Structural Phase Transition and Related Thermoelectric Properties in Sn Doped AgBiSe₂

Abstract

AgBiSe₂, which exhibits complex structural phase transition behavior, has recently been considered as a potential thermoelectric material due to its intrinsically low thermal conductivity. In this work, we investigate the crystal structure of Sn-doped AgBiSe₂ through powder X-ray diffraction and differential scanning calorimetry measurements. A stable cubic Ag_1−x/2Bi_1−x/2Sn_xSe₂ phase can be obtained at room temperature when the value of x is larger than 0.2. In addition, the thermoelectric properties of Ag_1−x/2Bi_1−x/2Sn_xSe₂ (x = 0.2, 0.25, 0.3, 0.35) are investigated, revealing that Ag_1−x/2Bi_1−x/2Sn_xSe₂ compounds are intrinsic semiconductors with a low lattice thermal conductivity. This work provides new insights into the crystal structure adjustment of AgBiSe₂ and shows that Ag_1−x/2Bi_1−x/2Sn_xSe₂ is a potentially lead-free thermoelectric material candidate.

1. Introduction

As devices that can convert heat and electricity, thermoelectric devices have received significant attention and have become a research hotspot in recent years [1,2,3]. The thermoelectric properties of materials can be evaluated by using the dimensionless quantity ZT. The value of ZT can be calculated with the formula S²σT/(к_ele + к_latt), where S, σ, T, к_ele, and к_latt are the Seebeck coefficient, electrical conductivity, temperature, electronic thermal conductivity, and lattice thermal conductivity, respectively. Accordingly, semiconductors with low lattice thermal conductivities have been widely studied and show excellent thermoelectric performance. These semiconductors include Zintl phases [4,5], fast ion conductors [6,7], complex oxides [8,9], and chalcogenides [10].

Ternary chalcogenides with the I-V-VI₂ formula (I = Cu, Ag; V = As, Sb, Bi; VI = S, Se, Te) have an intrinsically ultra-low lattice thermal conductivity due to their unique crystal structures [11,12,13]. For example, due to the anharmonicity caused by the repulsion between neighboring chalcogen ions and lone-pair electrons, the lattice thermal conductivity of CuSbS₂ is only about 0.5 W·m⁻¹·K⁻¹ at 627 K [14]. In AgSbSe₂ and AgSbTe₂, the mixing of Ag and Sb atoms can lead to a further decline in lattice thermal conductivity [15,16,17,18]. I-V-VI₂ compounds have a wide range of structural diversity. The crystal structure of Cu-containing compounds such as CuSbS₂ and CuSbSe₂ can be viewed as the stacking of [CuSb(S/Se)₂] layers in an AᾹAᾹ-type sequence along the c axis direction [19,20]. AgSbSe₂ and AgSbTe₂ crystallize in the cubic space group Fm$\overline{3}$m with disordered Ag and Sb cations [21], while AgBiVI₂ (VI = S, Se, Te) compounds exhibit complex temperature-dependent phase transition behavior [22,23,24,25].

AgBiSe₂ compound exists in three polymorphs: a disordered cubic phase, an ordered hexagonal phase, and a rhombohedral phase. Their structures are shown in Figure 1. As an n-type semiconductor with poor electrical conductivity, the thermoelectric properties of AgBiSe₂ can be enhanced by increasing the charge carrier concentration via doping In, Nb, or Ge at Ag sites [26,27,28] or by doping halogen elements at Se sites [29]. Both experimental and density functional theory calculation results indicate that p-type hexagonal AgBiSe₂ can potentially be used in room-temperature thermoelectric applications [30,31]. Additionally, the phase transition temperatures of AgBi_1−xSb_xSe₂ are determined by the doping concentration of Sb [32,33]. Very recently, Br-doped cubic (AgBiSe2)_0.7(PbSe)_0.3 phase has been proved to be potential material with fine thermoelectric properties in the range from 300 to 800 K [34]. The stable cubic AgBiSe₂ phase may potentially exhibit excellent thermoelectric properties due to its intrinsic crystal structures and related electronic structures. The high symmetry can result in energy band degeneracy, and thus lead to high power factor, while the material with disordered atoms usually exhibits ultra-low lattice thermal conductivity [35]. Accordingly, further investigation into methods for achieving a stable cubic AgBiSe₂ phase and enhancing its thermoelectric properties are of great interest.

Inspired by the research on (AgBiSe2)_1−x(PbSe)_x, we focus on another IV-A metal and find that a stabilized room-temperature cubic AgBiSe₂ phase can also be achieved via Sn substitution. In previous studies, both AgSbSe₂ and PbSe have cubic crystal structures, so it is not hard to understand the phase transition behavior in (AgBiSe₂)_1−x(AgSbSe₂)_x and (AgBiSe₂)_1−x(PbSe)_x solid solutions [32,33,34]. Herein, although the crystal structure of SnSe is not cubic [36], the solid solution between SnSe and AgBiSe₂ has a cubic structure at room temperature. Such a finding indicates that in-depth studies on structural phase transition and its related physical properties are needed. In addition, in this work the thermoelectric properties were also investigated, and the results indicate that Ag_1−x/2Bi_1−x/2Sn_xSe₂ is a potentially lead-free thermoelectric material with ultra-low lattice thermal conductivity.

2. Materials and Methods

Ag_1−x/2Bi_1−x/2Sn_xSe₂ samples were prepared via high-temperature solid-state reactions. Stoichiometric amounts of Ag (Alfa, 99.9%), Bi (Aladdin, 99.999%), Sn (Alfa, 99.99%), and Se (Alfa, 99.99%) were cut into small pieces and weighed in an argon-filled glovebox. The elements were mixed and loaded into evacuated silica tubes. The tubes were then heated to 1273 K at a rate of 80 K/h and homogenized at this temperature for 20 h using a programmable furnace. Finally, the tubes were slowly cooled down to room temperature over 20 h. The synthesized ingots were ground into fine powders for use in further measurements.

Powder X-ray diffraction (PXRD) patterns of Ag_1−x/2Bi_1−x/2Sn_xSe₂ were measured with a step size of 0.02° by using a Bruker D8 Advance X-ray powder diffractometer at room temperature and Cu Kα radiation. Rietveld refinements of AgBiSe₂, Ag_0.975Bi_0.975Sn_0.05Se₂, and Ag_0.95Bi_0.95Sn_0.10Se₂ were performed using Fullprof [37]. Differential scanning calorimetry (DSC) measurements were performed on the polycrystalline powders of Ag_1−x/2Bi_1−x/2Sn_xSe₂ (35–45 mg) using a NETZSCH STA 449 F3. DSC measurements were taken over a temperature range of 350 K to 650 K. The heating rate was 10 K/min. The microstructure and energy-dispersive spectroscopy (EDS) mapping of Ag_0.90Bi_0.90Sn_0.20Se₂ sample were carried out using a Zeiss Sigma 500 field emission scanning electron microscopy (SEM).

Ag_1−x/2Bi_1−x/2Sn_xSe₂ (x = 0.2, 0.25, 0.3, 0.35) powders were sintered into pellets with high relative densities by using a LABOX-325 spark plasma sintering instrument (Dr. Sinter Land). An axial compressive stress of 40 MPa was applied at 773 K for 7 min under 10 Pa. The Seebeck coefficients and electrical conductivity values at temperatures of 300 K to 773 K were measured using a NETZSCH SBA 458 instrument with a temperature gradient of 4 K across ~8.25 mm. Thermal conductivity was calculated using the standard formula κ = C_pDρ, where C_p is the specific heat calculated by the Dulong-Petit law, D is the thermal diffusivity measured using the laser flash method (NETZSCH LFA 457), and ρ is the measured mass density.

3. Results and Discussion

Theoretical diffraction patterns for hexagonal, rhombohedral, and cubic AgBiSe₂ are shown in Figure 2a, while Figure 2b shows PXRD patterns for Ag_1−x/2Bi_1−x/2Sn_xSe₂ (x = 0, 0.1, 0.15, 0.2, 0.25, 0.30, 0.35). The measured diffraction patterns of AgBiSe₂ are completely consistent with the theoretical patterns of the hexagonal phase, indicating that the pristine AgBiSe₂ is phase pure. The diffraction patterns change with increasing Sn concentration in the Sn-doped compounds. For instance, the (110) diffraction pattern at 43.26° and (018) diffraction pattern at 44.52° become closer with an increasing Sn ratio. Meanwhile, some small patterns such as (003) at 13.5°, (101) at 24.98°, and (006) at 27.18° disappear when x is higher than 0.15. The PXRD patterns for the samples with high Sn concentrations are similar to the theoretical diffraction patterns of cubic AgBiSe₂, revealing that a stable cubic phase can be achieved at room temperature via Sn doping. There are no impurity patterns in the measured data, indicating that solid solutions were formed in all the samples. It should be noticed that although the diffraction patterns of Ag_0.925Bi_0.925Sn_0.15Se₂ are consistent with the cubic phase, the broad pattern at about 43.82° potentially hints that Ag_0.925Bi_0.925Sn_0.15Se₂ is an intermediate phase between the hexagonal and cubic phases. Accordingly, only the thermoelectric properties of cubic Ag_1−x/2Bi_1−x/2Sn_xSe₂ (x = 0.2, 0.25, 0.3, 0.35) are discussed in this paper.

As shown in Figure 2c, the lattice parameters determined from the experimental PXRD patterns increase linearly with increasing Sn concentration. This is consistent with Vegard’s law. In general, variations in the lattice parameter of chalcogenide semiconductors can be explained by ionic radius differences between the intrinsic and doped elements. Since the ionic radius of Sn²⁺ is difficult to define [38], the increased lattice parameter cannot be directly explained by comparing ionic radii. However, a cubic SnSe phase which crystallizes in a rock salt structure may potentially help understand this change in lattice parameter [39]. The Sn-Se bonding distance in cubic SnSe is about 2.995 Å, much longer than the Ag/Bi-Se distance of 2.916 Å in cubic AgBiSe₂. Thus, the ionic radius of Sn²⁺ in the rock salt structure should be larger than the average ionic radius between Ag⁺ and Bi³⁺.

DSC measurements were performed for Ag_1−x/2Bi_1−x/2Sn_xSe₂ (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) in order to understand the polymorph change at room temperature. The results are shown in Figure 3. For pristine AgBiSe₂, one small peak at about 500 K indicates the phase transition from a hexagonal to a rhombohedral phase, while the sharp peak at 586 K represents the transformation to a cubic phase. For Sn-containing materials, the temperature corresponding to the sharp endothermic peak decreases with increasing Sn concentrations. Therefore, the thermal energy required to overcome the potential energy between the hexagonal and cubic phases also decreases. For Ag_0.925Bi_0.925Sn_0.15Se₂, a small and flat endothermic peak at about 520 K indicates the existence of the hexagonal phase, consistent with the prior discussion of PXRD data. The finding is interesting since neither AgBiSe₂ nor SnSe has a cubic structure at room temperature. Accordingly, in order to understand the phenomenon, detailed analysis of the crystal structures for Ag-V-Se (V = As, Sb, Bi) compounds was performed.

AgBiSe₂ shows multiple crystal phase transitions with increasing temperature, transforming from ordered hexagonal P$\overline{3}$m1 to rhombohedral R$\overline{3}$mR and then to the disordered rock salt structure [21]. AgSbSe₂ is a cubic phase material with mixed Ag and Sb atoms [21], while AgAsSe₂ crystallizes in the rhombohedral space group R$\overline{3}$mH [40]. Although these compounds crystallize in different space groups, their crystal structures are very similar, as shown in Figure 4. It is clear that the difference between Ag-Se and V-Se bonding distances plays a major role in determining the crystal structure of Ag-V-Se materials. In order to describe the bonding distance difference simply, the ratios between average Ag-Se and V-Se bonding distances in octahedrons are shown in the figure. The ratios of r_avg(Ag-Se)/r_avg(V-Se) for AgBiSe₂, AgSbSe₂ and AgAsSe₂ are 0.95, 1.00, and 1.09, respectively. The ratios affect the Se-As-Se angle directly, and further determine the distortion degree of the octahedral environments. The longer Bi-Se distance in AgBiSe₂ leads to a Se-Bi-Se angle of 87.2°. In contrast, the shorter As-Se distance of 2.710 Å in AgAsSe₂ results in a larger Se-As-Se angle (92.5°). Accordingly, for AgVSe₂ compounds, adjustment of Ag-Se and V-Se distances can change the crystal structure. The Sn-Se bonding distance is longer than the Ag-Se distances but shorter than the Bi-Se distances. In Ag_1−x/2Bi_1−x/2Sn_xSe₂, the bonding distance difference between Ag/Sn-Se and Bi/Sn-Se decreases with increasing Sn ratio. So the crystal structure of AgBiSe₂ can be transformed to cubic phase via Sn doping. The Rietveld refinement results of Ag_1−x/2Bi_1−x/2Sn_xSe₂ (x = 0, 0.05, 0.10) are consistent with the above analysis. As is shown in Figure S1, the Se-Bi-Se angle increases with increasing x value. Considering that these structures are extremely complex, further experimental studies on the crystal structures of metal-doped AgBiVI₂ (VI = S, Se, Te) are still needed.

As shown in Figure S2, there is no obvious pore or crack in the cross-sectional image, suggesting high density of the sintered sample. The distribution for all elements is homogeneous, indicating that the material is phase pure. Figure 5 shows the measured temperature-dependent thermoelectric transport properties of Ag_1−x/2Bi_1−x/2Sn_xSe₂. Tin doping leads to significant changes in both the Seebeck coefficient and electrical conductivity. Due to the phase transition behaviour, both Seebeck coefficient and electrical conductivity of pristine AgBiSe₂ do not change monotonically with increasing temperature. Pristine AgBiSe₂ is an intrinsic n-type semiconductor with a negative Seebeck coefficient over the entire measured temperature range. The Seebeck coefficients of the Sn-doped materials are positive at room temperature, indicating that the dominating charge carriers are holes. The room-temperature Seebeck coefficient of Ag_0.90Bi_0.90Sn_0.20Se₂ is about +130 μV/K, while that of Ag_0.875Bi_0.875Sn_0.25Se₂ increases to about +350 μV/K. A similar carrier type transition behaviour has been observed in AgBi_1-xSb_xSe₂ [33,34]. With increasing temperature, the Seebeck coefficients of the Ag_1−x/2Bi_1−x/2Sn_xSe₂ samples are converted into n-type negative values. The carrier type change from p-type to n-type can be attributed to the thermally generated extrinsic carriers at high temperatures. The absolute value of the Seebeck coefficients in the high-temperature region decreases with increasing Sn content. With increasing temperature, the electrical conductivity of all materials exponentially increases, indicating that the cubic crystal structure Sn-doped materials are intrinsic semiconductors. As shown in Figure S3, the logarithmic plot of ρ in the high temperature region versus the inverse temperature show linearity. According to the thermal activation model, the energy band gaps for all Sn-doped samples are calculated using the formula ln ρ_T = E_g/2k_BT + f, where E_g is the band gap, T is the absolute temperature, and k_B is the Boltzmann constant. The band gaps deduced from thermal activation model for Ag_0.90Bi_0.90Sn_0.20Se₂, Ag_0.875Bi_0.875Sn_0.25Se₂, Ag_0.85Bi_0.85Sn_0.30Se₂ and Ag_0.825Bi_0.825Sn_0.35Se₂ are 0.63 eV, 0.47 eV, 0.42 eV and 0.38 eV, respectively. The band gap gradually decreases with the increasing Sn ratio. In addition, in contrast to the changes to the Seebeck coefficients, the electrical conductivity of Ag_1−x/2Bi_1−x/2Sn_xSe₂ gradually increases with the increasing Sn ratio. It is clear that the carrier type change is related to the doping of Sn. The most likely reason for this phenomenon is that the participation of Sn modifies the intrinsic point defects of Ag_1−x/2Bi_1−x/2Sn_xSe₂. However, it is very difficult to confirm this conjecture from theoretical calculations or experiment measurements due to the disordered atomic distribution of these materials. The electrical conductivity of Sn-doped AgBiSe₂ falls in the range from 110 S/cm to 150 S/cm at 773 K, which is comparable with cubic (AgBiSe₂)_1−x(PbSe)_x [32]. Due to the change of carrier type, an extreme power factor (PF) value is observed at about 373 K. The maximum PF value of the Ag_1−x/2Bi_1−x/2Sn_xSe₂ materials with p-type transport properties is about 0.8 μW/cm·K², while Ag_0.875Bi_0.875Sn_0.25Se₂ exhibits the maximum n-type PF value of ~1.2 μW/cm·K² at 773 K.

The measured total thermal conductivity of Ag_1−x/2Bi_1−x/2Sn_xSe₂ samples is shown in Figure 6a. The room temperature thermal conductivity of Ag_1−x/2Bi_1−x/2Sn_xSe₂ ranges from 0.5–0.6 W/m·K, lower than the measured thermal conductivity of pristine AgBiSe₂ due to the enhanced point defect scattering. It should be noticed that the thermal conductivity of Sn-doped samples is inclined to contain less bipolar contribution due to the coexistence of holes and electrons. The thermal conductivity contributed from lattice vibration (κ_latt) and bipolar effect (κ_bip) is calculated by subtracting the electronic thermal conductivity from total thermal conductivity. The electronic thermal conductivity (κ_ele) is estimated by using the formula κ_ele = LσT, where L, σ, and T are the Lorenz factor, electrical conductivity, and absolute temperature, respectively. The Lorenz factors are calculated using a single parabolic band (SPB) model [41]. Considering that Ag_1−x/2Bi_1−x/2Sn_xSe₂ materials may exhibit multiple band behaviour, so the SPB model can only provide approximate estimates in this work. Figure 6b shows the calculated κ_latt + κ_bip results in the range of 300 K to 773 K. Due to the enhanced point defects scattering, the lattice thermal conductivity of Sn-doped AgBiSe₂ is lower than that of pristine AgBiSe₂ in the range from 300 K to 523 K. The κ_latt + κ_bip values of Ag_0.9Bi_0.9Sn_0.2Se₂, Ag_0.875Bi_0.875Sn_0.25Se₂, and Ag_0.825Bi_0.825Sn_0.35Se₂ are in the range of 0.45–0.6 W/m·K, while that of Ag_0.85Bi_0.85Sn_0.3Se₂ is much lower (only 0.37 W/m·K at 773 K). Such a low thermal conductivity is comparable with previous studies of cubic AgBiSe₂ materials [32,33,34]. This ultra-low κ_latt + κ_bip value can be explained by two reasons, namely the increased point defect scattering caused by doping Sn and the increased phonon–phonon scattering with increasing temperature. Figure 7 presents the calculated ZT values of the Ag_1−x/2Bi_1−x/2Sn_xSe₂ samples. The final ZT values are very low compared with state-of-the-art thermoelectric materials, ranging from 0.06 to 0.14 at 773 K. However, due to the high structural symmetry and low thermal conductivity of these materials, Ag_1−x/2Bi_1−x/2Sn_xSe₂ can still be viewed as a potential thermoelectric material candidate.

4. Conclusions

In summary, the polymorph changes and thermoelectric properties of Sn-doped AgBiSe₂ were investigated. The room-temperature crystal structure of Ag_1−x/2Bi_1−x/2Sn_xSe₂ was transformed from a hexagonal structure to a disordered cubic structure when the value of x was larger than 0.20. In addition, the carrier type of Ag_1−x/2Bi_1−x/2Sn_xSe₂ at room temperature was p-type, in contrast to the the n-type conductivity of pristine AgBiSe₂. The electrical conductivity exponentially increased with increasing temperature. The thermal conductivity of Ag_1−x/2Bi_1−x/2Sn_xSe₂ was ultra-low (in the range of 0.4 to 0.8 W/m·K) over the entire measured temperature range. Due to its high structural symmetry and low thermal conductivity, Ag_1−x/2Bi_1−x/2Sn_xSe₂ is a potentially lead-free thermoelectric material candidate. However, further in-depth investigation into enhancing the power factor of this material will be necessary.