skip to main content


This content will become publicly available on January 1, 2017

Title: Optic phonons and anisotropic thermal conductivity in hexagonal Ge2Sb2Te5

The lattice thermal conductivity ( ) of hexagonal Ge2Sb2Te5 (h-GST) is studied via direct first-principles calculations. We find significant intrinsic anisotropy of [( _a _c ~2)] in bulk h-GST along different transport directions. The dominant contribution to is from optic phonons, ~75%. This is extremely unusual as the acoustic phonon modes carry most of the heat in typical semiconductors and insulators with small unit cells. Very recently, Lee et. al. observed anisotropic in GST thin films and attributed this to thermal resistance of amorphous regions near grain boundaries. However, our results suggest an additional strong intrinsic anisotropy for the pure hexagonal phase. This derives from bonding anisotropy along different crystal directions, specifically from weak interlayer coupling, which gives anisotropic phonon dispersions. The phonon spectrum of h-GST has very dispersive optic branches with higher group velocities along the a-axis as compared to flat optic bands along the c-axis. The importance of optic mode contributions for the thermal conductivity in low- h-GST is unusual, and development of fundamental physical understanding of these contributions may be critical to better understanding of thermal conduction in other complex layered materials.
 [1] ;  [1] ;  [2]
  1. ORNL
  2. University of Missouri, Columbia
Publication Date:
OSTI Identifier:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Scientific Reports
Additional Journal Information:
Journal Volume: 6; Journal ID: ISSN 2045--2322
Research Org:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States