Pressure induced polymerization of acetylide anions in CaC2 and 107 fold enhancement of electrical conductivity† †Electronic supplementary information (ESI) available: Details of GC-MS analysis. See DOI: 10.1039/c6sc02830f Click here for additional data file.

aCenter for High Pressure Science and Technology Advanced Research (HPSTAR), PO Box 8009, Beijing, 100088, China bGeophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, United States cCOFCO Nutrition & Health Research Institute, Beijing Key Laboratory of Nutrition Health and Food Safety, Beijing 100209, China dAgilent Technologies (China) Co., Ltd., Wangjingbei Road, Chaoyang District, Beijing 100102, China eDepartment of Chemistry, Durham University, South Road, Durham, DH1 3 LE, UK fSpallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States gCentre for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States hHPSynC, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439, United States iDept. of Physics, Southern University of Science and Technology, ShenZhen, China jInstitute of Physics, Chinese Academy of Sciences, Beijing 100190, China kCollaborative Innovation Centre of Quantum Matter, Beijing, China † These authors contribute equally. § Present address: European Spallation Source ERIC, Lund, Sweden. Corresponding Author: *Kuo Li, Email: [email protected];