Spin-Polarized Study of the Structural, Optoelectronic, and Thermoelectric Properties of the Melilite-Type Gd₂Be₂GeO₇ Compound

Abstract

The present work is a theoretical study of the structural and spin-polarized dependent optoelectronic thermoelectric properties of the melilite-typeGd₂Be₂GeO₇ compound, using the full potential linearized augmented plane wave approach in the framework of density functional theory. The predicted structural parameters are in good accordance with the measured counterparts. It is found that the title compound is more stable in the ferromagnetic order than in the non-magnetic order. The calculated band structure using the modified Becke–Johnson potential reveals that the studied compound has a wide bandgap of 3.78 eV. The frequency-dependent linear optical spectra are studied in an energy range expanding from 0 to 30 eV. Finally, the semi classical Boltzmann theory as incorporated in the Boltztrap code is used to study the spin-polarized dependent transport properties. The obtained results show that Gd₂Be₂GeO₇ is a potential candidate for conversion energy device applications.

1. Introduction

Rare earth (RE melilite type RE₂Be₂GeO₇ compounds (RE = Pr, Nd, Gd, Yb, Dy) crystalize in a tetragonal structure with the $P\overline{4}{2}_{1 }m$space group. Yellow feldspar is a group of pyrosilicate compounds that may be found in high-temperature metamorphosed impure limestone and silica under soaked igneous rocks. [1,2,3]. They are employed in a variety of applications, including optical materials and fuel cell ingredients for the development of solid oxide fuel cell (SOFC) technology [4]. The 4f electrons in melilite compounds containing lanthanide atoms, in particular, are highly influenced by the surrounding coordination environment, and exhibit radically different behaviors compared with the single element. This structural characteristic has been widely used to create magnetic and optical materials with desired qualities [5,6]. Melilites have been a research focus for a long time [7,8,9] because of the ability to combine and crystallize atoms to achieve entirely new usabilities and allow the discovery of a compound with a new function. This opportunity is one of the most important contributions that solid-state chemistry field makes to society. Warren [10] was the first to determine the crystal structure of a synthesized melilite. Many different melilite compounds have been produced since then [11,12,13,14,15,16,17].

One of such compounds, the Gd₂Be₂GeO₇melilite type compound, has attracted attention; it was previously synthesized, and its crystal structure was characterized by Ochi et al. [18]. In addition, recently, Ashtaret et al. [19] reported the synthesis, structure, and magnetic properties of this compound and other families of melilite-type RE₂Be₂GeO₇ (RE = Pr, Nd, Dy, Yb) polycrystals using the solid-state reaction method. Few studies have highlighted the spin-polarized dependent optoelectronic and thermoelectric properties of melilite containing rare earth (RE) atoms which are considered to have a high degree of freedom of chemical composition. The discovery of new qualities resulting from the crystalline structure of the melilite-type compound was the focus of this research. The main goal of this study is to explain the spin-polarized contribution to the electronic band structure, optical response, and thermoelectric properties of Gd₂Be₂GeO₇ compound, using the FP-LAPW method based on density functional theory [20,21,22,23,24,25]. We find that the spin-polarized calculations with the modified Beck Johnson approach [26,27,28,29] raise the important wide-band gap magnetic semiconductor behavior in this compound.

This paper is organized as follows. A general introduction to the melilite compounds is given in Section 1. We summarize the significant parameters introduced in our theoretical methods used in this study in Section 2. In Section 3, we investigate the spin-polarized dependent calculations of the crystal structures, optoelectronics, and thermoelectric properties of Gd₂Be₂GeO₇ melilite-type compound. Furthermore, and in order to elucidate the optical coating properties, transmittance, reflectance, and absorbance at different thicknesses were performed. Concluding remarks are summarized in the last section.

2. Methodology and Calculation Settings

The full potential linear augmented plane wave (FP-LAPW) approach [22,23] based on density functional theory [20,21] as incorporated in the WIEN2k code [24] was used to investigate the structural, electronic, and magnetic characteristics of the melilite-type Gd₂Be₂GeO₇ compound. The GGA-PBE version of the generalized-gradient approximation (GGA) [25] and the modified Becke–Johnson potential by Tran and Blaha (TB-mBJ) [26,27,28,29] were used to treat the exchange-correlation effects. The muffin-tin sphere radius was chosen to be 1.8 Bohr for Gd and Ge, 1.4 Bohr for Be, and 1.35 Bohr for O. The energy convergence criterion was set to 10⁻⁴ Ry to ensure accurate computations. The electronic configuration of each element is taken as follows: Gd: [Xe] 6s² 4f⁷ 5d¹, Be: [He] 2s², Ge: [Ar] 4s² 3d¹⁰ 4p², and O: [He] 2s² 2p⁴. A dense Brillouin zone sampling of 18 × 18 × 28 k-mesh was used for the optical and thermoelectric characteristics. The transport parameters were determined through the semi classical Boltzmann theory as implemented in the BoltzTrap code [30].

3. Results and discussion

3.1. Structural Properties

Gd₂Be₂GeO₇ is isostructural to the melilitetype group A₂MM’₂O₇, where A symbolizes bigger cations such as alkali earth ions and lanthanides, and M and M’ denote smaller divalent to tetravalent cations. The melilite crystallizes in a tetragonal structure with space group $P\overline{4}{2}_{1}m$(SG No13) [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19], and can be represented as two-dimensional lattices in the ab-plane consisting of MO4 and M’O4 tetrahedra, with the larger A ions occupying the space between these lattices.

There are six crystallographic sites in the unit cell: one Wyckoff site 4e occupied by Gd atom, one site 2a occupied by Ge atom, one site 4e occupied by Be atom, and three Wyckoff sites 2c, 2a, 8f occupied by O atoms. Figure 1 depicts the crystal structure of melilite-type Gd₂Be₂GeO₇. Figure 2 shows that the title compound is more stable in the ferromagnetic order than in in the non-magnetic one, so only the physical properties of Gd₂Be₂GeO₇ in the ferromagnetic order were studied. The calculated total energy versus the volume data are fitted to the Birch–Murnaghan equation of state [31,32] and the obtained equilibrium structural parameters, such as the unit cell volume (V₀), lattice parameters (a, and c), bulk modulus (B₀) and its pressure derivative (B’) are summarized in

Table 1. Lattice constants a and c, bulk modulus B₀, and its pressure derivative B’ parameters optimization in both ferromagnetic (FM) and nonmagnetic orders (NM). For comparison, experimental and other theoretical values are included [18,19].

Gd2Be2GeO7	a ($\mathbf{\AA }$)	c ($\mathbf{\AA }$)	V ($\mathbf{\AA }$)	B0 (GPa)	B’	E0 (×106 eV)	Reference
	7.516	4.878	275.654	160.95	3.880	−1.371788	Present work	NM
	7.525	4.883	276.567	165.91	4.274	−1.371821	Present work	FM
	7.480	4.842	270.910	-	-	-	[18]
	7.494	4.842	271.894	-	-	-	[19]

. We notice that our results are very close to the reported experimental data [18,19].

3.2. Electronic Properties

The electron energy band structure of the Gd₂Be₂GeO₇ compound was examined at equilibrium lattice parameters, taking into consideration spin polarization, based on the stable melilite phase; the computational procedure was accomplished using PBE-GGA and PBE-GGA-mBJ techniques. This allows us to go beyond traditional GGA/LSDA methodologies and treat electron correlations more accurately, correcting the band gap and the contribution of various orbitals. The Brillouin zone of the melilite structure is represented in Figure 3, with the recommended band path taken from reference [33].

Including spin polarization with GGA-mBJ scheme, the calculated energy band dispersion and diagram of the total density of states (TDOS) of the Gd₂Be₂GeO₇ compound are illustrated in Figure 4 and Figure 5, respectively, according to the spin-up and spin-down channels. Analysis of the obtained curves shows that this compound presents an interesting band gap behavior. First, as seen in Figure 4, the material under discussion exhibits an insulating band gap behavior with an indirect Z−Γ band gap of 6.319 eV for the majority spin, while the semiconductor behavior of this compound predominates in the minority spin with a direct band gap Z−Z of 3.928 in which the VBM is located below the Fermi level.

Table 2. The HVB and the LCB electron energies in eV (in the present work), evaluated with the GGA + mBJ exchange potential according to the majority spin (spin-up) and minority spin (spin-down) panels at different high-symmetry k-points for the Gd₂Be₂GeO₇ compound.

k-Points	Majority Spin (Spin-up)		Minority Spin (Spin-down)
	HVB	LCB	HVB	LCB
Γ	−0.199	6.327	−0.183	3.792
X	−0.132	7.154	−0.246	3.792
M	−0.087	7.283	−0.212	3.796
Z	0.007	6.862	−0.139	3.788
R	−0.044	7.025	−0.191	3.79
A	−0.08	7.026	−0.164	3.803

shows the computed values for the highest valence band (HVB) and lowest conduction band (LCB) in the tetragonal BZ for the examined Gd₂Be₂GeO₇ material at various high-symmetry k-points. Therefore, the spin-polarized Gd₂Be₂GeO₇ material has a direct wide-band gap (Z_C − Z_V) of about 3.781 eV. For the first time, these results are presented in the context of the DFT technique with the spin-polarized GGA-mBJ exchange potential for the best correction of the electronic band energies and their localizations. Figure 5 displays the atom projected density of states (PDOS) for those atomic species whose orbitals may have substantial contributions around the Fermi level, spin-up and spin-down polarizations to better understand the electronic and magnetic characteristics of Gd₂Be₂GeO₇. The energy levels below the Fermi level are mainly consisting of Ge-4s4p, Be-2s as well as the Gd-p states. The electronic states of Be and O atoms cohere with each other throughout the area from the VB to the CB, and the states of Ge and O atoms are likewise totally overlapped, indicating that these orbitals strongly hybridize with each other due to interactions in the BeO₄ and GeO₄ tetrahedrons.

In addition, the occupied f states of Gadolinium fall below the O-2p valence bands, spin-up (majority), whereas unoccupied f states of Gd fall in the band gap, spin-down (minority); finally the 5d-Gdare mainly localized in the conduction band both in majority spin (spin-up) and minority spin (spin-down) channels. The obtained values of the total magnetic moment (μ) of the studied compound Gd₂Be₂GeO₇ using spin-polarized GGA + mBJ calculations are observed around 7.497 μ_B, which agrees well with those found by Ochi. et al. [18] (7.94–7.20 μ_B) and M. Ashtar et al. [19] (7.05 μ_B). As seen in

Table 3. Calculated magnetic moments of each atom, interstitial, and cell for Gd₂Be₂GeO₇ compound in μ_B unit compared to experimental data.

Gd2Be2GeO7	μInters	μGd	μBe	μGe	μO1	μO2	μO3	μTot (μB)
	0.661	6.729	0.000	0.001	0.049	0.018	0.039	7.49 Present work 7.94– 7.20 [18] 7.05 [19]

, most of the magnetic moments arise from the Gd-f states with small contributions from the interstitial region and the other constituent atoms.

3.3. Optical Properties

Knowledge of the optical properties of materials is necessary before their implication in optoelectronic device applications. Therefore, an accurate theoretical method to assess the optical properties of materials is required. The state-of-the-art FP-LAPW method [24] with the GGA + mBJ functional [25,26] might be a good theoretical tool for the investigation of the optical properties of the melilite-type Gd₂Be₂GeO₇ compound. Furthermore, adding spin-polarized calculations to this method, the optical properties of this compound give interesting discussion and useful information about its internal structure [34,35].

The dielectric function ε(ω) = ε₁ + iε₂ describes the linear response of a material to incident electromagnetic radiation, where ε₁ (the real part) describes the dispersion of radiation inside the medium and ε₂ (the imaginary part) describes the absorption of radiation by the medium [35,36]. All other optical coefficients, such as refractive indexes, reflectivity coefficient and absorption coefficient, can be calculated from the dielectric function through known relationships. Due to the tetragonal symmetry of the investigated material, all optical spectra are calculated for an electrical field vector E of the incident electromagnetic radiation polarized both parallel and perpendicular to the c-axis (E//c and E⊥c).

Our findings serve as a prediction because no previous study on the optoelectronic properties of the title compound has been made. Figure 6 depicts the ε₂spectrum for the Gd₂Be₂GeO₇ compound spin-up and spin-down channels in an energy range 0–30 eV. According to spin-polarized GGA + mBJ band structure computations, the compound under consideration’s valence band maximum and conduction band minimum exhibit divergent behavior, where it can be an insulator for spin-up polarization and a semiconductor for spin-down polarization. The ε₂ spectra for both spin-up and spin-down channels clearly show the above characteristics. The absorption edge (the first critical point of ε₂ (ω)), occurring at 3.9 eV for the minority spin channel and at 6.319 eV for the majority spin channel, is originated from the electronic transitions from the valence band maximum states to the conduction band minimum states. The band structure has a significant impact on the optical metrics given above, and our findings are consistent with the specific electronic properties outlined in the preceding section.

The ε₁ (ω)spectrum was calculated from that of ε₂ (ω) through the Kramers-Kronig relation [37,38]. In spin-polarized optics computations, the Kramers-Kronig (KK) transformation is not a simple additive quantity [34]. The real and imaginary components of the dielectric function as a function of photon energy are shown in Figure 7. The absorptive component ε₂ (ω) describes the optical transition process between the energy bands in materials. The optical spectra of materials are formed by the electronic transitions from the highest occupied orbitals in the valence band to the lowest vacant orbitals in the conduction band. The suitable optical transitions, according to the selection rules, are s → p, p → d, d → p, d → f, and so on. In the imaginary part ε₂ (ω) curve, the threshold energy of the dielectric function occurs at E₀ = 3.7 eV, which corresponds to the fundamental gap at equilibrium.

It is commonly accepted that materials with band gaps higher than 3.1 eV are suitable for uses in the ultraviolet (UV) spectrum. As a result, this broad band gap material might be appropriate for UV device applications. Furthermore, in the energy range of 7.5 to 17 eV, the ε₁ (ω) and ε₂ (ω) spectra exhibit a strong anisotropy in both directions of polarized light (parallel E//c as well as perpendicular E⊥c to the c-axis). This optical anisotropy is predicted due to the crystal’s poor symmetry. The principal peaks of the ε₂ (ω) spectra are caused by electronic transitions from the O-p occupied valence states to the Gd-f and Gd-d empty conduction states.

Figure 7a shows ε₁ (ω)spectra. This function provides information about a material’s electronic polarizability. The static dielectric constant ε₁ (0) values for E⊥c and E//c are 3.002 and 2.893, respectively. ε₁ (ω) begins to increase from its zero frequency limit to reach a maximum value of 5.5 at 10 eV, and then drops below 0 on the negative scale for photon energy ranging from 26 to 28 eV. Figure 7b and Figure 8b show a similar trend in the behavior of ε₂ (ω)and the extinction coefficient k (ω). The k (ω) threshold energy is roughly at 3.7 eV. At energies above this limit, k (ω) reaches its maximum value at 11 eV.

At higher energy, both components of the extinction coefficient show a significant drop. The refractive index (n(ω)) spectrum exhibits a noticeable anisotropy in the energy ranges 8–10 eV as can be seen in Figure 8a. The value of n(ω) becomes smaller than unity above 14 eV; this significant occurrence reflects the speed of light after passing through the substance. This effect could be beneficial in optoelectronics. The static refractive indices n_⊥(0) and n_||(0) are 1.732 and 1.701, respectively, and the corresponding birefringence Δn (0) is 0.031. The absorption of photons of light determines the excitation energy of an electron for passing over the band gap of a material. ε₂ (ω)is directly related to the absorption coefficient α(ω), which is defined as the amount of power absorbed when light passes through a solid. The optical absorption spectra of the described compound, as shown in Figure 9, demonstrate that it has a relatively high optical absorption (>150 cm⁻¹) throughout a large energy range in the ultraviolet range between 5 and 20 eV. Furthermore, the absorption coefficient α (ω) has peaked at 8 eV, 10 eV and 12 eV, which correspond to the inter-band transitions and then it gradually takes the higher values at higher photon energies. Despite the fact that the discussed compound (Gd₂Be₂GeO₇) has a wide-band gap and is structurally anisotropic; in lower photon energy region it is found to be optically isotropic, for which this feature makes it a potential candidate for the scintillator applications.

3.4. Optical Coating Characteristics

We attempted to examine the layer coating of the melilite-type compound Gd₂Be₂GeO₇ using the optical matrix approach, which is crucial for optical thin film design programs [39,40], in order to characterize the optical properties of this material to this extent.

This method considers each layer of the material as a 2 × 2 matrix, with the previously calculated extinction coefficient and refractive index of the optical properties (Section 3.3), resulting in thin films optical properties such as transmittance, absorbance, and reflectance, which are functions of the extinction coefficient (k) and refractive index (n), as well as geometry (thickness (d), surface roughness), etc. [41,42,43,44,45,46]. Two techniques were used in the optical cladding research: one on a clear substrate (glasses: n_glasses = 1.5, k_glasses = 0) and the other on a free-standing layer (n_air =1, k_air = 0). Figure 10, Figure 11 and Figure 12 show plots of optical coating spectra of the examined Gd₂Be₂GeO₇ thin film, such as Transmittance, Absorbance, and Reflectance, in the wavelength range of 150–1200 nm, for five different thicknesses: 100 nm, 300 nm, 600 nm, 1000 nm, and 1300 nm, respectively.

One can see from the plots of transmittance (Figure 10) that the Gd₂Be₂GeO₇ thin film exhibits a good transparency (between 90% and 95%) in the visible and infrared regions, and with increasing thickness, transmittance spectra diminish slowly at a higher wavelength level. Furthermore, the transmittance spectrum achieves the perfect shape for a sample of 100 nm thickness. Where for Gd₂Be₂GeO₇ thin film on transparent substrate, the maximum value of transmittance was found to be 92% at wavelengths around 350–450 nm and keeping this magnitude at a higher wavelength range, whereas the transmittance of 95% at wavelengths around 400 nm is found in the case of free-standing layers of this thin film and then decreases at higher wavelengths. Figure 11 shows the absorbance spectra of the Gd₂Be₂GeO₇ thin film under consideration. The plots show that the magnitude of this thin film’s absorbance increases with the gradual increase in film thickness due to an increase in the thin film’s absorbance for both cases of the investigated coating steps (free-standing layer and on transparent substrate), especially in the UV-visible spectrum (200–750 nm).

The absorbance peaks were found in the range of 200 nm to 300 nm as shown in Figure 11, for which we can conclude that the studied Gd₂Be₂GeO₇ thin film can be more efficient for applications in the ultraviolet (UV) region of the spectrum, and it could be useful for the high frequency UV device applications. Another important remark is that this thin film for very thin thicknesses of 100 nm can have good transparency in the visible spectrum (400–700 nm), which makes it a suitable candidate for optical coating design devices.

Figure 12 shows the computed optical reflectance spectra of our thin film under discussion for various layer thicknesses (d = 100 nm to d = 1300 nm) and both scenarios of the investigated coating steps: free-standing layer and transparent substrate. When moving from a free-standing layer to a layer on a glassy substrate, the form of the reflectance spectra changes dramatically. As a result, the usage of a glass substrate has an effect on the amplitude of interference oscillations. The interference phenomenon generated by the relative difference in refractive indices and thicknesses between the substrate and the film causes these rhythmic behaviors.

3.5. Thermoelectric Properties

In addition to detailed optoelectronic descriptions given obviously for the melilite-type Gd₂Be₂GeO₇ compound, in this subsection, we extended our investigation to explore its thermoelectric properties in order to study its ability to convert waste heat into useful electricity. The efficiency of thermoelectric power generation, on the other hand, is assessed by a dimensionless parameter called figure of merit (ZT), which is given as ZT = S²σT/(κe + κL), where S is the Seebeck coefficient, T is the absolute temperature, and κee and κLL are the material’s electronic and lattice thermal conductivities, respectively [47,48,49,50,51,52,53]. We used the BoltzTraP code [30] with a mesh of 18 × 18 × 28 k-points for this purpose. Variation of the Seebeck coefficient as a function of temperature of intrinsic (pure) Gd₂Be₂GeO₇ material is presented for the chemical potential μ = 0, which is in the middle of the band gap for spin-up and spin-down channels (see Figure 13a). The behavior of the Seebeck coefficient was relatively similar in the two spin channels, even though we stated different values, but the variation with temperature demonstrated a declining tendency in both spin-up and spin-down channels, respectively, from a very high value of ~270 μV/K and ~1700 μV/K. The Seebeck coefficients in both spin channels are positive, indicating that the holes are substantially dominating in this material.

Furthermore, we calculated the absolute Seebeck coefficient given by the following formula [47,48,49,50,51,52,53]:

$${\mathrm{S}}_{\mathrm{tot}}=\frac{\mathsf{\sigma }(\uparrow )\mathrm{S}(\uparrow )+\mathsf{\sigma }(\downarrow )\mathrm{S}(\downarrow )}{\mathsf{\sigma }(\uparrow )+\mathsf{\sigma }(\downarrow )}$$

The Seebeck coefficient is plotted as a function of temperature in Figure 13b, and it was positive throughout the temperature range for the examined compound, implying that holes are the charge carriers and that the spin-down state dominates the spin-up state. The presence of a low carrier concentration is indicated by the sharply higher values of the computed total S at lower temperatures <300 K. It has a value of 224.68 μV/K at room temperature, and we see a little drop in the total Seebeck value at higher temperatures. As a result, the computed Seebeck coefficients of Gd₂Be₂GeO₇ compound in this study make it a promising material for thermoelectric devices.

To make a good thermoelectric device, the TE material used should have a high S, as well as high σ and low k = κ_e + κ_L at the same time [47,48,49,50,51,52,53]. The results of temperature dependence of the electronic thermal conductivity (κ_el(W/K⋅m)) for the two spin channels are shown in Figure 14 for the considered compound. We can clearly see that the κ_el after saturated values at low temperatures starts to increase rapidly with increasing temperature for the two spin channels (down and up). The κ_el values at room temperature are 1.984 × 10¹² W/K⋅m and 2.718 × 10¹³ W/K⋅m for spin-down and spin-up channels, respectively. One can state that the compound under discussion shows important thermal behavior near room temperature where the thermal conductivity has lower values and higher Seebeck coefficient as expected, as shown in Figure 13b and Figure 14.

The higher the ZT value, the higher the thermoelectric conversion efficiency. The variation of ZT with temperature is shown in Figure 15 for the compound under discussion for the spin channels. Our results show that reasonable performance associated with ZT values around 1 is probably likely for the minority spin channel, particularly at 100K. Furthermore, it can be clearly seen that the ZT value decreases gradually with increasing temperature, while, for the majority spin channel, ZT reaches a value of about 0.89 at low temperature and then starts to decrease with increasing temperature. Higher values of ZT (approximately 0.99 and 0.70) for a large temperature range indicate that the examined compound has good thermoelectric performance and could be a suitable material for thermoelectric generator applications. The mobility of free charge carriers causes an electrical conductivity (σ/τ) in transport properties, which is a key component that must be high for desired thermoelectric materials to lower the Joule heating effect [47,48,49,50,51,52,53,54]. Figure 16 shows the predicted electrical conductivity (σ/τ) of the Gd₂Be₂GeO₇ compound as a function of temperature for both spins (up and down) configurations. The material exhibits approximately the same behavior in both channel configurations and σ/τ increases at given temperatures showing semiconductor behavior. Despite that, in the case of spin-down channel the value of σ/τ remains constant at low temperatures but above 300 K, a strong variation of σ/τ is observed with temperature.

The reported value of electrical conductivity at ambient temperature is 1.69 × 10¹⁸ (Ω m s)⁻¹, which increases to 4.51 × 10¹⁸ (Ω m s)⁻¹ around 1000 K in the spin-up channel. The value of the spin-down channel, on the other hand, climbs dramatically from 1.59 × 10¹⁶ (Ω m s)⁻¹ at room temperature to 1.58 × 10¹⁸ (Ω m s)⁻¹ at 1000 K. When compared to the spin-down arrangement, the electrical conductivity of the spin-up configuration is higher.

4. Conclusions

The structural, optoelectronics, and transport characteristics of the melilite-type Gd₂Be₂GeO₇ compound have been investigated using the FP-LAPW method with the GGA-PBE and TB-mBJ functionals. From the obtained results, the important following conclusions can be drawn:

• The calculated lattice constants agree well with the available experimental data.
• The studied Gd₂Be₂GeO₇ as thin film could be a good transparent in the visible spectrum (400–700 nm) for very thin thickness of 100 nm which makes it a suitable candidate for optical coating design devices; furthermore, it can be more efficient for applications in the ultraviolet (UV) region of the spectrum, and it could be useful for the high frequency UV device applications regarding the high absorbance peaks found in the range of 200 nm to 300 nm.
• From spin-polarized thermoelectric calculations, the material under discussion presents adequate Seebeck coefficients around room temperature; furthermore, we found higher values of the figure of merit (around 0.99 and 0.70) over large temperature ranges which suggest that this material exhibits good thermoelectric performance and it could be a promising material for applications in thermoelectric generators.
• Finally, the electrical conductivity (σ/τ) as a function of temperature exhibits the behavior of a typical semiconductor.