Defect Chemistry and Doping of Lead Phosphate Oxo Apatite Pb₁₀(PO₄)₆O

Lead phosphate oxo apatite Pb10(PO4)6O is claimed to host room-temperature superconductivity when doped with copper. However, unsuccessful attempts to reproduce this claim have raised many questions about the composition, off-stoichiometry, and copper doping itself, which are related to native defect chemistry. We perform first-principles defect calculations to provide much needed insights into the defect chemistry and doping of Pb10(PO4)6O. We find that Fermi energy pinning in the midgap region occurs due to Pb and O vacancies. Our calculations also suggest the plausible existence of closely related off-stoichiometric phase(s); we predict one such phase. We predict only moderate levels of Cu doping, which calls into question the experimental claim of 10% incorporation on the Pb sites. Cu substitution on the Pb(1) and Pb(2) Wyckoff sites is possible, resulting in Cu d9 and d10 electronic configurations, respectively. We predict unintentional S incorporation is highly possible. Our findings emphasize the need for careful characterization of the parent composition and the identification of synthesis conditions that will maximize (minimize) intentional (unintentional) doping

Synthesis, Structural Characterization, and Physical Properties of the Type‑I Clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈

The first arsenide clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈ have been synthesized in high yields via a two-step route. These compounds adopt the type-I structure and exhibit structural characteristics different from the recently reported antimonide clathrates Cs₈Zn₁₈Sb₂₈ and Cs₈Cd₁₈Sb₂₈. In arsenide clathrates, Zn (or Cd) and As atoms are statistically mixed at the three framework sites: 6<i>c</i>, 16<i>i</i>, and 24<i>k</i>; the alkali metals reside inside the cages at the 2<i>a</i> and 6<i>d</i> sites, with the 2<i>a</i> site being only partially filled. Single-crystal X-ray diffraction studies confirm that the Cd atoms preferably occupy the 6<i>c</i> and 24<i>k</i> sites over the 16<i>i</i> site, with more than 80% of Cd found at the former two positions. A unique structural feature is a framework disorder coupled with the partial occupancy of the cage’s 2<i>a</i> site. Optical absorption measurements and electronic property measurements reveal a semimetallic-like behavior for Cs₈Cd₁₈As₂₈ and semiconductor-like behavior for <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = Rb, Cs)

Effect of Isovalent Substitution on the Thermoelectric Properties of the Cu₂ZnGeSe_4–<i>x</i>S_<i>x</i> Series of Solid Solutions

Knowledge of structure–property relationships is a key feature of materials design. The control of thermal transport has proven to be crucial for the optimization of thermoelectric materials. We report the synthesis, chemical characterization, thermoelectric transport properties, and thermal transport calculations of the complete solid solution series Cu₂ZnGeSe_4–<i>x</i>S_<i>x</i> (<i>x</i> = 0–4). Throughout the substitution series a continuous Vegard-like behavior of the lattice parameters, bond distances, optical band gap energies, and sound velocities are found, which enables the tuning of these properties adjusting the initial composition. Refinements of the special chalcogen positions revealed a change in bonding angles, resulting in crystallographic strain possibly affecting transport properties. Thermal transport measurements showed a reduction in the room-temperature thermal conductivity of 42% triggered by the introduced disorder. Thermal transport calculations of mass and strain contrast revealed that 34% of the reduction in thermal conductivity is due to the mass contrast only and 8% is due to crystallographic strain

A Chemical Understanding of the Band Convergence in Thermoelectric CoSb₃ Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions

N-Type skutterudites, such as Yb_<i>x</i>Co₄Sb₁₂, have recently been shown to exhibit high valley degeneracy with possible band convergence, explaining the excellent thermoelectric efficiency of these materials. Using a combined theoretical and experimental approach involving temperature-dependent synchrotron diffraction, molecular orbital diagrams, and computational studies, the chemical nature of critical features in the band structure is highlighted. We identify how n-type doping on the filler site induces structural changes that are observed in both the diffraction data and computational results. Additionally, we show how chemical n-type doping slightly alters the electronic band structure, moving the high-valley degeneracy secondary conduction band closer to the primary conduction band and thus inducing band convergence

Synthesis, Structural Characterization, and Physical Properties of the Type‑I Clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈

The first arsenide clathrates <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = K, Rb, Cs) and Cs₈Cd₁₈As₂₈ have been synthesized in high yields via a two-step route. These compounds adopt the type-I structure and exhibit structural characteristics different from the recently reported antimonide clathrates Cs₈Zn₁₈Sb₂₈ and Cs₈Cd₁₈Sb₂₈. In arsenide clathrates, Zn (or Cd) and As atoms are statistically mixed at the three framework sites: 6<i>c</i>, 16<i>i</i>, and 24<i>k</i>; the alkali metals reside inside the cages at the 2<i>a</i> and 6<i>d</i> sites, with the 2<i>a</i> site being only partially filled. Single-crystal X-ray diffraction studies confirm that the Cd atoms preferably occupy the 6<i>c</i> and 24<i>k</i> sites over the 16<i>i</i> site, with more than 80% of Cd found at the former two positions. A unique structural feature is a framework disorder coupled with the partial occupancy of the cage’s 2<i>a</i> site. Optical absorption measurements and electronic property measurements reveal a semimetallic-like behavior for Cs₈Cd₁₈As₂₈ and semiconductor-like behavior for <i>A</i>₈Zn₁₈As₂₈ (<i>A</i> = Rb, Cs)

A Chemical Understanding of the Band Convergence in Thermoelectric CoSb₃ Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions

N-Type skutterudites, such as Yb_<i>x</i>Co₄Sb₁₂, have recently been shown to exhibit high valley degeneracy with possible band convergence, explaining the excellent thermoelectric efficiency of these materials. Using a combined theoretical and experimental approach involving temperature-dependent synchrotron diffraction, molecular orbital diagrams, and computational studies, the chemical nature of critical features in the band structure is highlighted. We identify how n-type doping on the filler site induces structural changes that are observed in both the diffraction data and computational results. Additionally, we show how chemical n-type doping slightly alters the electronic band structure, moving the high-valley degeneracy secondary conduction band closer to the primary conduction band and thus inducing band convergence

Les droits de l'homme et la Convention du 28 juillet 1951 relative au Statut des réfugiés.

Controlling extrinsic defects to tune the carrier concentration of electrons or holes is a crucial point with regard to the engineering application of thermoelectric semiconductors. To understand the defect-controlled electronic structure in thermoelectric materials, we apply density functional theory (DFT) to investigate the defect chemistry of dopants M (M = O, S, Se, or Te) in CoSb₃. DFT predicts that the breakage of Sb₄ rings induced by these dopants produces the unexpected (n- or p-type) conductivity behavior in CoSb₃. For example, energetically dominant O interstitials (O_i) chemically break Sb₄ rings and form O–4Sb five-membered rings, leading to the charge neutral behavior of O_i. While S interstitials (S_i) collapse Te–3Sb four-membered rings within Te doped CoSb₃ leading to p-type conduction behavior, Se substitution on Sb (Se_Sb) breaks the Se–Te–2Sb four-membered ring, resulting in a charge neutral behavior of the Se_Sb+Te_Sb complex defect. Furthermore, the solubility limits of M dopants (M = S, Se, or Te) are also calculated to provide essential information about single-phase material design. This study provides new insight into understanding the complicated chemical structure in doped CoSb₃, which is beneficial for devising effective doping strategies for the development of high-performance bulk thermoelectric materials

Brittle Failure Mechanism in Thermoelectric Skutterudite CoSb₃

Skutterudites based on CoSb₃ have high thermoelectric efficiency, but the low fracture strength is a serious consideration for commercial applications. To understand the origin of the brittleness in CoSb₃, we examine the response along various shear and tensile deformations using density functional theory. We find that the Co–Sb bond dominates the ideal strength. Among all the shear and tensile deformation paths, shearing along the (001)/⟨100⟩ slip system has the lowest ideal strength, indicating it is the most likely slip system to be activated under pressure. We also find that, because the Sb–Sb covalent bond is softer than the Co–Sb bond, the Sb-rings are less rigid than the Co–Sb frameworks, which leads to the Sb-rings softening before the Co–Sb frameworks. Further deformation leads to deconstruction of Sb-rings and collapse of Co–Sb frameworks, resulting in structural failure. Moreover, we find that filling of the CoSb₃ void spaces with such typical fillers as Na, Ba, or Yb has little effect on the ideal strength and failure mode, which can be understood because they have little effect on the Sb-rings

<i>n</i>‑Type Bi₂Te_3–<i>x</i>Se_<i>x</i> Nanoplates with Enhanced Thermoelectric Efficiency Driven by Wide-Frequency Phonon Scatterings and Synergistic Carrier Scatterings

Driven by the prospective applications of thermoelectric materials, massive efforts have been dedicated to enhancing the conversion efficiency. The latter is governed by the figure of merit (<i>ZT</i>), which is proportional to the power factor (<i>S</i>²σ) and inversely proportional to the thermal conductivity (κ). Here, we demonstrate the synthesis of high-quality ternary Bi₂Te_3–<i>x</i>Se_<i>x</i> nanoplates using a microwave-assisted surfactant-free solvothermal method. The obtained <i>n</i>-type Bi₂Te_2.7Se_0.3 nanostructures exhibit a high <i>ZT</i> of 1.23 at 480 K measured from the corresponding sintered pellets, in which a remarkably low κ and a shift of peak <i>S</i>²σ to high temperature are observed. By detailed electron microscopy investigations, coupled with theoretical analysis on phonon transports, we propose that the achieved κ reduction is attributed to the strong wide-frequency phonon scatterings. The shifting of peak <i>S</i>²σ to high temperature is due to the weakened temperature dependent transport properties governed by the synergistic carrier scatterings and the suppressed bipolar effects by enlarging the band gap

Grain Boundaries Softening Thermoelectric Oxide BiCuSeO

Engineering grain boundaries (GBs) are effective in tuning the thermoelectric (TE) properties of TE materials, but the role of GB on mechanical properties, which is important for their commercial applications, remains unexplored. In this paper, we apply ab initio method to examine the ideal shear strength and failure mechanism of GBs in TE oxide BiCuSeO. We find that the ideal shear strength of the GB is much lower than that of the ideal single crystal. The atomic rearrangements accommodating the lattice and neighbor structure mismatch between different grains leads to the much weaker GB stiffness compared with grains. Failure of the GBs arises from either the distortion of the Cu–Se layers or the relative slip between Bi–O and Cu–Se layers. This work is crucial to illustrate the deformation of GBs, laying the basis for the development and design of mechanically robust polycrystalline TE materials