Mg 3 Sb 2 –Mg 3 Bi 2 alloys have been heavily studied as a competitive alternative to the state-of-the-art n-type Bi 2 (Te,Se) 3 thermoelectric alloys. Using Mg 3 As 2 alloying, we examine another dimension of exploration in Mg 3 Sb 2 –Mg 3 Bi 2 alloys and the possibility of further improvement of thermoelectric performance was investigated. While the crystal structure of pure Mg 3 As 2 is different from Mg 3 Sb 2 and Mg 3 Bi 2 , at least 15% arsenic solubility on the anion site (Mg 3 ((Sb 0.5 Bi 0.5 ) 1−x As x ) 2 : x = 0.15) was confirmed. Density functional theory calculations showed the possibility of band convergence by alloying Mg 3 Sb 2 –Mg 3 Bi 2 with Mg 3 As 2 . Because of only a small detrimental effect on the charge carrier mobility compared to cation site substitution, the As 5% alloyed sample showed zT = 0.6–1.0 from 350 K to 600 K. This study shows that there is an even larger composition space to examine for the optimization of material properties by considering arsenic introduction into the Mg 3 Sb 2 –Mg 3 Bi 2 system.
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Limits of Cation Solubility in AMg2Sb2 (A = Mg, Ca, Sr, Ba) Alloys
A M 2 X 2 compounds that crystallize in the CaAl 2 Si 2 structure type have emerged as a promising class of n- and p-type thermoelectric materials. Alloying on the cation (A) site is a frequently used approach to optimize the thermoelectric transport properties of A M 2 X 2 compounds, and complete solid solubility has been reported for many combinations of cations. In the present study, we investigate the phase stability of the AMg 2 Sb 2 system with mixed occupancy of Mg, Ca, Sr, or Ba on the cation (A) site. We show that the small ionic radius of Mg 2 + leads to limited solubility when alloyed with larger cations such as Sr or Ba. Phase separation observed in such cases indicates a eutectic-like phase diagram. By combining these results with prior alloying studies, we establish an upper limit for cation radius mismatch in A M 2 X 2 alloys to provide general guidance for future alloying and doping studies.
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- Award ID(s):
- 1709158
- NSF-PAR ID:
- 10110348
- Date Published:
- Journal Name:
- Materials
- Volume:
- 12
- Issue:
- 4
- ISSN:
- 1996-1944
- Page Range / eLocation ID:
- 586
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Compounds that crystallize in the layered CaAl 2 Si 2 structural pattern have rapidly emerged as an exciting class of thermoelectric materials with attractive n- and p-type properties. More than 100 AM 2 X 2 compounds that form this structure type – characterized by anionic M 2 X 2 slabs sandwiched between layers of octahedrally coordinated A cations – provide numerous potential paths to chemically tune every aspect of thermoelectric transport. This review highlights the chemical diversity of this structure type, discusses the rules governing its formation and stability relative to competing AM 2 X 2 structures ( e.g. , ThCr 2 Si 2 and BaCu 2 S 2 ), and attempts to bring some of the most recently discovered compounds into the spotlight. The discussion of thermoelectric transport properties in AM 2 X 2 compounds focuses primarily on the intrinsic parameters that determine the potential for a high figure of merit: the band gap, effective mass, degeneracy, carrier relaxation time, and lattice thermal conductivity. We also discuss routes that have been used to successfully control the carrier concentration, including controlling the cation vacancy concentration, doping, and isoelectronic alloying (approaches that are highly interdependent). Finally, we discuss recent progress made towards n-type doping in this system, highlight opportunities for further improvements, as well as open questions that still remain.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/ma12040586
