Structural Characterization Of Bi2te3 And Sb2te3 As A Function Of Temperature Using Neutron Powder Diffraction And Extended X-Ray Absorption Fine Structure Techniques

The structure of Bi2Te3 (Seebeck coefficient Standard Reference Material (SRM (TM) 3451)) and the related phase Sb2Te3 have been characterized as a function of temperature using the neutron powder diffraction (NPD) and the extended X-ray absorption fine structure (EXAFS) techniques. The neutron structural studies were carried out from 20K to 300K for Bi2Te3 and from 10K to 298K for Sb2Te3. The EXAFS technique for studying the local structure of the two compounds was conducted from 19K to 298 K. Bi2Te3 and Sb2Te3 are isostructural, with a space group of R (3) over barm. The structure consists of repeated quintuple layers of atoms, Te2-M-Te1-M-Te2 (where M = Bi or Sb) stacking along the c-axis of the unit cell. EXAFS was used to examine the bond distances and static and thermal disorders for the first three shells of Bi2Te3 and Sb2Te3 as a function of temperature. The temperature dependencies of thermal disorders were analyzed using the Debye and Einstein models for lattice vibrations. The Debye and Einstein temperatures for the first two shells of Bi2Te3 are similar to those of Sb2Te3 within the uncertainty in the data. However, the Debye and Einstein temperatures for the third shell of Bi-Bi are significantly lower than those of the third shell of Sb-Sb. The Einstein temperature for the third shell is consistent with a soft phonon mode in both Bi2Te3 and Sb2Te3. The lower Einstein temperature of Bi-Bi relative to Sb-Sb is consistent with the lower value of thermal conductivity of Bi2Te3 relative to Sb2Te3.