Structural Complexity and Tuned Thermoelectric Properties of a Polymorph of the Zintl Phase Ca₂CdSb₂ with a Non-centrosymmetric Monoclinic Structure

Abstract

The Zintl phase Ca2CdSb2 was found to be dimorphic. Besides the orthorhombic Ca2CdSb2 (-o), here we report on the synthesis, the structural characterization, and the thermoelectric transport properties of its monoclinic form, Ca2CdSb2 (-m), and its Lu-doped variant Ca2–x Lu x CdSb2 (x ≈ 0.02). The monoclinic structure exhibits complex structural characteristics and constitutes a new structure type with the non-centrosymmetric space group Cm (Z = 30). The electrical resistivity ρ­(T) measured on single crystals of both phases portrays a transition from a semiconductor to a degenerate p-type semiconductor upon doping with Lu and with an attendant change in the Hall carrier concentration n H from 7.15 × 1018 to 2.30 × 1019 cm–3 at 300 K. The Seebeck coefficient S(T) of both phases are comparable and indicate a hole-dominated carrier transport mechanism with magnitudes of 133 and 116 μV/K at 600 K for Ca2CdSb2 (-m) and Ca2–x Lu x CdSb2, respectively. The convoluted atomic bonding with an attendant large unit cell volume of ∼4365 Å3 drives a putative low thermal conductivity in these materials resulting in a power factor PF of 1.63 μW/cm K2 and an estimated thermoelectric figure of merit zT of ∼0.5 for Ca2–x Lu x CdSb2 at 600 K. Differential scanning calorimetry results reveal the stability of these phases up to about 960 K, making them candidates for moderate temperature thermoelectric materials.

Observation of Anomalously High Seebeck Coefficients in the Family of Zintl Phase Semiconductors Ca10RECdSb9 (RE = Rare-Earth Metal)

“…Although the value achieved at 600 K is close to that of Ca 10 SmCdSb 9 , a striking difference between these two phases lies in their electrical transport properties captured in Figure b,d. The degenerate semiconducting state in Ca 10 NdCdSb 9 makes it satisfy one of the important criteria of a good thermoelectric material candidate. − This observation also attests to the fact that the structural/elemental configuration in the present case has resulted in near-optimal tuning conditions. This line of thought is further supported by the Hall coefficient measurements, which yield the following Hall concentrations n H at 300 K: 1.23 × 10 16 cm –3 (Ca 10 LuCdSb 9 ), 4.04 × 10 17 cm –3 (Ca 10 CeCdSb 9 ), 1.45 × 10 18 cm –3 (Ca 10 SmCdSb 9 ), and 4.68 × 10 20 cm –3 (Ca 10 NdCdSb 9 ).…”

Exploiting the fraternal twin nature of thermoelectrics and topological insulators in Zintl phases as a tool for engineering new efficient thermoelectric generators

“…[36][37][38] This interest led to the rapid expansion of Zintl phases into the realm of transition and rare-earth elements; when such elements of open core-shells are incorporated, the attendant structural diversity and physical properties can reach new heights. 20,30,[39][40][41][42][43][44] The actualization of closed-shell electronic configuration in these materials brings about their characteristic features as semiconductors or insulators as well as the diamagnetic or temperature independent paramagnetic behavior. 26 The Zintl phases combine the features of narrow band gap, intricate atomic bonding and amenability to a high degree of disorder and present a fertile playground to explore the various electronic and transport properties of materials of interests ranging from the topological phases of matter 45,46 to applications in thermoelectrics 47 and photovoltaics.…”

“…36–38 This interest led to the rapid expansion of Zintl phases into the realm of transition and rare-earth elements; when such elements of open core-shells are incorporated, the attendant structural diversity and physical properties can reach new heights. 20,30,39–44…”

Deciphering Defects in Yb2–xEuxCdSb2 and Their Impact on Thermoelectric Properties

“…Compounds with AM 2 Pn 2 and A 2 MPn 2 stoichiometry illustrate an example of the benefits and drawbacks of this generalized understanding of layered structures . Recent studies have emphasized the importance of cation bonding, identity, disorder, and defects in Yb 2 – x Ca x CdSb 2 , Ca 2 – x Eu x CdSb 2 , Yb 2– x Eu x CdSb 2 , Ca 2– x Lu x CdSb 2 , A 2 CdP 2 (A = Ba, Sr), and AZn 2 Sb 2 (A = Ca, Sr, Yb, Eu) in determining electronic properties and structure type. ,− In addition, studies on Mg 3 Sb 2 have revealed that the Zintl formalism does not adequately describe interactions between cations and [Mg 2 Sb 2 ] 2– slabs in AMg 2 Pn 2 compounds, which cannot be considered layered structures due to isotropic bonding networks, even though they adopt the CaAl 2 Si 2 structure type. , These examples highlight the need for better understanding of chemical bonding, defects, and local structure in each system to enable optimization of thermoelectric properties.…”