures, never observed at ambient conditions. Moreover, it opens a new route for synthesis of superhard materials based on novel stoichiometrie

Iodine in Metal–Organic Frameworks at High Pressure

Capture of highly volatile radioactive iodine is a promising application of metal–organic frameworks (MOFs), thanks to their high porosity with flexible chemical architecture. Specifically, strong charge-transfer binding of iodine to the framework enables efficient and selective iodine uptake as well as its long-term storage. As such, precise knowledge of the electronic structure of iodine is essential for a detailed modeling of the iodine sorption process, which will allow for rational design of iodophilic MOFs in the future. Here we probe the electronic structure of iodine in MOFs at variable iodine···framework interaction by Raman and optical absorption spectroscopy at high pressure (<i>P</i>). The electronic structure of iodine in the straight channels of SBMOF-1 (Ca-<i>sdb</i>, <i>sdb</i> = 4,4′-sulfonyldibenzoate) is modified irreversibly at <i>P</i> > 3.4 GPa by charge transfer, marking a polymerization of iodine molecules into a 1D polyiodide chain. In contrast, iodine in the sinusoidal channels of SBMOF-3 (Cd-<i>sdb</i>) retains its molecular (I₂) character up to at least 8.4 GPa. Such divergent high-pressure behavior of iodine in the MOFs with similar port size and chemistry illustrates adaptations of the electronic structure of iodine to channel topology and strength of the iodine···framework interaction, which can be used to tailor iodine-immobilizing MOFs

Aragonite-II and CaCO₃‑VII: New High-Pressure, High-Temperature Polymorphs of CaCO₃

The importance for the global carbon cycle, the <i>P</i>–<i>T</i> phase diagram of CaCO₃ has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO₃. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO₃-VII. CaCO₃-VII is a structural analogue of CaCO₃-<i>P</i>2₁/<i>c</i>-l, predicted theoretically earlier. The <i>P</i>–<i>T</i> phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO₃-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO₃-VI was found to be metastable in the whole pressure–temperature range