p-Type to n-Type Conversion through the “Bypass” Phase Transition in the Zintl-Phase Thermoelectric Materials

Abstract

Six rare-earth (RE) metal-doped n-type Zintl-phase thermoelectric (TE) compounds in the Ca 5−x−y Yb x RE y Al 2 Sb 6 (RE = Pr, Nd, and Sm; 1.26 ≤ x ≤ 3.03; 0.15 ≤ y ≤ 0.45) system have been prepared using arc melting followed by the post-heat treatment, and the isotypic and homotypic crystal structures were carefully determined by the powder and single-crystal analyses. Six title compounds adopted either the Ca 5 Al 2 Bi 6 -type or Ca 5 Ga 2 As 6 -type phase in the orthorhombic Pbam space group (Z = 2, Pearson code oP26) with seven crystallographically independent atomic sites. Interestingly, the Yb-rich compounds originally crystallized in the Ca 5 Al 2 Bi 6 -type phase and maintained their structure type even after the post-heat treatment. On the other hand, the Ca-rich compounds with particular compositions adopted the Ca 5 Al 2 Bi 6 -type phase first and then underwent phase transition to the Ca 5 Ga 2 As 6 -type phase after the post-heat treatment at the high temperature. Moreover, this single-crystal to single-crystal phase transition also brought the p-type to n-type conversion of electrical transport property for the two Ca 5 Ga 2 As 6 -type title compoundsCa 3.46 Yb 1.35 Pr 0.19 Al 2 Sb 6 and Ca 3.30 Yb 1.50 Sm 0.20 Al 2 Sb 6 according to Seebeck coefficient measurements. As far as we understand, this study is the first example of producing novel n-type Zintl TE compounds by the "bypass" method through the p-type to n-type conversion of identical Zintl compounds in the A 5 M 2 Pn 6 (A = Ca, Sr, Eu, and Yb; M = Al, Ga, In; Pn = As, Sb, and Bi) system. Theoretical calculations conducted for the three hypothetical models rationalized the specific site preference of RE and the overall electronic structures. Hall effect measurements proved the n-type carrier, and the carrier concentration and carrier mobility of this Ca 5 Ga 2 As 6 -type Ca 3.46 Yb 1.35 Pr 0.19 Al 2 Sb 6 were also evaluated.

From Three-Dimensional Clathrates to Two-Dimensional Zintl Phases AMSb2 (A = Rb, Cs; M = Ga, In) Composed of Pentagonal M–Sb Rings

“…Holes as major charge carriers are typical for many Ga–Sb framework compounds including clathrates Rb 8 Ga 27 Sb 19 (150 μV K –1 at 300 K) and Cs 8 Ga 27 Sb 19 (145 μV K –1 K at 300 K) . Rare exceptions of this trend are an n-type Zintl arsenide Eu 3 InAs 3 (−300 μV K –1 at 300 K) and rare-earth-doped 5–2–6 antimonide Ca 3.46 Yb 1.35 Pr 0.19 Al 2 Sb 6 (−115 μV K –1 at 300 K) . Switching of the majority carriers in the same compound is less common for semiconducting antimonides but was reported for Cs 8 M 18 Sb 28 (M = Zn, Cd) clathrates and other antimonides. ,, …”

“…16 Rare exceptions of this trend are an n-type Zintl arsenide Eu 3 InAs 3 (−300 μV K −1 at 300 K) 66 and rare-earth-doped 5− 2−6 antimonide Ca 3.46 Yb 1.35 Pr 0.19 Al 2 Sb 6 (−115 μV K −1 at 300 K). 67 Switching of the majority carriers in the same compound is less common for semiconducting antimonides but was reported for Cs 8 M 18 Sb 28 (M = Zn, Cd) clathrates 68 and other antimonides. 41,59,60 CsInSb 2 exhibits typical p-type semiconducting behavior, as shown by the positive Seebeck coefficient with a maximum value of 185 μV K −1 at 300 K (Figure 5F).…”

Thermoelectric Properties of p‐ and n‐type Eu5Sn2As6

“…Of the various compounds of the Ca 5 Ga 2 As 6 structure type, Ca 5 Al 2 Sb 6 has been one of the most studied to date for thermoelectric properties [9,10] . In the Ca 5 Al 2 Sb 6 phase the A site has been substituted with Na and the M site has been substituted with Zn and Mn [9] .…”

“…Of the various compounds of the Ca 5 Ga 2 As 6 structure type, Ca 5 Al 2 Sb 6 has been one of the most studied to date for thermoelectric properties. [9,10] In the Ca 5 Al 2 Sb 6 phase the A site has been substituted with Na and the M site has been substituted with Zn and Mn. [9] In all three studies, the zT of the material was improved from the > 0.1 zT of the parent compound to near 0.6 at ~900 K. Additionally, all of the Ca 5 Al 2 Sb 6 site substitutions have led to p-type Zintl phases.…”

“…[13] The altering of Zintl phases from p-to n-type semiconductors is limited by the native defects of a material having low formation energy; this type of acceptor defect, or "electron killer", can make it difficult or impossible to dope a material n-type. [13] Although Zintl phases are difficult to substitute to be n-type semiconductors, there are a few promising Zintl phases that are inherently n-type or can be further substituted n-type: Ca 5 Al 2 Sb 6 , [10] KGaSb 4 , [14,15] KAlSb 4 , [16,17] Zr 3 Ni 3 Sb 4 , [18] Ca 11 Sb 10 . [19] Eu 5 Sn 2 As 6 is of interest as a candidate to become an n-type Zintl phase due to the low temperature ptype to n-type transition of the material.…”