Exotic stable calcium carbides: theory and experiment

TL;DR

This study combines theoretical predictions and experimental validation to identify and synthesize stable calcium carbides under high pressure, revealing new structures and electronic properties of these materials.

Contribution

It systematically predicts all stable calcium carbides up to 100 GPa and successfully synthesizes two new compounds, confirming theoretical models with experimental data.

Findings

Ca2C and Ca2C3 synthesized successfully with structural confirmation

Discovery of a layered, negatively charged calcium structure in Ca2C

Identification of semimetallic behavior in Ca5C2

Abstract

It is well known that pressure causes profound changes in the properties of atoms and chemical bonding, leading to the formation of many unusual materials. Here we systematically explore all stable calcium carbides at pressures from ambient to 100 GPa using variable-composition evolutionary structure predictions. We find that Ca5C2, Ca2C, Ca3C2, CaC, Ca2C3, and CaC2 have stability fields on the phase diagram. Among these, Ca2C and Ca2C3 are successfully synthesized for the first time via high-pressure experiments with excellent structural correspondence to theoretical predictions. Of particular significance are the base-centered monoclinic phase (space group C2/m) of Ca2C, a quasi-two-dimensional metal with layers of negatively charged calcium atoms, and the primitive monoclinic phase (space group P21/c) of CaC with zigzag C4 groups. Interestingly, strong interstitial charge localization is found in the structure of R-3m-Ca5C2 with semimetallic behaviour.