ARTICLE Received 6 Aug 2012 | Accepted 8 Oct 2012 | Published 13 Nov 2012 Observation of a topological crystalline insulator phase and topological phase transition in Pb 1  x Sn x Te Su-Yang Xu 1 , Chang Liu 1 , N. Alidoust 1 , M. Neupane 1 , D. Qian 1,2 , I. Belopolski 1 , J.D. Denlinger 3 , Y.J. Wang 4 , H. Lin 4 , L.A. Wray 1,3 , G. Landolt 5,6 , B. Slomski 5,6 , J.H. Dil 5,6 , A. Marcinkova 7 , E. Morosan 7 , Q. Gibson 8 , R. Sankar 9 , F.C. Chou 9 , R.J. Cava 8 , A. Bansil 4 & M.Z. Hasan 1,10 A topological insulator protected by time-reversal symmetry is realized via spin–orbit inter- action-driven band inversion. The topological phase in the Bi 1  x Sb x system is due to an odd number of band inversions. A related spin–orbit system, the Pb 1  x Sn x Te, has long been known to contain an even number of inversions based on band theory. Here we experi- mentally investigate the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb 1  x Sn x Te class of materials that has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin- polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi 1  x Sb x . Our observation of the spin-polarized Dirac surface states in the inverted Pb 1  x Sn x Te and their absence in the non-inverted compounds related via a topo- logical phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices. DOI: 10.1038/ncomms2191 1 Joseph Henry Laboratory, Department of Physics, Princeton University, Princeton, New Jersey 08544, USA. 2 Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China. 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94305, USA. 4 Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA. 5 Swiss Light Source, Paul Scherrer Institute, CH-5232, Villigen, Switzerland. 6 Physik-Institute, Universitat Zurich-Irchel, CH-8057 Zurich, Switzerland. 7 Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA. 8 Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA. 9 Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan. 10 Princeton Center for Complex Materials, PRISM, Princeton University, Princeton, NJ 08544, USA. Correspondence and requests for materials should be addressed to M.Z.H. (email: mzhasan@princeton.edu). NATURE COMMUNICATIONS | 3:1192 | DOI: 10.1038/ncomms2191 | www.nature.com/naturecommunications 1 & 2012 Macmillan Publishers Limited. All rights reserved.