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CuGaTe 2 -based compounds show great promise in the application for high-temperature thermoelectric power generation; however, its wide bandgap feature poses a great challenge for enhancing thermoelectric performance via structural defects modulation and doping the system. Herein, it is discovered that the presence of Ga Cu antisite defects in the CuGaTe 2 compound promotes the formation of Cu vacancies, and vice versa, which tends to form the charge-neutral structure defects combination with one Ga Cu antisite defect and two Cu vacancies. The accumulation of Cu vacancies in the structure of the (Cu 2 Te) x (Ga 2 Te 3 ) 1- x compounds evolves into twins and stacking faults. This in conjunction with Ga Cu antisite defects intensify the point defects phonon scattering, yielding a dramatic reduction on lattice thermal conductivity from 6.95 W m -1 K -1 for the pristine CuGaTe 2 sample to 2.98 W m -1 K -1 for the (Cu 2 Te) 0.45 (Ga 2 Te 3 ) 0.55 sample at room temperature. Furthermore, the high concentration of charge-neutral defects combination narrows the band gap and increases the carrier concentration, leading to an improved power factor of 1.58 mW/mK 2 at 600 K for the (Cu 2 Te) 0.49 (Ga 2 Te 3 ) 0.51 sample, which is 41% higher than for the pristine CuGaTe 2 sample. Consequently, the highest ZT value of 0.82 is achieved at 915 K for Cu 0.015 (Cu 2 Te) 0.48 (Ga 2 Te 3 ) 0.52 , which represents an enhancement of about 22% over that of the pristine CuGaTe 2 compound.

The stabilization at low temperatures of the Ag 2 S cubic phase could afford the design of high-performance thermoelectric materials with excellent mechanical behavior, enabling them to withstand prolonged vibrations and thermal stress. In this work, we show that the Ag 2 Te x S 1- x solid solutions, with Te content within the optimal range 0.20 ≤ x ≤ 0.30, maintain a stable cubic phase across a wide temperature range from 300 to 773 K, thus avoiding the detrimental phase transition from monoclinic to cubic phase observed in Ag 2 S. Notably, the Ag 2 Te x S 1- x (0.20 ≤ x ≤ 0.30) samples showed no fractures during bending tests and displayed superior ductility at room temperature compared to Ag 2 S, which fractured at a strain of 6.6%. Specifically, the Ag 2 Te 0.20 S 0.80 sample demonstrated a bending average yield strength of 46.52 MPa at 673 K, significantly higher than that of Ag 2 S, whose bending average yield strength dropped from 80.15 MPa at 300 K to 12.66 MPa at 673 K. Furthermore, the thermoelectric performance of the Ag 2 Te x S 1- x (0.20 ≤ x ≤ 0.30) samples surpassed that of both InSe and pure Ag 2 S, with the Ag 2 Te 0.30 S 0.70 sample achieving the highest ZT value of 0.59 at 723 K. These results indicate substantial potential for practical applications due to enhanced durability and thermoelectric performance.

Carrier transport engineering in bulk semiconductors using inclusion phases often results in the deterioration of carrier mobility (μ) owing to enhanced carrier scattering at phase boundaries. Here, we show by leveraging the temperature-induced structural transition between the α-Cu 2 Se and β-Cu 2 Se polymorphs that the incorporation of Cu 4 TiSe 4 inclusions within the Cu 2 Se matrix results in a gradual large drop in the carrier mobility at temperatures below 400 K (α-Cu 2 Se), whereas the carrier mobility remains unchanged at higher temperatures, where the β-Cu 2 Se polymorph dominates. The sharp discrepancy in the electronic transport within the α-Cu 2 Se and β-Cu 2 Se matrices is associated with the formation of incoherent α-Cu 2 Se/Cu 4 TiSe 4 interfaces, owing to the difference in their atomic structures and lattice parameters, which results in enhanced carrier scattering. In contrast, the similarity of the Se sublattices between β-Cu 2 Se and Cu 4 TiSe 4 gives rise to coherent phase boundaries and good band alignment, which promote carrier transport across the interfaces. Interestingly, the different cation arrangements in Cu 4 TiSe 4 and β-Cu 2 Se contribute to enhanced phonon scattering at the interfaces, which leads to a reduction in the lattice thermal conductivity. The large reduction in the total thermal conductivity while preserving the high power factor of β-Cu 2 Se in the (1- x )Cu 2 Se/( x )Cu 4 TiSe 4 composites results in an improved ZT of 1.2 at 850 K, with an average ZT of 0.84 (500-850 K) for the composite with x = 0.01. This work highlights the importance of structural similarity between the matrix and inclusions when designing thermoelectric materials with improved energy conversion efficiency.

The ability to control the relative density of electronic point defects as well as their energy distribution in semiconductors could afford a systematic modulation of their electronic, optical, and optoelectronic properties. Using a model binary hybrid system Cu 2 Se-CuInSe 2 , we have investigated the correlation between phase composition, microstructure, and electronic transport behavior in the synthesized composites. We found that both Cu 2 Se and CuInSe 2 phases coexist at multiple length scales, ranging from sub-ten nanometer to several micrometers, leading to the formation of a hybrid hierarchical microstructure. Astonishingly, the electronic phase diagram of the (1 - x )Cu 2 Se-( x )CuInSe 2 (15% ≤ x ≤ 100%) hierarchical composites remarkably deviates from the trend normally expected for composites between a heavily doped semiconductor (Cu 2 Se) and a poorly conducting phase (CuInSe 2 ). A sudden 3-fold increase in the electrical conductivity and carrier concentration along with a marginal increase in the carrier mobility is observed for composites at the vicinity of equimolar composition (48% ≤ x ≤ 52%). The carrier concentration increases from ∼1.5 × 10 20 cm -3 for the composites with x ≤ 45% to 5.0 × 10 20 cm -3 for x = 50%, and remains constant at 4.5 × 10 20 cm -3 with x value in the range of 52% < x ≤ 90%, then quickly drops to 8 × 10 18 cm -3 for pristine CuInSe 2 phase ( x = 100%). The atypical electronic behavior was rationalized in the light of the formation of an inter-band (IB) within the band gap, which arises from the hybridization of native electronic point defects from both Cu 2 Se and CuInSe 2 phases in the resulting hierarchical composites. The result points to a new strategy to modulate the electronic structure of semiconductor composites to maximize interaction and coupling between two fundamentally contrasting properties enabling access to electronic hybrid systems with potential applications as interactive and stimuli-responsive multifunctional materials., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

Atomic-scale incorporation of CuAlSe 2 inclusions within the Cu 2 Se matrix, achieved through a solid-state transformation of CuSe 2 template precursor using elemental Cu and Al, enables a unique temperature-dependent dynamic doping of the Cu 2 Se matrix. The CuAlSe 2 inclusions, due to their ability to accommodate a large fraction of excess metal atoms within their crystal lattice, serve as a "reservoir" for Cu ions diffusing away from the Cu 2 Se matrix. Such unidirectional diffusion of Cu ions from the Cu 2 Se matrix to the CuAlSe 2 inclusion leads to the formation, near the CuAlSe 2 /Cu 2 Se interface, of a high density of Cu-deficient β-Cu 2-δ Se nanoparticles within the α-Cu 2 Se matrix and the formation of Cu-rich Cu 1+ y AlSe 2 nanoparticles with the CuAlSe 2 inclusions. This gives rise to a large enhancement in carrier concentration and electrical conductivity at elevated temperatures. Furthermore, the nanostructuring near the CuAlSe 2 /Cu 2 Se interface, as well as the extensive atomic disorder in the Cu 2 Se and CuAlSe 2 phases, significantly increases phonon scattering, leading to suppressed lattice thermal conductivity. Consequently, a significant improvement in ZT is observed for selected Cu 2 Se/CuAlSe 2 composites. This work demonstrates the use of in situ-formed interactive secondary phases in a semiconducting matrix as an elegant alternative approach for further improvement of the performance of leading thermoelectric materials.

Designing crystalline solids in which intrinsically and extremely low lattice thermal conductivity mainly arises from their unique bonding nature rather than structure complexity and/or atomic disorder could promote thermal energy manipulation and utilization for applications ranging from thermoelectric energy conversion to thermal barrier coatings. Here, we report an extremely low lattice thermal conductivity of ∼0.34 W m -1 K -1 at 300 K in the new complex sulfosalt MnPb 16 Sb 14 S 38 . We attribute the ultralow lattice thermal conductivity to a synergistic combination of scattering mechanisms involving (1) strong bond anharmonicity in various structural building units, owing to the presence of stereoactive lone-electron-pair (LEP) micelles and (2) phonon scattering at the interfaces between building units of increasing size and complexity. Remarkably, low-temperature heat capacity measurement revealed a C p value of 0.206 J g -1 K -1 at T > 300 K, which is 22% lower than the Dulong-Petit value (0.274 J g -1 K -1 ). Further analysis of the C p data and sound velocity ( ν = 1834 m s -1 ) measurement yielded Debye temperature values of 161 and 187 K, respectively. The resulting Grüneisen parameter, γ = 1.65, further supports strong bond anharmonicity as the dominant mechanism responsible for the observed extremely low lattice thermal conductivity.

Crystal polymorphism selection during synthesis is extremely challenging. However, promoting the formation of a specific metastable polymorph enables modulation of the functional properties of phase-change materials through alteration of the relative abundance of various polymorphs. Here, we demonstrate the stabilization of the superionic β-Cu 2 Se phase under ambient conditions and the direct control over the relative ratio between the α-Cu 2 Se and β-Cu 2 Se polymorphs in ( x )CuGaSe 2 /(1- x )Cu 2 Se composites using CuGaSe 2 nanoseeds. We found that the small lattice mismatch between β-Cu 2 Se (cubic) and the ab plane of tetragonal CuGaSe 2 nanoseeds promotes the formation of low-energy coherent CuGaSe 2 /β-Cu 2 Se interfaces, leading to preferential stabilization of β-Cu 2 Se. Astonishingly, the hierarchical microstructure of the resulting composites enables a remarkable decoupling of charge and heat transport, which is manifested by a breakdown of the Wiedemann-Franz law.

Cd-containing polycrystalline Bi 0.46 Sb 1.54 Te 3 samples with precisely controlled phase composition were synthesized by conventional melting-quenching-annealing technique and a melt-spinning method. The pseudo ternary phase diagram for Cd-Bi/Sb-Te in the region near Bi 0.46 Sb 1.54 Te 3 was systematically studied. Cd serves as an acceptor dopant contributing holes, whereas for samples doped with CdTe, the combined effects of the substitution of Sb/Bi with Cd and the formation of Sb/Bi Te antisite defects leads to the increase in hole concentration. Moreover, upon doping with Cd, the lattice thermal conductivity decreases significantly owing to the intensified point defect phonon scattering. The sample with Cd content of 0.01 attains the maximum ZT of 1.15 at 425 K. The utilization of melt-spinning method brings about the in situ nanostructured CdTe and grain size refinement, which further reduce the lattice thermal conductivity while preserving excellent electrical performance. As a result, a higher ZT of 1.30 at 425 K is realized with CdTe content x = 0.005.

Lead telluride (PbTe) is one of the best thermoelectric materials in the intermediate temperature range, which shows great potential for waste heat recycling. However, its low strength and high brittleness limit its large-scale application because the thermoelectric device usually undergoes mechanical vibration, mechanical and/or thermal cycling, and thermal shock in service. In this study, the enhanced mechanical properties and thermoelectric properties of PbTe are realized simultaneously through introducing dispersive transition-metal dichalcogenide MoTe 2 (molybdenum telluride). The in-situ-formed MoTe 2 precipitations with a size in the range from 2 to 5 μm and the tight and smooth interface between the PbTe matrix and precipitates contribute to the obvious crack deflection, crack bridging, and pull-out of long grains, dissipating more energy during crack propagation and resulting in a tortuous propagation path. Because of the toughening and the dispersion strengthening effect, the compressive strength, bending strength, and fracture toughness of the sample with a composite amount of 1% are 109 MPa and 50 MPa and 0.65 MPa·m 1/2 , respectively, which are increased by about 37, 117, and 67% compared to the Na 0.02 Pb 0.98 Te matrix. Additionally, the in situ MoTe 2 precipitates intensify the interface phonon scattering and thus decrease the lattice thermal conductivity. As a result, the Na 0.02 Pb 0.98 Te-1%MoTe 2 sample achieves a maximum ZT value of 1.46 at 700 K, which is 11% higher than that of Na 0.02 Pb 0.98 Te without any MoTe 2 nanoprecipitation.

Ferromagnetic semiconductors (FMSs) featuring a high Curie transition temperature ( T c ) and a strong correlation between itinerant carriers and localized magnetic moments are of tremendous importance for the development of practical spintronic devices. The realization of such materials hinges on the ability to generate and manipulate a high density of itinerant spin-polarized carriers and the understanding of their responses to external stimuli. In this study, we demonstrate the ability to tune magnetic ordering in the p-type FMS FeSb 2- x Sn x Se 4 (0 ≤ x ≤ 0.20) through carrier density engineering. We found that the substitution of Sb by Sn FeSb 2- x Sn x Se 4 increases the ordering of metal atoms within the selenium crystal lattice, leading to a large separation between magnetic centers. This results in a decrease in the T c from 450 K for samples with x ≤ 0.05 to 325 K for samples with 0.05 < x ≤ 0.2. In addition, charge disproportionation arising from the substitution of Sb 3+ by Sn 2+ triggers the partial oxidation of Sb 3+ to Sb 5+ , which is accompanied by the generation of both electrons and holes. This leads to a drastic decrease in the electrical resistivity and thermopower simultaneously with a large increase in the magnetic susceptibility and saturation magnetization upon increasing Sn content. The observed bipolar doping induces a very interesting temperature-induced quantum electronic transition (Lifshitz transition), which is manifested by the presence of an anomalous peak in the resistivity curve simultaneously with a reversal of the sign of a majority of the charge carriers from hole-like to electron-like at the temperature of maximum resistivity. This study suggests that while there is a strong correlation between the overall magnetic moment and free carrier spin in FeSb 2- x Sn x Se 4 FMSs, the magnitude of the Curie temperature strongly depends on the spatial separation between localized magnetic centers rather than the concentration of magnetic atoms or the density of itinerant carriers.

Single-phase polycrystalline powders of Sr 1- x Sb x HfSe 3 ( x = 0, 0.005, 0.01), a new member of the chalcogenide perovskites, were synthesized using a combination of high temperature solid-state reaction and mechanical alloying approaches. Structural analysis using single-crystal as well as powder X-ray diffraction revealed that the synthesized materials are isostructural with SrZrSe 3 , crystallizing in the orthorhombic space group Pnma (#62) with lattice parameters a = 8.901(2) Å; b = 3.943(1) Å; c = 14.480(3) Å; and Z = 4 for the x = 0 composition. Thermal conductivity data of SrHfSe 3 revealed low values ranging from 0.9 to 1.3 W m -1 K -1 from 300 to 700 K, which is further lowered to 0.77 W m -1 K -1 by doping with 1 mol % Sb for Sr. Electronic property measurements indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe 3 is a p-type semiconductor with thermopower values reaching a maximum of 287 μV/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr 1- x Sb x HfSe 3 samples, as determined by density functional theory calculations and the diffuse reflectance method, is ∼1.00 eV and increases with Sb concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe 3 is essentially determined by the Se-4p and Hf-5d orbitals with little to no contribution from Sr atoms. Typically, band edges of p- and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized p and d orbitals. This points to potential application of SrHfSe 3 as absorbing layer in photovoltaic devices.

Earth-abundant solar absorber materials with large optical absorption coefficients in the visible enable the fabrication of low-cost high-efficiency single and multi-junction thin-film solar cells. Here, we report a new p-type semiconductor, Cu 4 TiSe 4 (CTSe), featuring indirect (1.15 eV) and direct (1.34 eV) band gaps in the optimal range for solar absorber materials. CTSe crystallizes in a new noncentrosymmetric cubic structure (space group F 4[combining macron]3 c ) in which CuSe 4 tetrahedra share edges and corners to form octahedral anionic clusters, [Cu 4 Se 4 ] 4- , which in turn share corners to build the three-dimensional framework, with Ti 4+ ions located at tetrahedral interstices within the channels. The unique crystal structure and the Ti 3d orbital character of the conduction band of CTSe give rise to near-optimal band gap values and ultra-large absorption coefficients (larger than 10 5 cm -1 ) throughout the visible range, which are promising for scalable low-cost high-efficiency CTSe-based thin-film solar cells.