Enhanced Thermoelectric Properties of Cu 3 SbSe 4 Compounds via Gallium Doping

In this study, the p-type Ga-doped Cu3Sb1−xGaxSe4 compounds were fabricated by melting, annealing, grinding, and spark plasma sintering (SPS). The transport properties of Ga-doped Cu3Sb1−xGaxSe4 compounds were investigated. As Ga content increased, the hole concentration of Cu3Sb1−xGaxSe4 compounds increased, which led to an increase in electrical conductivity. Meanwhile, the Seebeck coefficient of the Cu3Sb1−xGaxSe4 compounds decreased as Ga content increased. The extra phonon scattering originating from Ga-doping effectively depressed the lattice thermal conductivity of the Cu3Sb1−xGaxSe4 compounds. The ZT value of Cu3SbSe4 markedly improved, which is primarily ascribed to the depressed lattice thermal conductivity and the increased electrical conductivity. The highest ZT value for the Cu3Sb0.985Ga0.015Se4 compound was 0.54 at 650 K, which is two times higher than that of a pure Cu3SbSe4 compound.


Introduction
With the global environmental issue and energy crisis becoming more and more serious, developing renewable and eco-friendly technologies for the sustainable development has gained more attention.Moreover, substantial amounts of waste heat from industrial, private, and transport sectors in modern society should be effectively recovered.Thermoelectric material provides a possibility to solve the issues mentioned above.Thermoelectric material is a kind of energy conversion material, which can realize the conversion between heat energy and electric energy.Thermoelectric material is expected to play a significant role in the field of electronic cooling, power generation, and waste heat recovery.The efficiency of thermoelectric material is usually characterized by the dimensionless figure of merit ZT.The ZT value can be calculated using the equation ZT = σα 2 T/κ, where κ, T, α, and σ are the total thermal conductivity, absolute temperature, Seebeck coefficient, and electrical conductivity, respectively [1][2][3][4][5].The total thermal conductivity consists of a carrier part (κ c ) and a phonon part (κ l ).Therefore, a large ZT requires the thermoelectric material to have a low κ, a high α, and a high σ.Nowadays, developing high ZT material has been a research focus in the field of thermoelectric materials.As the α, σ, and κ e of a material are associated closely with carrier concentration, how to optimize the carrier concentration to realize the maximum ZT is a key issue in this field.To improve ZT, many feasible methods have been developed and applied.Band engineering including electric band structure and valley degeneracy has been regarded as an efficient approach to improve the power factor (PF = α 2 σ), thereby enhancing the ZT.Doping or nanostructuring are also effective ways of enhancing the ZT by introducing extra phonon scattering centers [6][7][8][9][10].
Recently, copper-based chalcogenide semiconductors have attracted much attention because of their relatively high carrier mobility (µ H ) and low κ, such as CuGa(In)Te 2 , Cu 2 CdSnX 4 (X = Se, S), Cu 2 SnSe 3 , and Cu 3 SbSe 4 [11][12][13][14].Among these compounds, ternary Cu 3 SbSe 4 semiconductor has emerged as a promising thermoelectric material because of its narrow band gap and large carrier effective mass.Cu 3 SbSe 4 has a superlattice of a zinc-blended structure and is of the type Cu 2 FeSnS 4 with space group I-42m.The Cu/Se atoms form an electrically conductive framework and the remaining Sb atoms form the one-dimensional [SbSe 4 ] tetrahedra.This special tetrahedra in the Cu 3 SbSe 4 crystal structure can enhance phonon scattering, similar to the "rattling atom" in skutterudite, resulting in a decrease in lattice thermal conductivity.Therefore, Cu 3 SbSe 4 has a relatively low thermal conductivity.However, the electrical properties of intrinsic Cu 3 SbSe 4 is poor due to its low hole concentration (p), which decreases the thermoelectric performance and leads to a low ZT value in the middle temperature range [15][16][17][18].Theoretically, partial substitution on the Sb site of the Cu 3 SbSe 4 can tune its electrical conductivity so as to enhance the thermoelectric performance.Previous studies about doping on the Sb site have been carried out, and some valuable work has been achieved [19,20].Qin et al. synthesized the Al-doped Cu 3 Sb 1−x Al x Se 4 compounds and the maximum ZT reached 0.58 @ 600 K [21].Similarly, Ge-doping or In-doping on the Sn site of Cu 3 SbSe 4 was carried out, and the ZT value was enhanced to some extent [22,23].Gallium has been shown to be a promising dopant in copper-based chalcogenide systems [24], but very little literature on Cu 3 SbSe 4 has been reported.In the present work, the Ga substation on the Sb site is investigated in synthesized Cu 3 Sb 1−x Ga x Se 4 compounds, and our experimental results demonstrate that Ga-doping can effectively optimize carrier concentration (p) and decrease κ simultaneously.The paper investigated the phase composition, microstructure, and transport properties of Cu 3 Sb 1−x Ga x Se 4 compounds.The highest ZT of 0.54 was obtained for the Cu 3 Sb 0.985 Ga 0.015 Se 4 compound.

Experimental Procedures
Cu 3 Sb 1−x Ga x Se 4 (x = 0, 0.005, 0.010, 0.015) compounds were conventionally synthesized via melting, annealing, grinding, and spark plasma sintering (SPS).The stoichiometric mixtures of pure elements Cu (powder, 99.98%), Sb (powder, 99.998%), Ga (granule, 99.998%), and Se (granule, 99.998%) were loaded in a graphite crucible.Then, the graphite crucible was sealed in a quartz tube, heated to 1173 K, and left for 720 min.The quartz tube was slowly cooled to 773 K at the rate of 0.5 K/min and subsequently quenched in salt water.Then, the sample was annealed at 573 K and the holding time is 48 h to ensure homogeneity.Lastly, the resultant alloys were ground in ethyl alcohol in an agate mortar.The obtained powder was sintered via SPS at 683 K in a vacuum of 0.1 Pa.The axial pressure and holding time were 50 MPa and 5 min, respectively.The Archimedes method was adopted to measure the density (d) of samples.
X-ray diffractometer equipment with Cu K α radiation (Rigaku Rint 2000) was used to analyze the phase composition of the Cu 3 Sb 1−x Ga x Se 4 samples.Scanning electron microscopy (SEM, JXA-8200, JEOL, Tokyo, Japan) was employed to characterize the microstructure of Cu 3 Sb 1−x Ga x Se 4 samples.ZEM-3 apparatus (ULVAC-RIKO, Yokohama, Japan) was used to measure the σ and α in the temperature range of 300-650 K in an argon atmosphere.The measurement of thermal diffusivity (λ) of Cu 3 Sb 1−x Ga x Se 4 compounds was carried out using a laser flash equipment (Netzsch, LFA427) in an argon atmosphere under a vacuum of 0.001 Pa.A differential scanning calorimetry (Netzsch, DSC404, Munich, Germany) was used to measure the specific heat capacity (C p ) of Cu 3 Sb 1−x Ga x Se 4 compounds.The thermal conductivity was then obtained by the equation κ = dλC p .Van der Pauw's method was adopted to measure the Hall coefficient (R H ). Hall measurement was carried out in a vacuum of 0.1 Pa with a constant magnetic strength of 0.5 T. The p can be calculated using the equation p H = 1/(R H e), where e is the electron charge.The µ H was obtained using the equations of µ H = R H σ.

XRD Analysis and Microstructure
The X-ray diffraction patterns of Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds is present in Figure 1.All major XRD peaks coincide well with the stand JCPDS card of Cu 3 SbSe 4 (No.01-085-0003).Therefore, the Ga-doped Cu 3 Sb 1−x Ga x Se 4 compounds are single phase and have the same crystallographic structure with a pure Cu 3 SbSe 4 phase.In addition, no impurity phase was detected in the XRD results, suggesting the amount of Ga-doping in this study is within the doping limit.However, as the Ga content in Cu 3 Sb 1−x Ga x Se 4 increases, no obvious peak shift is found.On the one hand, the Ga content is very low; on the other hand, it is possibly related with the similar atomic radius of Ga and Sb.Chen et al. synthesized the Cu 3 Sb 1−x Ge x Se 4 compounds and the small atomic radius of Ge resulted in a decrease in the lattice constant of the Cu 3 Sb 1−x Ge x Se 4 compounds [22].The SEM image and elemental distribution maps, including Cu, Sb, and Se elements for the Cu 3 Sb 0.985 Ga 0.015 Se 4 compound is displayed in Figure 2. It can be seen that each element (Cu, Sb and Se) was uniform with no notable brighter regions, indicating that all elements distributed homogeneously in the matrix.Meanwhile, no visible other phase can be found in the SEM, which is also in agreement with the XRD result above.
Energies 2017, 10, 1524 3 of 8 Therefore, the Ga-doped Cu3Sb1−xGaxSe4 compounds are single phase and have the same crystallographic structure with a pure Cu3SbSe4 phase.In addition, no impurity phase was detected in the XRD results, suggesting the amount of Ga-doping in this study is within the doping limit.However, as the Ga content in Cu3Sb1−xGaxSe4 increases, no obvious peak shift is found.On the one hand, the Ga content is very low; on the other hand, it is possibly related with the similar atomic radius of Ga and Sb.Chen et al. synthesized the Cu3Sb1−xGexSe4 compounds and the small atomic radius of Ge resulted in a decrease in the lattice constant of the Cu3Sb1−xGexSe4 compounds [22].The SEM image and elemental distribution maps, including Cu, Sb, and Se elements for the Cu3Sb0.985Ga0.015Se4compound is displayed in Figure 2. It can be seen that each element (Cu, Sb and Se) was uniform with no notable brighter regions, indicating that all elements distributed homogeneously in the matrix.Meanwhile, no visible other phase can be found in the SEM, which is also in agreement with the XRD result above.Therefore, the Ga-doped Cu3Sb1−xGaxSe4 compounds are single phase and have the same crystallographic structure with a pure Cu3SbSe4 phase.In addition, no impurity phase was detected in the XRD results, suggesting the amount of Ga-doping in this study is within the doping limit.However, as the Ga content in Cu3Sb1−xGaxSe4 increases, no obvious peak shift is found.On the one hand, the Ga content is very low; on the other hand, it is possibly related with the similar atomic radius of Ga and Sb.Chen et al. synthesized the Cu3Sb1−xGexSe4 compounds and the small atomic radius of Ge resulted in a decrease in the lattice constant of the Cu3Sb1−xGexSe4 compounds [22].The SEM image and elemental distribution maps, including Cu, Sb, and Se elements for the Cu3Sb0.985Ga0.015Se4compound is displayed in Figure 2. It can be seen that each element (Cu, Sb and Se) was uniform with no notable brighter regions, indicating that all elements distributed homogeneously in the matrix.Meanwhile, no visible other phase can be found in the SEM, which is also in agreement with the XRD result above.

Electrical Performance
The temperature dependence of σ for the Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds is present in Figure 3.As the temperature increases, the σ of the Cu 3 Sb 1−x Ga x Se 4 samples increases, indicating typical heavily doped semiconducting behavior.Moreover, the σ of these samples increases as the Ga content increases.The improvement in σ for Cu 3 Sb 1−x Ga x Se 4 should be ascribed to an increase in carrier concentration (p) resulting from the Ga-doping.The calculated carrier concentration of pure Cu 3 SbSe 4 was about 1.90 × 10 18 cm −3 .The thermoelectric properties and structural parameters of Cu 3 Sb 1−x Ga x Se 4 compounds at room temperature are listed in Table 1.The hole concentration of Ga-doped Cu 3 Sb 1−x Ga x Se 4 is higher than that of pure Cu 3 SbSe 4 .The hole concentration increases rapidly from 1.90 × 10 18 to 12.7 × 10 18 cm −3 when the Ga content increases from 0 to 0.015.Meanwhile, the corresponding µ H decreases from cm 2 /Vs for pure Cu 3 SbSe 4 to 30.8 cm 2 /Vs for the Cu 3 Sb 0.985 Ga 0.015 Se 4 sample.The extra ionized impurity scattering and alloy scattering should result in a decrease in µ H .The µ H of the Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds is present in Figure 4.It can be seen that the µ H of these compounds shows a gradual downward trend with the increase in temperature.In addition, the relationship of µ H ∞ T −3/2 can be found at high temperature, which indicates that the dominant scattering mechanism of Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds is phonon scattering.As the Ga content increases, the relationship of µ H ∞ T −3/2 of these compounds becomes weak, indicating that the dominant mechanism is mixed scattering for the Cu 3 Sb 1−x Ga x Se 4 compounds at high temperature [8].Moreover, the µ H of Ga-doped Cu 3 Sb 1−x Ga x Se 4 samples in this study is between 30 and 40 cm 2 V −1 s −1 at room temperature.Shi et al. calculated the µ H of Cu 2 SnSe 3 materials, and the results showed that the µ H was about 52 cm 2 V −1 s −1 at room temperature.The similar Hall mobility is possibly related to the similar density of states effective mass [25,26].

Electrical Performance
The temperature dependence of σ for the Cu3Sb1−xGaxSe4 (0 ≤ x ≤ 0.015) compounds is present in Figure 3.As the temperature increases, the σ of the Cu3Sb1−xGaxSe4 samples increases, indicating typical heavily doped semiconducting behavior.Moreover, the σ of these samples increases as the Ga content increases.The improvement in σ for Cu3Sb1−xGaxSe4 should be ascribed to an increase in carrier concentration (p) resulting from the Ga-doping.The calculated carrier concentration of pure Cu3SbSe4 was about 1.90 × 10 18 cm −3 .The thermoelectric properties and structural parameters of Cu3Sb1−xGaxSe4 compounds at room temperature are listed in Table 1.The hole concentration of Ga-doped Cu3Sb1−xGaxSe4 is higher than that of pure Cu3SbSe4.The hole concentration increases rapidly from 1.90 × 10 18 to 12.7 × 10 18 cm −3 when the Ga content increases from 0 to 0.015.Meanwhile, the corresponding μH decreases from 76.2 cm 2 /Vs for pure Cu3SbSe4 to 30.8 cm 2 /Vs for the Cu3Sb0.985Ga0.015Se4sample.The extra ionized impurity scattering and alloy scattering should result in a decrease in μH.The μH of the Cu3Sb1−xGaxSe4 (0 ≤ x ≤ 0.015) compounds is present in Figure 4.It can be seen that the μH of these compounds shows a gradual downward trend with the increase in temperature.In addition, the relationship of μH ∞ T −3/2 can be found at high temperature, which indicates that the dominant scattering mechanism of Cu3Sb1−xGaxSe4 (0 ≤ x ≤ 0.015) compounds is phonon scattering.As the Ga content increases, the relationship of μH ∞ T −3/2 of these compounds becomes weak, indicating that the dominant mechanism is mixed scattering for the Cu3Sb1−xGaxSe4 compounds at high temperature [8].Moreover, the μH of Ga-doped Cu3Sb1−xGaxSe4 samples in this study is between 30 and 40 cm 2 V −1 s −1 at room temperature.Shi et al. calculated the μH of Cu2SnSe3 materials, and the results showed that the μH was about 52 cm 2 V −1 s −1 at room temperature.The similar Hall mobility is possibly related to the similar density of states effective mass [25,26].Figure 5 demonstrates the Seebeck coefficients for Cu3Sb1−xGaxSe4 (0 ≤ x ≤ 0.015) compounds.All of the Cu3Sb1−xGaxSe4 samples exhibit a p-type character, and the major charge carriers are holes.As the temperature increases, the α of the pure Cu3SbSe4 samples decreases, from 405 μV/K at 300 K to 291 μV/K at 650 K. Nevertheless, the α of Ga-doped samples firstly increases approximately linearly to a maximum value, and then decreases, suggesting a heavily degenerate semiconductor behavior.For example, the peak value of α for the Cu3Sb0.985Ga0.015Se4compound is 295 μV/K at 500 K.The α decreases linearly to 260 μV/K at 650 K. Similar behaviors have been reported in In-doped Cu3SbSe4 samples [26].In addition, the α of Ga-doped samples decreases as the Ga-doped content increases because of the increase in hole concentration.Generally, the Seebeck coefficient can be written as: where m* is the density of states effective mass, h is Planck's constant, and kB is Boltzmann constant [4][5][6].As the increase in hole concentration has a more significant effect than the increase in the density of states effective mass (m*, Table 1), the α of the Ga-doped Cu3Sb1−xGaxSe4 compounds decreases as Ga-doped content increases.As the temperature increases, the α of the pure Cu 3 SbSe 4 samples decreases, from 405 µV/K at 300 K to 291 µV/K at 650 K. Nevertheless, the α of Ga-doped samples firstly increases approximately linearly to a maximum value, and then decreases, suggesting a heavily degenerate semiconductor behavior.For example, the peak value of α for the Cu 3 Sb 0.985 Ga 0.015 Se 4 compound is 295 µV/K at 500 K.The α decreases linearly to 260 µV/K at 650 K. Similar behaviors have been reported in In-doped Cu 3 SbSe 4 samples [26].In addition, the α of Ga-doped samples decreases as the Ga-doped content increases because of the increase in hole concentration.Generally, the Seebeck coefficient can be written as: where m* is the density of states effective mass, h is Planck's constant, and k B is Boltzmann constant [4][5][6].As the increase in hole concentration has a more significant effect than the increase in the density of states effective mass (m*, Table 1), the α of the Ga-doped Cu 3 Sb 1−x Ga x Se 4 compounds decreases as Ga-doped content increases.Figure 5 demonstrates the Seebeck coefficients for Cu3Sb1−xGaxSe4 (0 ≤ x ≤ 0.015) compounds.All of the Cu3Sb1−xGaxSe4 samples exhibit a p-type character, and the major charge carriers are holes.As the temperature increases, the α of the pure Cu3SbSe4 samples decreases, from 405 μV/K at 300 K to 291 μV/K at 650 K. Nevertheless, the α of Ga-doped samples firstly increases approximately linearly to a maximum value, and then decreases, suggesting a heavily degenerate semiconductor behavior.For example, the peak value of α for the Cu3Sb0.985Ga0.015Se4compound is 295 μV/K at 500 K.The α decreases linearly to 260 μV/K at 650 K. Similar behaviors have been reported in In-doped Cu3SbSe4 samples [26].In addition, the α of Ga-doped samples decreases as the Ga-doped content increases because of the increase in hole concentration.Generally, the Seebeck coefficient can be written as: where m* is the density of states effective mass, h is Planck's constant, and kB is Boltzmann constant [4][5][6].As the increase in hole concentration has a more significant effect than the increase in the density of states effective mass (m*, Table 1), the α of the Ga-doped Cu3Sb1−xGaxSe4 compounds decreases as Ga-doped content increases.

Thermal Conductivity
Figure 6a,b show the temperature dependences of the total thermal conductivity κ and the phonon part κ l for Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds, respectively.The κ of all Cu 3 Sb 1−x Ga x Se 4 compounds decreases as the temperature increases.In addition, the κ of Ga-doped Cu 3 Sb 1−x Ga x Se 4 samples is markedly lower that of pure Cu 3 SbSe 4 , which should be attributed to the decrease in κ l , resulting from the increase of point defect scattering.The κ of the material consists of a carrier part (κ c ) and a lattice part (κ l ).The electron part (κ c ) can be obtained using the Wiedemann-Franz equation, κ c = L 0 σT, where L 0 is the Lorenz number.As the Lorenz number varies with the temperature and the composition of materials, the precise Lorenz number is adopted according to the method in [27].Therefore, the κ l can be obtained by subtracting the κ c from the κ.The κ l of Ga-doped Cu 3 Sb 1−x Ga x Se 4 compounds drastically decreases with increasing Ga content, as shown in Figure 6b.In addition, the κ l shows a temperature dependence of T −1 , as illustrated by the blue dotted line, indicating that phonon-phonon scattering is the dominant scattering for the pure Cu 3 SbSe 4 sample and the Ga-doped Cu 3 Sb 1−x Ga x Se 4 samples.For the Cu 3 Sb 0.985 Ga 0.015 Se 4 sample, the κ l is 2.27 W/mK at room temperature, which is reduced by 30% than that of pure Cu 3 SbSe 4 .The minimum κ l of the Cu 3 Sb 0.985 Ga 0.015 Se 4 sample in this study is 0.62 W/mK at 650K.As far as is known, the theoretical minimal value of lattice thermal conductivity, κ lmin , can be evaluated according to the equation κ lmin = 1/3lν m C v , where l, C v , and ν m are the mean free path of the phonon, the isochoric specific heat, and the mean sound velocity, respectively.The red dashed line in Figure 6b presents the theoretical minimal value of lattice thermal conductivity for pure Cu 3 SbSe 4 and the obtained κ lmin is 0.47 W m −1 K −1 , as shown in the red dashed line.It also can be concluded from Figure 6b that there is still a potential possibility to further decrease the κ l of the Cu 3 SbSe 4 compound.

Thermal Conductivity
Figures 6a,b show the temperature dependences of the total thermal conductivity κ and the phonon part κl for Cu3Sb1−xGaxSe4 (0 ≤ x ≤ 0.015) compounds, respectively.The κ of all Cu3Sb1−xGaxSe4 compounds decreases as the temperature increases.In addition, the κ of Ga-doped Cu3Sb1−xGaxSe4 samples is markedly lower that of pure Cu3SbSe4, which should be attributed to the decrease in κl, resulting from the increase of point defect scattering.The κ of the material consists of a carrier part (κc) and a lattice part (κl).The electron part (κc) can be obtained using the Wiedemann-Franz equation, κc = L0σT, where L0 is the Lorenz number.As the Lorenz number varies with the temperature and the composition of materials, the precise Lorenz number is adopted according to the method in [27].Therefore, the κl can be obtained by subtracting the κc from the κ.The κl of Ga-doped Cu3Sb1−xGaxSe4 compounds drastically decreases with increasing Ga content, as shown in Figure 6b.In addition, the κl shows a temperature dependence of T −1 , as illustrated by the blue dotted line, indicating that phonon-phonon scattering is the dominant scattering for the pure Cu3SbSe4 sample and the Ga-doped Cu3Sb1−xGaxSe4 samples.For the Cu3Sb0.985Ga0.015Se4sample, the κl is 2.27 W/mK at room temperature, which is reduced by 30% than that of pure Cu3SbSe4.The minimum κl of the Cu3Sb0.985Ga0.015Se4sample in this study is 0.62 W/mK at 650K.As far as is known, the theoretical minimal value of lattice thermal conductivity, κlmin, can be evaluated according to the equation κlmin = 1/3lνmCv, where l, Cv, and νm are the mean free path of the phonon, the isochoric specific heat, and the mean sound velocity, respectively.The red dashed line in Figure 6b presents the theoretical minimal value of lattice thermal conductivity for pure Cu3SbSe4 and the obtained κlmin is 0.47 W m −1 K −1 , as shown in the red dashed line.It also can be concluded from Figure 6b that there is still a potential possibility to further decrease the κl of the Cu3SbSe4 compound.

Figure of Merit
The ZT value for Cu3Sb1−xGaxSe4 (0 ≤ x ≤ 0.015) compounds is present in Figure 7.The ZT value of Cu3SbSe4 sample increases from 0.02 to 0.18 in the temperature ranged from room temperature to 650 K. Compared with the ZT value of pure Cu3SbSe4, the ZT of Ga-doped Cu3Sb1−xGaxSe4 sample is obviously improved.For example, the ZT of the Cu3Sb0.995Ga0.005Se4compound is 0.36 at 650 K, which is one higher than the ZT of pure Cu3SbSe4.The maximal ZT value of the Cu3Sb0.985Ga0.015Se4compound can reach 0.54 at 650 K, which is about 3 times as large as that of the pure Cu3SbSe4 compound.

Figure of Merit
The ZT value for Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds is present in Figure 7.The ZT value of Cu 3 SbSe 4 sample increases from 0.02 to 0.18 in the temperature ranged from room temperature to 650 K. Compared with the ZT value of pure Cu 3 SbSe 4 , the ZT of Ga-doped Cu 3 Sb 1−x Ga x Se 4 sample is obviously improved.For example, the ZT of the Cu 3 Sb 0.995 Ga 0.005 Se 4 compound is 0.36 at 650 K, which is one higher than the ZT of pure Cu 3 SbSe 4 .The maximal ZT value of the Cu 3 Sb 0.985 Ga 0.015 Se 4 compound can reach 0.54 at 650 K, which is about 3 times as large as that of the pure Cu 3 SbSe 4 compound.

Conclusions
In this study, p-type Ga-doped Cu3Sb1−xGaxSe4 compounds were fabricated by melting, annealing, and SPS.Compared with a pure Cu3SbSe4 compound, Ga-doped Cu3Sb1−xGaxSe4 compounds showed a large increase in electrical conductivity resulting from the substantial increase in carrier concentration.However, the Seebeck coefficient of the Cu3Sb1−xGaxSe4 compounds decreased as the Ga content increased.The Seebeck coefficient of Ga-doped samples firstly increased approximately linearly to a maximum value and then decreased.Meanwhile, the thermal conductivity of the Cu3Sb1−xGaxSe4 compounds markedly decreased because of the extra phonon scattering originating from the Ga-doping on the Sb site.Therefore, the increased electrical conductivity and the depressed lattice thermal conductivity effectively enhanced the ZT value of Cu3SbSe4.The maximum ZT value for the Cu3Sb0.985Ga0.015Se4compounds was 0.54 at 650 K, which is around two times larger than that of pure Cu3SbSe4 compounds.

Conclusions
In this study, p-type Ga-doped Cu 3 Sb 1−x Ga x Se 4 compounds were fabricated by melting, annealing, and SPS.Compared with a pure Cu 3 SbSe 4 compound, Ga-doped Cu 3 Sb 1−x Ga x Se 4 compounds showed a large increase in electrical conductivity resulting from the substantial increase in carrier concentration.However, the Seebeck coefficient of the Cu 3 Sb 1−x Ga x Se 4 compounds decreased as the Ga content increased.The Seebeck coefficient of Ga-doped samples firstly increased approximately linearly to a maximum value and then decreased.Meanwhile, the thermal conductivity of the Cu 3 Sb 1−x Ga x Se 4 compounds markedly decreased because of the extra phonon scattering originating from the Ga-doping on the Sb site.Therefore, the increased electrical conductivity and the depressed lattice thermal conductivity effectively enhanced the ZT value of Cu 3 SbSe 4 .The maximum ZT value for the Cu 3 Sb 0.985 Ga 0.015 Se 4 compounds was 0.54 at 650 K, which is around two times larger than that of pure Cu 3 SbSe 4 compounds.

Figure 2 .
Figure 2. SEM image and elemental distribution maps of Cu 3 Sb 0.985 Ga 0.015 Se 4 compounds.

Figure 5
Figure 5 demonstrates the Seebeck coefficients for Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds.All of the Cu 3 Sb 1−x Ga x Se 4 samples exhibit a p-type character, and the major charge carriers are holes.As the temperature increases, the α of the pure Cu 3 SbSe 4 samples decreases, from 405 µV/K at 300 K to 291 µV/K at 650 K. Nevertheless, the α of Ga-doped samples firstly increases approximately linearly to a maximum value, and then decreases, suggesting a heavily degenerate semiconductor behavior.For example, the peak value of α for the Cu 3 Sb 0.985 Ga 0.015 Se 4 compound is 295 µV/K at 500 K.The α decreases linearly to 260 µV/K at 650 K. Similar behaviors have been reported in In-doped Cu 3 SbSe 4 samples[26].In addition, the α of Ga-doped samples decreases as the Ga-doped content increases because of the increase in hole concentration.Generally, the Seebeck coefficient can be written as:

Table 1 .
Thermoelectric properties and structural parameter of Ga-doped Cu 3 Sb 1−x Ga x Se 4 (0 ≤ x ≤ 0.015) compounds at room temperature.