New n-Type Zintl Phases for Thermoelectrics: Discovery, Structural Characterization, and Band Engineering of the Compounds A₂CdP₂ (A = Sr, Ba, Eu)

Abstract

Zintl phases, owing to their complex crystal structures and intricate chemical bonding, have recently been recognized as promising candidates for thermoelectric (TE) applications. Band engineering, including band convergence, has been shown to be an effective way to enhance the thermoelectric performance of such materials. In this work, a series of emerging TE materials, the isostructural Zintl phases with the general formula A2CdP2 (A = Sr, Ba, Eu), are presented for the first time. Their structures, established from single-crystal X-ray diffraction methods, show them to crystallize with the orthorhombic Yb2CdSb2 structure type, with first-principles calculations on phase stability confirming that Ba2CdP2 and Sr2CdP2 are thermodynamically stable. Computationally, it was found that both Ba2CdP2 and Sr2CdP2 have the potential to exhibit high n-type TE performance (0.6 and 0.7 relative to the n-type PbTe, a reference TE material). To optimize the TE performance, band engineering strategies, including isovalent substitution and cation mutations, were investigated. From the band engineering of Ba2CdP2 via isovalent substitution of Sr on a single Ba site, leading to the quaternary composition SrBaCdP2, it can be suggested that increasing the conduction band valley degeneracy is an effective way to improve the n-type TE performance by 3-fold. Moreover, first-principles defect calculations reveal that both Ba2CdP2 and SrBaCdP2 are n-type dopable, adding these compounds to a small list of rare n-type dopable Zintl phases. The band engineering strategies used in this work are equally applicable to other TE materials, either for optimization of existing TE materials or designing new materials with desired properties.

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.…”

“…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. These parent compounds feature a layered structure comprised of CdP 4 tetrahedra, forming [CdP 2 ] 4− layers (Fig.…”

“…There are an equal number of sites 1 and 2 available in the structure. Since Ba 2(1− x ) Sr 2 x CdP 2 are iso-structural alloys with no observed changes in the layering and coordination, the reader is referred to our previous work 30 for the overall structural description.…”

“…The computational framework builds upon well-established alloy modeling techniques, including special quasi-random structures (SQS) 27 and effective band structure (EBS) 28 to quantify the effect of alloying on the transport properties. To assess the dopability of alloys, we extend a standard approach for computing point defect energetics 29,30 in ordered structures, akin to our prior work on defect modeling of complex disordered structures. 31 Through the application of this computational framework, we identify Ba 2(1− x ) Sr 2 x CdP 2 alloy compositions that minimize alloy scattering of electrons, promote conduction band convergence, lower lattice thermal conductivity and are n-type dopable.…”

Computational Design of Thermoelectric Alloys Through Optimization of Transport and Dopability

“…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 3, DG m is negative for all values of x suggesting the formation of a complete solid solution. More interestingly, our calculations reveal site selectivity of the disorder depending on whether the alloy composition is Ba-rich (x < 0.5) or Sr-rich (x > 0.5).…”