Solid Solution Yb_2–xCa_xCdSb₂: Structure, Thermoelectric Properties, and Quality Factor

Abstract

Solid solutions of Yb 2−x A x CdSb 2 (A = Ca, Sr, Eu; x ≤ 1) are of interest for their promising thermoelectric (TE) properties. Of these solid solutions, Yb 2−x Ca x CdSb 2 has end members with different crystal structures. Yb 2 CdSb 2 crystallizes in the polar space group Cmc2 1 , whereas Ca 2 CdSb 2 crystallizes in the centrosymmetric space group Pnma. Other solid solutions, Yb 2−x A x CdSb 2 (A = Sr, Eu), crystallize in the polar space group for x ≤ 1, and compositions with x ≥ 1 have not been reported. Both structure types are composed of corner-sharing CdSb 4 tetrahedra condensed into sheets that differ by the stacking of the layers. Single crystals of the solid solution Yb 2−x Ca x CdSb 2 (x = 0−1) were studied to elucidate the structural transition between the Yb 2 CdSb 2 and Ca 2 CdSb 2 structure types. For x ≤ 1, the structures remain in the polar space group Cmc2 1 . As the Ca content is increased, a positional disorder arises in the intralayer cation sites (Yb2/Ca2) and the Cd site, resulting in inversion of the CdSb 4 tetrahedral chain. This phenomenon could be indicative of an intergrowth of the opposing space group. The TE properties of polycrystalline samples of Yb 2−x Ca x CdSb 2 (x ≤ 1) were measured from 300 to 525 K. The lattice thermal conductivity is extremely low (0.3−0.4 W/m•K) and the Seebeck coefficients are high (100−180 μV/K) across the temperature range. First-principles calculations show a minimum in the thermal conductivity for the x = 0.3 composition, in good agreement with experimental data. The low thermal conductivity stems from the acoustic branches being confined to low frequencies and a large number of phonon scattering channels provided by the localized optical branches. The TE quality factor of the Yb 1.7 A 0.3 CdSb 2 (A = Ca, Sr, Eu) series has been calculated and predicts that the A = Ca and Sr solid solutions may not improve with carrier concentration optimization but that the Eu series is worthy of additional modifications. Overall, the x = 0.3 compositions provide the highest zT because they provide the best electronic properties with the lowest thermal conductivity.

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

“…Such [CdSb 4 ] tetrahedra are highly distorted as evidenced from the Cd–Sb bond distances, which range from 2.72 to 3.13 Å (Table S3). These values are comparable to the sum of respective covalent radii and to those in the recently reported Zintl antimonides featuring [CdSb 4 ] tetrahedra. ,,− The upper values for the Cd–Sb interatomic distances between 3.05 and 3.13 Å are noticeably elongated but cannot be neglected in the context of chemical bonding.…”

Computational design of thermoelectric alloys through optimization of transport and dopability

“…There are 5 unique atomic positions, all of which are 4 a Wyckoff sites (site symmetry m ). The structures belong to a family of compounds, 9,30,32,33 which crystallize in the well-established Yb 2 CdSb 2 structure type. 34 This collection of compounds have been the subject of investigation in the past, including a detailed study 30 conducted by our team on the parent compounds (Sr 2 CdP 2 and Ba 2 CdP 2 ) of the Ba–Sr–Cd–P quaternaries that we presently discuss.…”

“…Designing thermoelectric (TE) alloys is particularly challenging because it is a multi-property optimization problem, where the properties that contribute to high TE performance are interdependent. Alloying has been employed to optimize a variety of properties that directly or indirectly affect the TE performance: (1) enhance phonon scattering to reduce lattice thermal conductivity, 8,9 (2) promote band convergence to improve power factor, 8,10–12 (3) tune band gap to suppress bipolar conduction, 8,10,11 and (4) optimize dopability. 13 Fig.…”

Synthesis-enabled exploration of chiral and polar multivalent quaternary sulfides

“…Lowering of the reaction temperature to stabilize complex metastable or stable compounds is a common goal for solid state synthesis, [2][3][4][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] such as formation of metastable forms of RE oxychlorides via low-temperature pyrohydrolysis of (NH4)3REC16. 49 The mixing of the rare-earth and transition metals in the silicide precursors is somewhat akin to eutectic fluxes used in the Latturner group to make metal-rich intermetallics, [29][30][31][32][33][34] as well as flux syntheses of complex intermetallics performed by other groups.…”

“…49 The mixing of the rare-earth and transition metals in the silicide precursors is somewhat akin to eutectic fluxes used in the Latturner group to make metal-rich intermetallics, [29][30][31][32][33][34] as well as flux syntheses of complex intermetallics performed by other groups. [2][3][4][35][36][37][38][39][40][41][42][43][44][45][46][47][48] In-situ powder X-ray diffraction study was used to elucidate the mechanism of formation of such quaternaries. Detailed structural characterization by single crystal X-ray diffraction and 29 Si and 45 Sc solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies are reported.…”