Spectroscopic investigations of polycrystalline In_xSb_20−xAg₁₀Se₇₀ (0 ⩽ × ⩽ 15) multicomponent chalcogenides

The elemental composition of each bulk sample was determined by energy dispersive X-ray spectroscopy (EDS) and the EDS spectra are shown in Fig. 1. The atomic percentages of all elements present in the bulk alloys are given in Table 1. These results suggest that the samples are nearly stoichiometric with those of the initial compositions used for the synthesis.

Table 1

Compositions of as-prepared bulk In_xSb_20−xAg₁₀Se₇₀ (0 ⩽ x ⩽ 15) chalcogenide alloys obtained by using EDS spectroscopy.

The structural analysis of devitrified glassy alloys and their phases is important for furthering the science of chalcogenide alloys. Fig. 2 shows the X-ray diffraction patterns of In_xSb_20−xAg₁₀Se₇₀ chalcogenide alloys. The existence of sharp diffraction peaks for different phases in the multicomponent chalcogenide alloys reveals their polycrystalline nature. The crystalline phases are indexed to tetragonal phases of AgSbSe₂ (JCPDF Card No. 12-0379) and AgInSe₂ (JCPDF Card No. 75-0118) having a body centered lattice. The structural analysis of Se₇₀Sb₂₀Ag₁₀ reveals the devitrification of single phase AgSbSe₂ [12] without denomination of other phases due to their large intensity. The addition of small amount of indium additive (x = 5) results in a decrease in the intensity of diffraction peaks of AgSbSe₂ without forming new phases. However, further increase in x (10 at.%) favors the appearance of new diffraction peaks with increased intensity for AgInSe₂ phase, accompanied with peaks of decreased intensity for AgSbSe₂ phase.

Further addition of indium (x = 15) results in an increase in the AgInSe₂ phase and decrease in AgSbSe₂ phase.

Raman spectra yield valuable information about the dynamics and structure at the molecular level. The Raman spectra have been shown in Fig. 3. The Raman spectrum for the parent Sb₂₀Ag₁₀Se₇₀ chalcogenide alloy exhibits three dominant peaks at 82 cm⁻¹, 188 cm⁻¹ and 252 cm⁻¹, and two weak bands at 117 cm⁻¹ and 371 cm⁻¹. The Raman spectra of In–Ag–Sb–Se systems have been discussed by considering AgSbSe₂ and AgInSe₂ basic structural units. For simplifying the discussion, the R-value has been defined as a ratio of covalent bonding possibility of chalcogen atoms to the covalent bonding possibility of the non-chalcogen atoms [20]. The value of R = 1 represents the case of existence of only hetero-nuclear bonds in a given system which unequivocally indicates the occurrence of the chemical threshold. Since the R-value is 1.75 for the present system it suggests Se rich material having heteropolar bonds and chalcogen-chalcogen bonds. The heteropolar bonds are also more favorable than homopolar bonds due to their larger bond energies. The weak bands up to 136 cm⁻¹ are reported to be of mixed character with identical contributions from Ag, Se and In atoms [21]. Therefore, the observation of weak band can be ascribed to the contributions from Se and Ag atoms in the parent ternary system. Holubova et al. [22] have assigned the peak at 194 cm⁻¹ to heteropolar Sb–Se bond vibrations in SbSe_3/2 pyramids so, the peak observed at 188 cm⁻¹ can be ascribed to Sb–Se bond vibrations of AgSbSe₂ structural units due to the incorporation of Ag in this ternary system. However, the broadening of this peak with indium content can be ascribed to the replacement of Sb by In in AgSbSe2 structural units. The peak at 252 cm⁻¹ may correspond to the homopolar Sb–Sb bonds in Se₂Sb–Se₂Sb structural units [23]. The band at 369 cm⁻¹ has been ascribed to the bond bending modes of Se [24]. Therefore, the band at 371 cm⁻¹ can be assigned to this mode. The observed shift may be due to the presence of Sb or Ag. This band was found to fade with the increase in the indium concentration due to breaking of Se chains.

The peak at 82 cm⁻¹ has been found to broaden for all the concentrations, while the band at 117 cm⁻¹ broadens for x = 5 and disappears for x = 10 and 15 at.%. The intensity ratio for 188 cm⁻¹ and 252 cm⁻¹ peaks have been found to increase up to x = 5, and thereafter, this value decreased with the appearance of new band at 238 cm⁻¹ for x = 10 sample. The B₂ mode of AgInSe₂ chalcopyrite structure has been reported at 230 cm⁻¹[25] and therefore, the shift can be ascribed to the presence of Sb atoms in this system. It has further been observed that when the In content increases (x = 15), the intensity for 238 cm⁻¹ peak increases as compared to the intensity of 252 cm⁻¹ peak, indicating a decrease of the homopolar Sb–Sb bonds and increase in the heteropolar In–Se bonds. The relative decrease in the intensity for the 252 cm⁻¹ peak at higher In content can be ascribed to the segregation of AgInSe₂ phase. Also, the new weak band at 142 cm⁻¹ can be assigned to the B2 mode corresponding to the antiphase motion between In and Se atoms of AgInSe₂ chalcopyrite structure [25].

The relation between absorption coefficient and diffuse reflectance has been given by Kubelka-Munk [26]: α<!-- a -->=ks=(1−<!-- - -->R∞<!-- 8 -->)22R∞<!-- 8 --> $$\alpha {\rm{ = }}\frac{{{\rm{ k}}}}{{\rm{s}}}{\rm{ = }}\frac{{{{{\rm{(1 - }}{{\rm{R}}_\infty }{\rm{)}}}^{\rm{2}}}}}{{{\rm{2}}{{\rm{R}}_\infty }}} $$ (1)

where s and k are the scattering and absorption coefficients. Fig. 4 shows the variation of (αhv)² versus photon energy (E) for the present system.

The band gap can also be calculated by the following equation [27]: α<!-- a -->=B(hv−<!-- - -->Eg)hvn $$\alpha = B{\frac{{( hv - Eg)}}{{hv}}^n} $$ (2)

where B is a constant, E_g is the band gap and n = 1/2 corresponds to the allowed direct transitions [28]. The parameter B depends upon the transition probability, the inverse of which is known as tailing parameter describing the disorder in the network systems [29, 30]. Fig. 4 reveals direct band transitions for In_xSb_20−xAg₁₀Se₇₀ alloys. The values of band gap and tailing parameter are summarized in Table 2. The value of E_g for Sb₂₀Ag₁₀Se₇₀ alloy is 1.59 eV and decreases to 1.30 eV for 5 at.% of indium with the increase in tailing parameter. This can be correlated with the decrease in disorder of the system. The dominant contribution for the states near the band edge of the chalcogenide alloys comes from their lone pair p-orbital [31]. The electronegativity of Sb, Ag, Se and In are 2.05, 1.93, 2.55 and 1.78 respectively [32]. When Sb atom is replaced by the electropositive In atom, the energy of lone pair of electrons gets enhanced and hence, the valence band edge moves towards the forbidden gap to cause an increase in the tailing parameter and decrease in the optical gap for this system [33]. Further addition of In have been found to increase the band gap to 1.71 and 2.06 eV for x = 10 and 15, respectively. This can be explained by considering the bond formation energies. For hetero-nuclear bonds, the bond energy (BE), E_A−B has been calculated by using the relation [34]: EA - B=(EA - A×EB - B)1/2+30(χA - χB)2 $${E_{A{\text{ - }}B}}\; = \;{{\text{(}}{E_{A{\text{ - }}A}}\; \times \;{E_{B{\text{ - }}B}})^{1/2}}\; + \;30{({\chi _A}\;{\text{ - }}\;{\chi _B}{\text{)}}^{\text{2}}} $$ (3)

where E_A-A and E_B-B are the BE of the homonuclear bonds and χA, χB are the electronegativities of the atoms respectively. The calculated bond energies are Se-Se (44.02 kcal/mol), Sb–Sb (30.02 kcal/mol), In-In (24.2 kcal/mol), Sb–Se (40.15 kcal/mol), Sb–In (30.72 kcal/mol) and In–Se (47.33 kcal/mol) [35]. According to the chemically ordered network model, an atom combines preferably with the atoms of different kind as compared with the same atoms and the bonds are formed in the sequence of the BE [2]. The increase in In and decrease in Sb lead to the relative increase in AgInSe₂ phase at the expense of AgSbSe₂ phase or replacing the weaker Sb–Se and Sb–In bonds by stronger In–Se bonds in this system. Therefore, the band gap is found to decrease for lower indium concentrations where the system forms the solid solutions. Similar effects associated with the incorporation of In on the electrical properties have been observed in Se_{90_x}>Te₅Sn₅ln_x glassy alloys [36]. The XRD studies have also revealed the increase in the relative increase in AgInSe₂ phase at the expense of AgSbSe₂ phase as earlier discussed. This leads to an increase in the average bond energy and hence, an increase in the optical gap for this system.