Dilute Sb Doping Yields Softer p‐Type Bi2Te3 Thermoelectrics

In this study, the Sb content within p‐type Bi2Te3 by employing phase diagram engineering is strategically tuned. This method retains the advantages of Sb doping but mitigated the brittleness typically seen in high‐Sb Bi0.5Sb1.5Te3 (BST). The as‐constructed phase diagram demonstrates the asymmetrical homogeneity of (Bi, Sb)2Te3, guiding focus toward developing an optimized p‐type (Bi2Te3)0.96(Sb2Te)0.04 with reduced Sb content. The resulting crystal of (Bi2Te3)0.96(Sb2Te)0.04 exhibit an exceptional peak zT of 1.3 at 303 K, surpassing the mechanical robustness of standard high‐Sb BST. Additionally, it matches the energy conversion efficiency of traditional BST, achieving 2.3% at a temperature difference ΔT of 150 K. This significant advance makes (Bi2Te3)0.96(Sb2Te)0.04 a potential competitor to the well‐established BST, thanks to its enhanced thermoelectric performance owing to the elevated carrier concentration and a less brittle nature due to the diluted Sb dopant.


Introduction
The outbreaks of SARS and COVID-19 have underscored refrigeration's pivotal role in vaccine storage, increasing the demand for cooling solutions. [1]Traditional cooling methods will likely cause greenhouse gas emissions and climate change, making sustainable and solid-state cooling techniques important. [2]Thermoelectric cooling (TEC) using the Peltier effect is emerging as a promising candidate.[5] Optimal TE materials necessitate an elevated figure of merit, represented by the equation zT = S 2 T/.8] DOI: 10.1002/aelm.202300793Bi 2 Te 3 has historically been the material for room-temperature TE applications. [9,10]It is frequently alloyed, resulting in p-type (Bi, Sb) 2 Te 3 or n-type Bi 2 (Te, Se) 3 compounds. [11]Notably, Antimony (Sb) is a preferred p-type dopant due to its comparable electronegativity and covalent radius to Te compared with Bi. [12][13][14] Consequently, the p-type Bi 0.5 Sb 1.5 Te 3 (BST) alloy, enriched with Sb, has consistently been used in cuttingedge TEC devices.The high amount of Sb dopants induces the formation of Sb Te antisite defects, thereby increasing the hole carriers. [15,16]Recent innovations have led to significant improvements in the TE performance of p-type BST materials, as documented in bulk forms [17,18] and thin-film structures. [19]These advancements have achieved a notable peak zT greater than 1.2 at ambient temperatures, with a substantial increase in the average zT across the measurement temperature from 303 to 473 K.
While the BST performs a superior zT in contrast to the undoped Bi 2 Te 3 , its peak zT is recorded at 400 K, which lowers its efficacy at ambient temperatures. [20,21]Furthermore, adding a high dose of Sb makes the Bi 2 Te 3 fracture or cleavage, imposing constraints on its performance and lifetime. [22]Hence, this study seeks to minimize Sb concentration in a p-type Bi 2 Te 3 via phase diagram engineering, [23,24] retaining benefits from Sb doping while rectifying drawbacks such as the cleaving susceptibility of BST and the low zT of pure Bi 2 Te 3 .Moreover, the asymmetrical homogeneity of (Bi, Sb) 2 Te 3 was depicted by constructing the phase diagram, enabling the development of an optimal p-type (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 .Compared to BST, our (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 crystal demonstrates an enhanced peak zT of 1.3 at 303 K with reduced brittleness.Furthermore, the singleleg conversion efficiency  of (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 akin to BST under a small temperature difference ∆T. [25]These findings propose that the (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 with a dilute Sb content emerges as a potential alternative to commercial BST, encompassing superior TE performance and softness.

Results & Discussion
An equilibrium phase diagram for the ternary Bi-Te-Sb system, depicted in Figure 1a, was constructed based on extensive microstructural analyses and structural characterization of various ternary alloys.These alloys were subjected to an annealing process at 523 K for no less than nine months.The solid lines represent the confirmed phase regions, while the dashed lines indicate the regions that lack experimental confirmations.The emphasis is especially drawn to the presence of the two-phase regions: (Bi, Sb) 2 Te 3 + Te and (Bi, Sb) 2 Te 3 + Bi 4 Te 5 , and the (Bi, Sb) 2 Te 3 solid solution.Notably, these phase boundaries are established based on the phase equilibrium information acquired from thermally equilibrated alloys (as collected in Table S1, Supporting Information).Taking alloy#1 (Sb-80 at%Te-10 at%Bi) and alloy#7 (Sb-58 at%Te-28 at%Bi) as examples, their microstructures and corresponding XRD patterns, shown in Figure S1a,d (Supporting Information), confirm the phase regions of (Bi, Sb) 2 Te 3 + Te and (Bi, Sb) 2 Te 3 + Bi 4 Te 5 , respectively.
Noteworthy are the phase relationships nearing the singlephase Bi 2 Te 3 , which hold particular significance and are featured within the enlarged inset at the upper-left corner of Figure 1.By the Bi─Te binary phase diagram, [9] the single-phase Bi 2 Te 3 exists in the compositional range of 39.8-40.As shown in Figure 2b, adding a small amount of Sb enhances the stability of p-type conduction, as illustrated in a schematic phase diagram set at 1.63 at% Sb.Additionally, as the Sb content rises, the S and  curves, represented in Figure 2c,d, are suppressed due to the increasing hole carrier concentration.Significantly, the alloy with c = 0.04 exhibits the lowest  with a minimal S.This is credited to its elevated carrier concentration, with n H = 1.44 × 10 19 cm −3 , standing out as the most pronounced within the c-series alloys, as noted in Table 1.It is worth noting  that the PF value for the c-series alloys outperforms the undoped Bi 2 Te 3 .In particular, the c = 0.04 achieves the highest PF value of S 2  −1 = 7.75 mWm −1 K −2 at 303 K.These findings underscore the effectiveness of dilute Sb doping in tuning the electrical transport properties.
Regarding the thermal conduction, it is evident that both  and  L , as shown in Figure 2e,f, are lower than undoped Bi 2 Te 3 , suggesting that incorporating Sb dopant likely enhances the phonon scattering.The  of the c-series alloys is reduced to 1.8 Wm −1 K −1 at 303 K, leading to an improved zT, even with a small amount of Sb dopants.Among all the Sb-─Bi 2 Te 3 alloys, the c = 0.04 alloy achieves the highest zT value of 1.3 at 303 K, owing to its enhanced PF and reduced .][28][29][30][31] The (Bi 2 Te 3 ) 1-c (Sb 2 Te) c series shows improved p-type TE properties due to the decrease in  and .
As previously mentioned, the zT value of (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 surpasses that of all other Sb-doped Bi 2 Te 3 .This leads us to explore whether it could be a viable alternative to the state-of-theart Bi 0.5 Sb 1.5 Te 3 (BST).Our discussion begins by assessing the efficiency of a single-leg device incorporating the c = 0.04 under small temperature differences, thereby simulating practical applications near room temperature.
An inset of Figure 3a shows a single-leg device of c = 0.04 crystal.While maintaining the cold-side T c temperature at 295 K, we incrementally raised the hot-side temperature T h to 468 K.As illustrated in Figure 3a, the TE efficiency  increases with the rise in temperature difference (ΔT = T h −T c ) and ultimately reaches its peak value of 2.3% at ΔT = 150 K. Consequently, when the T h reaches 445 K, the generated output power P reaches as high as 24 mW, as shown in Figure 3b.This is accompanied by the dissipation of heat Q amounting to 1.1 W, as demonstrated in Figure 3c.Comparing our singleleg device to a commercial BST module, [25] we observe that our single-leg device exhibits a comparable  for ΔT below 150 K.This suggests that our p-type crystal, (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 , which contains a significantly lower concentration of Sb, holds promise as a potential alternative to commercial BST as a TE material.
Another challenge in utilizing Sb─Bi 2 Te 3 crystals for practical applications is their mechanical properties, which directly impact their lifetime and usage conditions.To assess this aspect, we   4a), indicating its inherent brittleness.In contrast, the indentation mark on the surface of (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 displayed a mud-like appearance, as shown in Figure 4b.This implies that dilute Sb doping reduces hardness and mitigates the pronounced anisotropy observed in Bi 2 Te 3 .Considering the small portion of Bi 4 Te 5 in the composition with c = 0.04, as indicated in Figure S6 (Supporting Information), the coexistence of Bi bilayer (Bi 2 ) and Bi 2 Te 3 quintuple layer within Bi 4 Te 5 is plausible.This coexistence suggests a strong interaction between the Bi 2 and Bi 2 Te 3 layers, combining metallic bonding from the Bi 2 layer with the unconventional metavalent bonding within Bi 2 Te 3 .[34] Consequently, a softer and superior (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 crystal was synthesized in this study.
Our newly developed (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 exhibits the potential to replace the well-established BST material.It offers softer, more cost-effective, and more durable advantages, making it a promising candidate for TE applications.For future practical applications, the aim will be to leverage the improved mechanical properties of this material to fabricate dependable, enduring TE modules capable of performing under diverse environmental stresses.This advancement is anticipated to broaden the material's applicability in various sectors, including industrial, automotive, and consumer electronics, where the longevity of the material is just as important as its TE efficiency.

Conclusion
In conclusion, this study presents an empirically obtained equilibrium phase diagram for the Bi-Sb-Te ternary system, pinpointing the homogeneity region of (Bi, Sb) 2 Te 3 .By determining the solubility limits of Sb within single-phase (Bi, Sb) 2 Te 3 , we highlight the feasibility of using dilute doping, spotlighting the potential of (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 .Impressively, in comparison to BST, the (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 demonstrates an advanced peak zT value of 1.3 at 303 K and exhibits a commendable conversion efficiency  of 2.3% under a temperature difference ΔT of 150 K when assembled into a single-leg device.Furthermore, this crystal displays decreased hardness while preserving its soft nature, minimizing possibilities for cleavage and cracking.The adoption of light-doping, assisted by phase diagram engineering, unveils a promising p-type Bi 2 Te 3 incorporated with a dilute amount of Sb, offering a competitive candidate over the conventional BST.

Experimental Section
Phase Diagram Construction: Pure elements of Bi (99.99%),Sb (99.99%), and Te (99.99%) were weighed according to the predetermined nominal composition with a total mass of 1 g (Table S1, Supporting Information).Then, the elements were placed in the evacuated quartz tubes (≈10 −3 torr), heated to 1173 K with a 25 K/10 min heating rate, and homogenized for 2 h.The melted samples were solidified by a waterquenching process, and then the alloys were annealed at 523 K for at least 400 days.These thermally equilibrated alloys were cooled by waterquenching and subjected to metallographic observations.Synthesis: For the bulk TE properties, three series alloys with the chemical configuration of Bi 2-a Sb a Te 3 , Bi 2 Te 3 Sb b , and (Bi 2 Te 3 ) 1-c (Sb 2 Te) c series were grown via the Bridgman method with a total weight of 8 g.Before the Bridgman growth, pure elements Bi (99.99%),Sb (99.99%), and Te (99.99%) were loaded into a carbon-coated quartz tube and sealed under a vacuum of 8 × 10 −3 torr.The samples were then gradually heated to 973 K and homogenized for 12 h before air-cooling.The samples were subsequently placed in a Bridgman furnace and heated up to 913 K to form a liquid melt of the alloy.Then, samples were slowly moved downward at a constant growth rate of 2.8 K h −1 until complete solidification of the alloy.The as-growth alloys were cut into rod-shaped specimens (7.0 mm in diameter and 12 mm in height) and pellet-shaped specimens (7.0 mm in diameter and 2 mm in thickness).These specimens were then ground to #4000 SiC paper in preparation for thermoelectric measurement.
Characterization: Both thermal-equilibrated alloys and Bridgmangrown samples were mounted in an epoxy resin and polished through SiC paper (#800 to #4000) and Al 2 O 3 powder (particle size ranges from 1 to 0.05 μm) for metallographic observation.Microstructure and element compositions of bulk samples were observed by field-emission Electron Probe Microanalyzer (EPMA, JEOL JXA-8530F).The error bar in the chemical composition for each equilibrium phase was obtained based on five data points collected by the EPMA from areas exhibiting the same phase contrast.The morphology of the thin film samples was observed by fieldemission scanning electron microscopy (FE-SEM, Hitachi SU-8010), and the composition was detected by dispersive X-ray spectroscopy (EDS).The crystal structures of bulk samples were analyzed by powder X-ray diffraction (XRD, Bruker, D2 PHASER X-ray Diffractometer) with Cu K as a target at angles (2) of 10-90°.The hardness of the Bridgman-grown samples was measured by a Vickers hardness tester.
Thermoelectric Property Measurement: The electrical resistivity  and Seebeck coefficient S of the Bi 2 Te 3 -based alloys were measured parallel to the growth direction using a commercial ZEM-3 (ULVAC, Japan) apparatus in a helium-filled atmosphere.The thermal conductivity  of the bulk samples was obtained from  = D × C p × d, where D is the thermal diffusivity, C p is the heat capacity ( C p = 3R/M, R is the gas constant, and M is the average molecular weight), and d is the sample density, respectively.Thermal diffusivity D was obtained using a commercial apparatus (LFA-467, NETZSCH, Germany), while the d was determined by the Archimedes method.The carrier concentration n c and mobility μ were determined by a commercial Hall effect measurement system (Ecopia, HMS-3000) under 5 mA and 0.49T.The TE conversion efficiency was measured by the mini-PEM (Ulvac-Riko, Japan).Sn 63 Pb 37 tin paste, which has a melting point of 456 K, was employed to bond the copper plates and TE material to assemble a TE single-leg device.Meanwhile, the SAC305 solder (Sn 96.5 Ag 3 Cu 0.5 ) was utilized to join the copper plates and copper wires.The overall height of this single-leg device is ≈1 cm.

Figure 1 .
Figure 1.Isothermal section of the Sb-Bi-Te ternary system at T = 523 K.The enlarged area emphasized the asymmetric single-phase region of (Bi, Sb) 2 Te 3 .
2 at%Bi at 523 K.The addition of Sb further establishes the solid solution phase, denoted as (Bi, Sb) 2 Te 3 , signifying the varying mutual solubility between the Bi and Sb.Significantly, the region of mutual solubility increases from 0.4 at%Bi in binary Bi 2 Te 3 to ≈1.0 at%Bi when the doping concentration exceeds 1.63 at%Sb.As depicted in Figure 2a, the homogeneity of (Bi, Sb) 2 Te 3 extends asymmetrically toward the Bi-rich side, indicating a tendency for Sb to substitute Bi.This established solubility boundary for (Bi, Sb) 2 Te 3 serves as a blueprint for designing Sb-doped Bi 2 Te 3 alloys.Leveraging this phase diagram, we synthesized three sets of Sb─ Bi 2 Te 3 crystals with designated compositions: Bi 2-a Sb a Te 3 , Bi 2 Te 3 Sb b , and (Bi 2 Te 3 ) 1-c (Sb 2 Te) c .The Bi 2-a Sb a Te 3 series alloys, ranging from 0 to 1.5, cover the state-of-the-art ptype Bi 0.5 Sb 1.5 Te 3 and span widely across the (Bi, Sb) 2 Te 3 solidsolution region.At 303 K, the  decreases with increasing Sb content (Figure S4b, Supporting Information), corresponding to the decreasing S (Figure S4a, Supporting Information).Moreover, the  drops with rising due to the alloying effect (Figure S2c, Supporting Information).The substitution of Bi by Sb in Bi 2 Te 3 optimizes the hole carrier and reduces the , leading to the enhanced zT of 1.0 for Bi 1.5 Sb 0.5 Te 3 at 300 K.In contrast to the effectiveness in zT enhancement of Bi 2-a Sb a Te 3 , the zT curves of Bi 2 Te 3 Sb b exhibit strikingly similar to those of undoped Bi 2 Te 3 .This similarity can be attributed to the counterbalance effect of reduced S 2  −1 and decreased  (Figure S4, Supporting Information).While the Bi 1.5 Sb 0.5 Te 3 performed an enhanced zT compared with undoped Bi 2 Te 3 , it is essential to note that the high dose of antimony makes the sample brittle.Therefore, we explored the development of a third series, (Bi 2 Te 3 ) 1-c (Sb 2 Te) c , which, it turns out, emerges as the most promising group of p-type Sb─Bi 2 Te 3 among the three alloy series.As revealed in the expanded pseudo-binary phase diagram, the nominal composition of p-type (Bi 2 Te 3 ) 1-c (Sb 2 Te) c resides close to the lower limit of the (Bi, Sb) 2 Te 3 homogeneity region.As depicted in Figure 2a, most of the c-series alloys fall within the (Bi, Sb) 2 Te 3 single-phase region, as evidenced by their powder XRD pattern in Figure S6 (Supporting Information).Given the reduced covalent radius of Sb (1.40 Å) relative to Bi (1.46 Å), replacing Bi with Sb is expected to reduce the lattice volume.This assertion is verified by the decline in the lattice constants along the a-axis and c-axis as the Sb content rises, as illustrated in Figure S7 (Supporting Information).

Figure 2 .
Figure 2. a) (Bi 2 Te 3 ) 1-c (Sb 2 Te) c compositions on the Sb-Bi-Te ternary phase diagram b) Phase diagram of the Sb-Bi-Te ternary system with Sb fixed at 1.63 at%, and its p-type and n-type characteristics concerning temperature.Temperature-dependent of c) Seebeck coefficient, d) resistivity, e) thermal conductivity, f) lattice thermal conductivity, g) power factor PF = S 2  −1 , and h) figure-of-merit zT.

Figure 3 .
Figure 3. a) Efficiency  b) output power c) heat flow of (Bi 2 Te 3 ) 0.96 (Sb 2 Te) 0.04 sample with a temperature difference from 25 to 150 K, as well as the schematic module and the actual module d) Comparison of the thermoelectric conversion efficiency with the commercially available Bi 0.5 Sb 1.5 Te 3 as reported in reference.[25]