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Phase Stability and Defect Behavior in Complex Thermoelectric Zinc-Antimonides


Pomrehn, Gregory Schoelerman (2013) Phase Stability and Defect Behavior in Complex Thermoelectric Zinc-Antimonides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0AWH-4P69.


The Zn-Sb binary phase system has been of interest for many years in the search for efficient and low-cost thermoelectric materials. Of primary interest has been the Zn4Sb3 phase which exhibits a thermoelectric figure of merit, zT, in excess of 1 in an intermediate temperature range. In this study, Zn4Sb3 is shown to be entropically stabilized with respect to decomposition to Zn and ZnSb through the effects of configurational disorder and phonon free energy. Single-phase stability is predicted for a range of compositions and temperatures. Retrograde solubility of Zn is predicted on the two-phase boundary region between Zn4Sb3 and Zn. The complex temperature-dependent solubility can be used to explain the variety of nanoparticle formation observed in the system: formation of ZnSb on the Sb-rich side, Zn on the far Zn-rich side, and nano-void formation due to Zn precipitates being reabsorbed at lower temperatures.

A new binary compound, Zn8Sb7, known only in nanoparticulate form, is also studied using density functional calculations. The free energies of formation, including effects from vibrations and configurational disorder, are calculated to compare with the relevant phases ZnSb, Zn, and Zn4Sb3, yielding insight into the phase stability of Zn8Sb7. Band structure calculations predict Zn8Sb7, much like ZnSb and Zn4Sb3, to be an intermetallic semiconductor with similar thermoelectric properties. If sufficient entropy or surface energy exists to stabilize the bulk material, it would be stable in a limited temperature window at high temperature.

In the AZn2Sb2 series of materials—A = Ca, Sr, Yb, and Eu—I show that a large concentration of thermodynamically stable cation vacancies leads to high extrinsic carrier concentrations. The stable defect level depends on the choice of A, and is consistent with experimentally observed carrier concentrations in these materials. These results demonstrate that point defects are the primary mechanism by which the covalency of the cation bond can influence carrier concentration in nominally valence-precise AZn2Sb2compounds. This mechanism may be generally applicable to other Zintl phases, perhaps explaining similar trends seen in A14MSb11, A2MSb2 (A=2+ cation, M = 2+ or 3+ metal),and similar materials.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Thermoelectrics; density functional theory; thermodynamics; computational materials science; cluster expansion
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • van de Walle, Axel (co-advisor)
  • Snyder, G. Jeffrey (co-advisor)
Thesis Committee:
  • Fultz, Brent T. (chair)
  • van de Walle, Axel
  • Snyder, G. Jeffrey
  • Johnson, William Lewis
  • Goddard, William A., III
Defense Date:7 January 2013
Record Number:CaltechTHESIS:01252013-135311552
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Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7439
Deposited By: Gregory Pomrehn
Deposited On:14 Feb 2013 22:03
Last Modified:03 Oct 2019 23:58

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