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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

On the Effects of Aliovalent Substitutions in Thermoelectric Zintl Pnictides. Varied Polyanionic Dimensionality and Complex Structural Transformations–The Case of Sr 3 ZnP 3 vs Sr 3 Al x Zn 1– x P 3

The structures and the transport properties of a novel family of Zintl phosphides and arsenides with the formula AE 3 ZnPn 3 and the solid solutions AE 3 Al x Zn 1–x Pn 3 , AE 3 ZnAs y P 1–y (AE = Sr, Eu; Pn = P, As) are reported. Crystals of nine new phases have been obtained via Pb-flux reactions and used for structural work by means of single-crystal X-ray diffraction methods. The derived orthorhombic structure is without a direct analog, and features unusual structural units, where the Zn atoms are in both distorted tetrahedral and trigonal-planar coordination of pnictogens. Electronic structure calculations reveal moderately wide bandgaps for Sr 3 ZnP 3 and Sr 3 ZnAs 3 , on the order of 0.70 and 0.63 eV, respectively. Electrical transport measurements above room temperature indicate relatively high resistivity values above 500 K (ρ ≈ 4.8 Ω cm and above), but some of the samples exhibit very high Seebeck coefficients, as large as 300 μV/K at 560 K for Sr 3 ZnAs 3 . Aliovalent substitutions in AE 3 ZnPn 3 , achieved by the partial replacement of Zn 2+ with Al 3+ cations promote occupational and positional disorder, which causes structural transformation towards the disordered variant of the Sr 5 Al 2 Sb 6 structure type. Such substitutions also change the dimensionality of the polyanionic sub-lattice in the resulting quaternary AE 3 Al x Zn 1–x Pn 3 phases. Furthermore, preliminary transport property data on the latter reveal nine times lower electrical resistivity (ρ 500 ≈ 0.5 Ω cm) together with a significantly enhanced Seebeck coefficient, αmax ≈ 430 μV/K at 560 K.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Tuning the Radius Ratio to Enhance Thermoelectric Properties in the Zintl Compounds AM 2 Sb 2 (A = Ba, Sr; M = Zn, Cd)

Five novel Zintl phase solid solutions in the Ba 1–x Sr x Zn 2–y Cd y Sb 2 (0 ≤ x ≤ 0.13(1); 0 ≤ y ≤ 0.32(2)) system were successfully synthesized by the molten Pb metal-flux method, and the powder X-ray diffraction and single-crystal X-ray diffraction analyses proved that all five title compounds adopted the BaCu 2 S 2 -type phase having the orthorhombic Pnma space group (Z = 4, Pearson code oP20) with five crystallographically independent atomic sites. The previously studied BaCu 2 S 2 -type antimonides demonstrated a limited tolerance for doping in contrast to the CaAl 2 Si 2 -type antimonides. To understand the relatively narrower phase width and limited dopability of the title BaCu 2 S 2 -type phase than the CaAl 2 Si 2 -type phase in the overall Ba 1–x Sr x Zn 2–y Cd y Sb 2 system, the radius ratio of cations and anionic elements r + /r – for two structure types were thoroughly investigated. For the first time, the r + /r – ratio was identified as a critical factor for the phase selectivity: (1) r + /r – > 1 favored the BaCu 2 S 2 -type phase, and (2) r + /r – < 1 favored the CaAl 2 Si 2 -type phase. Further, we also revealed the structural transformation mechanism from the more widely observed CaAl 2 Si 2 -type phase to the title BaCu 2 S 2 -type phase as the relatively larger cationic elements were introduced to the system. A series of DFT calculations using the three hypothetical models indicated that a resonance peak near EF in the density of states curves was descended from the relatively flat band structure at several special symmetry points rationalizing the enhanced Seebeck coefficients of Ba 0.94(1) Sr 0.06 Zn 1.86(3) Cd 0.14 Sb 2 and Ba 0.96(1) Sr 0.04 Zn 1.68(2) Cd 0.32 Sb 2 . Electron localization function analysis rationalized the correlation between the polarity change of anionic Zn/Cd–Sb bonds and the charge carrier mobility on the anionic frameworks. Temperature-dependent thermoelectric properties were studied for the four title compounds, and the results proved that the Sr and Cd doping in the title Ba 1–x Sr x Zn 2–y Cd y Sb 2 system successfully enhanced the ZT values through the increased Seebeck coefficients and the reduced total thermal conductivities.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Synthesis and Structural Study of Substituted Ternary Nitrides for Ammonia Production

Over the past few decades, inorganic nitride materials have grown in importance in part due to their potential as catalysts for the synthesis of NH 3 , a key ingredient in fertilizer and precursor to industrial chemicals. Of particular interest are the ternary (ABN) or higher-order nitrides with high metal-to-nitrogen ratios that show promise in enhancing NH 3 synthesis reaction rates and yields via heterogeneous catalysis or chemical looping. Although metal nitrides are predicted to be numerous, the stability of nitrogen triple bonds found in N 2 , especially in comparison to the metal–nitrogen bonds, has considerably hindered synthetic efforts to produce complex nitride compounds. In this study, we present an exhaustive down-selection process to identify ternary nitrides for a promising chemical looping NH 3 production mechanism. We also report on a facile and efficient two-step synthesis method that can produce well-characterized η-carbide Co 3 Mo 3 N/Fe 3 Mo 3 N or filled β-manganese Ni 2 Mo 3 N ternaries, as well as their associated quaternary, (Co,Fe) 3 Mo 3 N, (Fe,Ni) 2 Mo 3 N, and (Co,Ni) 2 Mo 3 N, solid solutions. To further explore the quaternary space, syntheses of (Co,Ni) 3 Mo 3 N (Ni ≤ 10 mol %) and Co 3 (Mo,W) 3 N (W ≤ 10 mol %) were also investigated. The structures of the nitrides were characterized via X-ray powder diffraction. The morphology and compositions were characterized with scanning electron microscopy. In conclusion, the multitude of chemically unique, but structurally related, nitrides suggests that properties such as nitrogen activity may be tunable, making the materials of great interest for NH 3 synthesis schemes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Accelerating the Discovery of New, Single Phase High Entropy Ceramics via Active Learning

High-entropy ceramics have garnered interest due to their remarkable hardness, compressive strength, thermal stability, and fracture toughness; yet the discovery of new high-entropy ceramics (out of a tremendous number of possible elemental permutations) still largely requires costly, inefficient, trial-and-error experimental and computational approaches. The entropy forming ability (EFA) factor was recently proposed as a computational descriptor that positively correlates with the likelihood that a 5-metal high-entropy carbide (HECs) will form the desired single phase, homogeneous solid solution; however, discovery of new compositions is computationally expensive. If you consider 8 candidate metals, the HEC EFA approach uses 49 optimizations for each of the 56 unique 5-metal carbides, requiring a total of 2744 costly density functional theory calculations. Here, we describe an orders-of-magnitude more efficient active learning (AL) approach for identifying novel HECs. To begin, we compared numerous methods for generating composition-based feature vectors (e.g., magpie and mat2vec), deployed an ensemble of machine learning (ML) models to generate an average and distribution of predictions, and then utilized the distribution as an uncertainty. Here we then deployed an AL approach to extract new training data points where the ensemble of ML models predicted a high EFA value or was uncertain of the prediction. Our approach has the combined benefit of decreasing the amount of training data required to reach acceptable prediction qualities and biases the predictions toward identifying HECs with the desired high EFA values, which are tentatively correlated with the formation of single phase HECs. Using this approach, we increased the number of 5-metal carbides screened from 56 to 15,504, revealing 4 compositions with record-high EFA values that were previously unreported in the literature. Our AL framework is also generalizable and could be modified to rationally predict optimized candidate materials/combinations with a wide range of desired properties (e.g., mechanical stability, thermal conductivity).

36 MATERIALS SCIENCE↗
DOE OSTI2015

Crystal Chemistry of Electrochemically and Chemically Lithiated Layered α I -LiVOPO 4

LiVOPO 4 is an attractive cathode for lithium-ion batteries with a high operating voltage and the potential to achieve the reversible insertion of two lithium ions between VOPO 4 and Li 2 VOPO 4 . Among the three known forms of LiVOPO 4 (α, β, and αI), the α I -LiVOPO 4 has a layered structure that could promote better ionic mobility and reversibility than others. However, a comprehensive study of its lithiated product is not available as αI-LiVOPO 4 is metastable and difficult to prepare by conventional approaches. We present here a facile synthesis of highly crystalline αI-LiVOPO 4 and α I -LiVOPO 4 /rGO nanocomposite by a microwave-assisted solvothermal method and its electrochemical/chemical lithiation. The LiVOPO 4 /rGO cathodes exhibit a high reversible capacity of 225 mAh g –1 , indicating the insertion of more than one lithium into VOPO 4 . Both electrochemical and chemical lithiation imply a solid-solution reaction mechanism on inserting the second lithium into α I -LiVOPO 4 , but a two-phase reaction feature could also occur under certain conditions such as insufficient time for equilibration of Li + diffusion in the structure. The fully lithiated new α I -Li 2 VOPO 4 phase was characterized by combined Rietveld refinement of neutron diffraction and X-ray diffraction data and by bond-valence sum maps. The results suggest that αI-Li 2 VOPO 4 retains the tetragonal P4/nmm symmetry of the parent α I -LiVOPO 4 structure, where the second lithium ions are located in the lithium layers rather than in the VOPO 4 layers

25 ENERGY STORAGE↗
DOE OSTI2015

Synthesis, Structure, and Electrochemical Performance of High Capacity Li 2 Cu 0.5 Ni 0.5 O 2 Cathodes

Orthorhombic Li 2 NiO 2 , Li 2 CuO 2 , and solid solutions thereof have been studied as potential cathode materials for lithium-ion batteries due to their high theoretical capacity and relatively low cost. While neither endmember shows good cycling stability, the intermediate composition, Li 2 Cu 0.5 Ni 0.5 O 2 , yields reasonably high reversible capacities. A new synthetic approach and detailed characterization of this phase and the parent Li 2 CuO 2 are presented. The cycle life of Li 2 Cu 0.5 Ni 0.5 O 2 is shown to depend critically on the voltage window. The formation of Cu 1+ at low voltage and oxygen evolution at high voltage limit the electrochemical reversibility. In situ X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and gas evolution measurements are used to follow the chemical and structural changes that occur as a function of cell voltage.

25 ENERGY STORAGE↗
DOE OSTI2016

Influence of Rotational Distortions on Li + - and Na + -Intercalation in Anti-NASICON Fe 2 (MoO 4 ) 3

Anti-NASICON Fe 2 (MoO 4 ) 3 (P2 1 /c) shows significant structural and electrochemical differences in the intercalation of Li + and Na + ions. To understand the origin of this behavior, we have used a combination of in situ X-ray and high-resolution neutron diffraction, total scattering, electrochemical measurements, density functional theory calculations, and symmetry-mode analysis. We find that for Li + -intercalation, which proceeds via a two-phase monoclinic-to-orthorhombic (Pbcn) phase transition, the host lattice undergoes a concerted rotation of rigid polyhedral subunits driven by strong interactions with the Li + ions, leading to an ordered lithium arrangement. Na + -intercalation, which proceeds via a two-stage solid solution insertion into the monoclinic structure, similarly produces rotations of the lattice polyhedral subunits. Furthermore, using a combination of total neutron scattering data and density functional theory calculations, we find that while these rotational distortions upon Na + -intercalation are fundamentally the same as for Li + -intercalation, they result in a far less coherent final structure, with this difference attributed to the substantial difference between the ionic radii of the two alkali metals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Thermoelectric Performance and Defect Chemistry in n-Type Zintl KGaSb 4

The rise of high-throughput calculations has accelerated the discovery of promising classes of thermoelectric materials. In prior work, we identified the n-type Zintl pnictides as one such material class. To date, however, a lack of detailed defect calculations and chemical intuition has led the community to investigate p-type Zintls almost exclusively. Here, we investigate the synthesis, thermoelectric properties, and defect structure of the complex Zintl KGaSb 4 . We find that KGaSb 4 is successfully doped n-type with Ba and has the potential for p-type doping with Zn. Our calculations reveal the fundamental defect structure in KGaSb 4 that enables n-type and p-type doping. We find that Ba doped KGaSb4 exhibits high electronic mobility (~50 cm 2 V -1 s -1 ) and near minimum lattice thermal conductivity (<0.5 Wm -1 K -1 ) at 400 °C. Samples doped with 1.5% Ba achieve zT > 0.9 at 400 °C, promising for a previously unstudied material. Here, we also briefly investigate the series of alloys between KGaSb 4 and KAlSb 4 , finding that a full solid solution exists. Altogether our work reinforces motivation for the exploration of n-type Zintl materials, especially in tandem with high-throughput defect calculations to inform selection of effective dopants and systems amenable to n-type transport.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Reaction Heterogeneity in LiNi 0.8 Co 0.15 Al 0.05 O 2 Induced by Surface Layer

Through operando synchrotron powder X-ray diffraction (XRD) analysis of layered transition metal oxide electrodes of composition LiNi0.8Co0.15Al0.05O2 (NCA), we decouple the intrinsic bulk reaction mechanism from surface-induced effects. For identically prepared and cycled electrodes stored in different environments, we demonstrate that the intrinsic bulk reaction for pristine NCA follows solid-solution mechanism, not a two-phase as suggested previously. By combining high resolution powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and surface sensitive X-ray photoelectron spectroscopy (XPS), we demonstrate that adventitious Li2CO3 forms on the electrode particle surface during exposure to air, through reaction with atmospheric CO2. This surface impedes ionic and electronic transport to the underlying electrode, with progressive erosion of this layer during cycling giving rise to different reaction states in particles with an intact vs an eroded Li2CO3 surface-coating. This reaction heterogeneity, with a bimodal distribution of reaction states, has previously been interpreted as a “two-phase” reaction mechanism for NCA, as an activation step that only occurs during the first cycle. Similar surface layers may impact the reaction mechanism observed in other electrode materials using bulk probes such as operando powder XRD.

Grenier, Antonin↗
DOE OSTI2017

Direct Evidence of Significant Cation Intermixing in Upconverting Core@Shell Nanocrystals: Toward a New Crystallochemical Model

Core@shell design represents an important class of architectures because of its capability to dramatically increase the absolute upconversion quantum yield (UCQY) of upconverting nanocrystals (UCNCs) but also to tune energy migration pathways. A relatively new trend towards the use of very thick optically inert shells affording significantly higher absolute UCQYs raises the question of the crystallographic and chemical characteristics of such nanocrystals (NCs). In this article, local chemical analyses performed by scanning transmission electron microscopy (STEM) combined with energy dispersive x-ray spectroscopy (EDXS) and x-ray total scattering experiments together with pair distribution function (PDF) analyses were used to probe the local chemical and structural characteristics of hexagonal β-NaGd0.78Yb0.2Er0.02F4@NaYF4 core@shell UCNCs. The investigations lead to a new crystallochemical model to describe core@shell UCNCs that considerably digresses from the commonly accepted epitaxial growth concept with sharp interfaces. The results obtained on ultra-small (4.8 ± 0.5 nm) optically active cores (β-NaGd0.78Yb0.2Er0.02F4) surrounded by an optically inert shell (NaYF4) of tunable thickness (roughly 0, 1, 2, and 3.5 nm) clearly indicate the massive dissolution of the starting seeds and the inter-diffusion of the shell element (such as Y) into the Gd/Yb/Er-containing core giving rise to the formation of a non-homogeneous solid solution characterized by concentration gradients and the lack of sharp interfaces. Independently of the inert shell thickness, core/interface/shell architectures were observed for all synthesized UCNCs. The presented results constitute a significant step towards the comprehensive understanding of the “structure - property” relationship of upconverting core@shell architectures, which is of prime interest not only in the development of more efficient structures but also to provide new physical insights at the nanoscale to better explain upconversion (UC) properties alterations.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Designing High-Efficiency Nanostructured Two-Phase Heusler Thermoelectrics

Nanostructured systems formed by two distinct phases are particularly promising for high efficiency thermoelectrics due to the reduction in thermal conductivity afforded by the nanostructured phase. However, the choice of the matrix and nanostructured phases represents a challenging materials discovery problem due to the large compositional space involved. Heusler phase thermoelectrics are particularly promising candidates for nanostructuring since these compounds often possess favorable electronic thermoelectric properties but relatively high thermal conductivity. Here, we have developed a high-throughput screening strategy to predict promising candidates for nanostructuring systems based on two Heusler phases. Our search includes all two-phase systems involving full, half, and inverse Heusler in the Open Quantum Materials Database, in total a search space of ~10 11 possible combinations of two Heusler compounds. To reduce this space, our screening approach starts with a set of known thermoelectrics as matrix phases and screens for all second phase compounds that are stable and form a two-phase equilibrium with the matrix. We compute mixing energies for the resulting combinations of a matrix and a nanostructured phase, find systems that have a moderately large positive mixing energy, and hence show an appropriate balance between tendency for nanostructuring and solubility of the second phase. Furthermore, our screening approach gives 31 pairs, two of which have been explored experimentally (thus validating our screening strategy) and 29 of which represent new predictions of systems awaiting experimental synthesis. In addition, our results show that matrix/nanostructure pairs consisting of distinct crystal structures (e.g., mixing of half Heusler with full Heusler) typically have low mutual solubility, whereas isostructural matrix/nanostructured phase pairs (where both matrix and nanostructure have the same structure type, half Heusler or full Heusler) more often have energetics suitable for forming nanostructures or solid solutions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Surface Structural and Chemical Evolution of Layered LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) under High Voltage and Elevated Temperature Conditions

This work reports new insights into structural and chemical evolution of surface phases of LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) held at constant high voltages (up to 4.75 V) as well as high temperatures (60 °C) by correlating crystal structure using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging with chemistry using electron energy loss spectroscopy (EELS). We also followed the Al distribution within individual NCA particles by X-ray energy dispersive spectroscopy (EDS). The progression of these phases as a function of distance from the edge shows simultaneous evolution of crystal structures and chemistry from rocksalt to layered, forming a complete solid solution. We have also observed an extended disordered phase with rocksalt (Fm3m) symmetry in which quantitative electron energy loss spectroscopy reveals it to be an oxygen deficient cation disordered phase with chemical characteristics, as determined by EELS, similar to spinel. The formation of these disordered phases with cation and oxygen vacancies has been driven by surface oxygen loss caused by reactions with the electrolyte followed by cation migration from the octahedral 3a M (M = Ni, Co, Al) layer to the octahedral 3b Li layer. These surface rocksalt phases are not fully dense as they contain Al and Li as well as a high concentration of cation and oxygen vacancies. After discharge, Li is detected within these phases indicative that Li transport has occurred through these rocksalt phases. At 60 °C and 4.75 V a very large impedance rise is observed leading to complete cell irreversibility which is caused by significant metal dissolution from the cathode and formation of surface porosity. In the near surface region of some particles, a phase transformation from R3m (O3) to P3m1 (O1) is also observed which has become thermodynamically stable from complete delithiation as well as from local Al surface depletion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Origin of the High Capacity Manganese-Based Oxyfluoride Electrodes for Rechargeable Batteries

In the quest for high energy density rechargeable batteries, conversion-type cathode materials stand out with their appealing multielectron transfer properties. However, they undergo a series of complex phase transitions upon initial cycling as opposed to conventional intercalation-type materials. Within this category, iron-based mixed-anion solid solutions (FeO x F 2-x ) have captured the most attention of the battery community, owing to their high theoretical capacity and moderate cyclability. In the meantime, it was recently demonstrated, via a series of electrochemical cycling experiments, the in situ preparation of manganese-based mixed-anion cathode materials based on decomposition of electrolyte salt LiPF6 in the presence of MnO. To take a step forward, we herein report a routine protocol to prepare 220 mAh g -1 -class composite cathodes. In addition, we provide a comprehensive understanding of the in situ fluorination and locally reversible phase transitions using complementary analytical techniques. The charged phase, with an average Mn oxidation state of ca. +2.8, consists of a highly disordered O-rich cubic-spinel-like core and an F-rich amorphous shell. Upon discharge, lithiation induces further phase transition, forming LiF, MnO, and a lithiated rocksalt-like phase. Furthermore this work, which we also extended to the iron-based system, offers insights into modification of chemical and electronic properties of electrode materials by in situ fluorination.

25 ENERGY STORAGE↗
DOE OSTI2018

Metastable Li 1+δ Mn 2 O 4 (0 ≤ δ ≤ 1) Spinel Phases Revealed by in Operando Neutron Diffraction and First-Principles Calculations

Here, we examine the reaction mechanisms driven by the lithiation of Li 1+δ Mn 2 O 4 (0 ≤ δ ≤ 1) spinels via in operando neutron powder diffraction (NPD). New reaction mechanisms are proposed involving solid-solution regions within both cubic and tetragonal spinel phases in addition to a continuous phase transition between them. In operando NPD is an ideal tool to follow the light elements such as lithium and oxygen in cathode materials which are often the key to fully understand their structural evolutions. Here, we report a novel methodology to prepare an extremely thick electrode with ~378 mg·cm –2 loading density suitable for the in operando NPD studies. Enabled by such thick electrode, we find that the metal oxygen M–O (M = Li and Mn) bond lengths in both end members LiMn 2 O 4 and Li 2 Mn 2 O 4 experience pronounced changes larger than dictated by the change in lattice parameters because of the locally formed Jahn–Teller distorted Mn 3+ . First-principles density functional theory calculations confirm these metastable intermediates and further propose atomistic reaction pathways for the phase transition by coupling a global structure search algorithm. In conclusion, these findings redefine the conventional understandings on two-phase reactions of this spinel material.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Two-Dimensional CsAg 5 Te 3– x S x Semiconductors: Multi-anion Chalcogenides with Dynamic Disorder and Ultralow Thermal Conductivity

Metal chalcogenides underpin a wide variety of energyrelated applications and are ideal systems for probing lattice dynamics and fundamental transport phenomena. Here we describe the synthesis and transport properties of CsAg 5 TeS 2 and its solid solution CsAg 5 Te 3-x S x (x = 1-2), new semiconductors with tunable band gaps ranging from 0.17 to 0.30 eV. CsAg 5 TeS 2 has a fully ordered two-dimensional structure that includes a group of Ag atoms in a heteroleptic tetrahedral coordination geometry (AgTe 2 S 2 ). Single-crystal X-ray diffraction indicates that the compounds crystallize in the tetragonal space group P 4/ mmm , while pair distribution function (PDF) analysis reveals off-centering at the heteroleptic Ag sites, signifying the lower-symmetry I4/ mcm space group. The underlying disorder as a phonon-blocking mechanism that helps facilitate an ultralow lattice thermal conductivity below 0.40 Wm -1 • K -1 at , ~ 300 K , highlighting the importance of local disorder in thermal transport. In conclusion, density functional theory provides additional insight into the electronic and thermal properties of the materials, which are good candidates for p-type thermoelectrics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Cu-Catalyzed Synthesis of CdZnSe–CdZnS Alloy Quantum Dots with Highly Tunable Emission

Fabrication of quantum dots (QDs) with emission covering a wide spectral region has been persistently intriguing because of their potentials in a range of practical applications such as biological labeling and imaging, solar cells, light-emitting diodes, and next-generation displays. We report the synthesis of CdZnSe–CdZnS core–shell alloy QDs through a Cu-catalyzed solid solution alloying strategy starting from CdSe–CdS core–shell QDs. The resulting CdZnSe–CdZnS alloy QDs exhibit emission profiles covering a wide wavelength range of 470–650 nm while maintaining high photoluminescence quantum yields. In addition, high morphological uniformity of the starting CdSe–CdS QDs can be largely retained in the final alloy QDs. We attribute this alloying process to the high mobility nature of Cu cations in Cd-chalcogenide crystals at elevated reaction temperatures, which allows Cu cations to act as transporting agents to transfer a Zn component into the CdSe–CdS QDs while maintaining the particle integrity. We show that this unique alloying strategy is independent of the shape of the starting QDs and can also be applied to the synthesis of CdZnSe–CdZnS nanorods. We anticipate that our study will instigate the synthesis of various high-quality alloy QDs and other alloy nanocrystals beyond what can be achieved currently.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electronic Structure and Phase Stability of Yb-filled CoSb 3 Skutterudite Thermoelectrics from First Principles

Filling the large voids in the crystal structure of the skutterudite CoSb$_3$ with rattler atoms $R$ provides an avenue for both increasing carrier concentration and disrupting lattice heat transport, leading to impressive thermoelectric performance. While the influence of $R$ on the lattice dynamics of skutterudite materials has been well studied, the phase stability of $R$-filled skutterudite materials and the influence of the presence and ordering of $R$ on the electronic structure remain unclear. Here, focusing on the Yb-filled skutterudite Yb$_x$Co$_4$Sb$_{12}$, we employ first-principles methods to compute the phase stability and electronic structure. Yb-filled CoSb$_3$ exhibits (1) a mild tendency for phase separation into Yb-rich and Yb-poor regions and (2) a strong tendency for chemical decomposition into Co--Sb and Yb--Sb binaries (i.e., CoSb$_3$, CoSb$_2$, and YbSb$_2$). We find that, at reasonable synthesis temperatures, configurational entropy stabilizes single-phase solid solutions with limited Yb solubility, in agreement with experiments. Filling CoSb$_3$ with Yb increases the band gap, enhances the carrier effective masses, and generates new low-energy ``emergent'' conduction band minima, which is distinct from the traditional band convergence picture of aligning the energies of existing band extrema. The explicit presence of $R$ is necessary to achieve the emergent conduction band minima, though the rattler ordering does not strongly influence the electronic structure. The emergent conduction bands are spatially localized in the Yb-rich regions, unlike the delocalized electronic states at the Brillouin zone center that form the unfilled skutterudite band edges.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗