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Open access publications by faculty, postdocs, and graduate students in the Department of Chemistry and Biochemistry.
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Browsing Open Access Publications by Subject "antimonides"
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Item Enhanced thermoelectric performance in the zintl antimonides (Ca,RE)9Cd4Sb9 (RE = rare-earth metal). Synergy between increased structural complexity and drive towards optimized chemical bonding(Materials Today Advances, 2022-10-27) Ogunbunmi, M. O.; Baranets, S.; Bobev, S.The interplay of structural complexity, high carrier mobility, and high density of states effective mass can play a pivotal role in achieving enhanced thermoelectric (TE) performance in candidate materials. In this regard, the Zintl phases represent a class of compounds that are susceptible to harboring these key ingredients. This, in addition to their amenability to various forms of chemical substitution mechanisms makes them a good choice of systems to explore systematically. Here we demonstrate the role-play of these ingredients in achieving excellent TE properties on single-crystals of Ca9–xREyCd4+δSb9 (RE = Ce, Pr, Nd, Sm, Gd, Tb; x ≈ 0.5–0.8, y ≈ 0.5–0.7, δ ≈ 0.25). These phases represent a new addition to the “9–4–9” family with intricate chemical bonding arising from both a purposely introduced disorder on Ca sites and the inherent presence of interstitial Cd positions. Many of the newly synthesized and characterized phases show moderate values of the Seebeck coefficient, lying in the range of 71–116 μV/K at 600 K and evolving as degenerate semiconductors. Simultaneously, the electrical resistivity ρ(T) of the measured samples can be as low as 0.18 mΩ cm at this temperature. As a result, the observed TE power factors in the Ce-, Nd-, and Sm-samples are in the range 6–46 μW/cm.K2. For Ca9–xCexCd4+δSb9, the estimated thermoelectric quality factor B > 0.4 at 300 K, which corresponds to a figure of merit zT ≥ 1. Calculations based on the single parabolic band (SPB) model show that the optimum region for thermoelectric performance requires carrier concentration n = 2–6 × 1019 cm−3 thus providing for an open window to further tune the TE properties.Item Synthesis and Crystal Structure of the Zintl Phases Na2CaCdSb2, Na2SrCdSb2 and Na2EuCdSb2(Inorganics, 2022-12-18) Saparov, Bayram; Bobev, SvilenThis work details the synthesis and the crystal structures of the quaternary Zintl phases Na2CaCdSb2, Na2SrCdSb2 and Na2EuCdSb2. They are isostructural and their noncentrosymmetric structure is with the space group Pmc21 (Pearson code oP12). All structural work is carried out via single-crystal X-ray diffraction methods. The structure features [CdSb2]4– layers of corner-shared CdSb4 tetrahedra, which are stacked along the b-crystallographic axis and are separated by cations. The results from the structure refinements suggest that in addition to full cation ordering, which is typical for this structure, there also exists a possibility for an accommodation of a small degree of cation disorder.Item Synthesis and Crystal Structure of the Zintl Phases NaSrSb, NaBaSb and NaEuSb(Materials, 2023-02-08) Wang, Yi; Bobev, SvilenThis work details the synthesis and the crystal structures of the ternary compounds NaSrSb, NaBaSb and NaEuSb. They are isostructural and adopt the hexagonal ZrNiAl-type structure (space group P6¯2m; Pearson code hP9). The structure determination in all three cases was performed using single-crystal X-ray diffraction methods. The structure features isolated Sb3– anions arranged in layers stacked along the crystallographic c-axis. In the interstices, alkali and alkaline-earth metal cations are found in tetrahedral and square pyramidal coordination environments, respectively. The formal partitioning of the valence electrons adheres to the valence rules, i.e., Na+Sr2+Sb3–, Na+Ba2+Sb3– and Na+Eu2+Sb3– can be considered as Zintl phases with intrinsic semiconductor behavior. Electronic band structure calculations conducted for NaBaSb are consistent with this notion and show a direct gap of approx. 0.9 eV. Additionally, the calculations hint at possible inverted Dirac cones, a feature that is reminiscent of topological quantum materials.