The exceptional ability of carbon to form sp 2 and sp 3 bonding states leads to a great structural and chemical diversity of carbon-bearing phases at non-ambient conditions. Here we use laser-heated diamond anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in CaCO 3 at P > 40GPa. We find that post-aragonite CaCO 3 transforms to the previously predicted P2 1 /c-CaCO 3 with sp 3 -hybridized carbon at 105 GPa (~30 GPa higher than the theoretically predicted crossover pressure). The lowest enthalpy transition path to P2 1 /c-CaCO 3 includes reoccurring sp 2 -and sp 3 -CaCO 3 intermediate phases and transition states, as reveled by our variable-cell nudged elastic band simulation. Raman spectra of P2 1 /c-CaCO 3 show an intense band at 1025 cm -1 , which we assign to the symmetric C-O stretching vibration based on empirical and first principles calculations. This Raman band has a frequency that is ~20 % lower than the symmetric C-O stretching in sp 2 -CaCO 3 , due to the C-O bond length increase across the sp 2 -sp 3 transition, and can be used as a fingerprint of tetrahedrally-coordinated carbon in other carbonates.
Nitrides, carbides, and borides of transition metals are an attractive class of hard materials. Our recent preliminary explorations of the binary chemical compounds indicated that chromium-based materials are among the hardest transition metal compounds. Motivated by this, here we explore in detail the binary Cr-B, Cr-C, and Cr-N systems using global optimization techniques. Calculated enthalpy of formation and hardness of predicted materials were used for Pareto optimization to define the hardest materials with the lowest energy. Our calculations recover all numerous known stable compounds (except CrC with its large unit cell) and discover a novel stable phase Pmn2-CrC. We resolve the structure of CrN and find it to be of anti-CaCl type (space group Pnnm). Many of these phases possess remarkable hardness, but only CrB is superhard (Vickers hardness 48 GPa). Among chromium compounds, borides generally possess the highest hardnesses and greatest stability. Under pressure, we predict stabilization of a layered TMDC-like phase of CrN, a WC-type phase of CrN, and a new compound CrN. Nitrogen-rich chromium nitride CrN is a high-energy-density material featuring polymeric nitrogen chains. In the presence of metal atoms (e.g., Cr), polymerization of nitrogen takes place at much lower pressures; CrN becomes stable at ∼15 GPa (cf. 110 GPa for synthesis of pure polymeric nitrogen).
The study of van der Waals interactions plays a central role in the understanding of bonding across a range of biological, chemical and physical phenomena. The presence of van der Waals interactions can be identified through analysis of the reduced density gradient, a fundamental parameter at the core of Density Functional Theory. An extension of Bader’s Quantum Theory of Atoms in Molecules is developed here through combination with the analysis of the reduced density gradient. Through this development, a new quantum chemical topological tool is presented: the volumetric source function. This technique allows insight into the atomic composition of van der Waals interactions, offering the first route towards applying the highly successful source function to these disperse interactions. A new algorithm has been implemented in the open-source code, CRITIC2, and tested on acetone, adipic and maleic acids molecular crystals, each stabilized by van der Waals interactions. This novel technique for studying van der Waals interactions at an atomic level offers unprecedented opportunities in the fundamental study of intermolecular interactions and molecular design for crystal engineering, drug design and bio-macromolecular processes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.