title: Investigation of exotic stable calcium carbides using theory and experiment.
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 using the USPEX code. 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 is the base-centred 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 semi-metallic behaviour.
authors.affiliation: Laboratory for Quantum Design of Functional Materials, School of Physics and Electronic Engineering, Jiangsu Normal University, 221116 Xuzhou, China.
authors.forename: Yan-Ling
authors.initials: YL
authors.lastname: Li
authors.affiliation: Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, USA.
authors.forename: Sheng-Nan
authors.initials: SN
authors.lastname: Wang
authors.affiliation: 1] Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, USA [2] Center for Materials by Design, Institute for Advanced Computational Science, State University of New York, Stony Brook, New York 11794-2100, USA [3] Moscow Institute of Physics and Technology, 9 Institutskiy lane, Dolgoprudny city, Moscow Region 141700, Russia [4] Northwestern Polytechnical University, Xi'an 710072, China.
authors.forename: Artem R
authors.initials: AR
authors.lastname: Oganov
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, District of Columbia 20015, USA.
authors.forename: Huiyang
authors.initials: H
authors.lastname: Gou
authors.affiliation: High Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, USA.
authors.forename: Jesse S
authors.initials: JS
authors.lastname: Smith
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, District of Columbia 20015, USA.
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Strobel