Figure 1 shows the x-ray diffraction XRD patterns for as-quenched melt-spun Sn-5Bi-xAg, (x = 0, 1, 2, 3, and 4 in at %) alloys. The XRD for Sn-5Bi alloy is shown in Figure 1(a), Bi is precipitated as indicated by (012) peak and all the other peaks are for Sn. The matrix is β-Sn solid solution, the unit cell of β-Sn is body centered tetragonal (I4₁/amd).The crystal structure of Bi is rhombohedral-hexagonal (S.G.: R3m) with a = 4.5491, c = 11.9485 Å.

Figure 1(b) shows for Sn-5Bi-1Ag alloy the same structure, i.e. precipitation of Bi in the Sn matrix as indicated by (012) peak. For Sn-5Bi-2Ag alloy (Figure 1(c)) the peak due to Bi is disappeared and instead a peak due to Ag₃Sn (211) is appeared. As the Ag concentration increases the intensity of the peak due to Ag₃Sn increases (Figure 1(d) and Figure 1(e)). The crystal structure of Ag₃Sn is orthorhombic (S.G.: Pmmn) with lattice parameters a = 5.9680, b =4.7802, and c =5.1843. The detail of XRD analysis is shown in Table 1.

The axial ratio c/a is very important parameter in determination the properties of Sn-based alloys since the work of [10] . Figure 2(a) shows the variation of the axial ratio c/a with Ag concentration. The lattice parameters a, c, and the axial ratio c/a for pure Sn rapidly solidified were found by [9] to be 5.809 Å, 3.169 Å, and 0.5455 respectively.

The addition of Bi to Sn expands the β-Sn tetragonal unit cell, both a and c are increased (see Table 1) and the c/a ratio is decreased in agreement with [10] . It is evident that the addition of Ag increases the axial ratio from 0.5436 to 0.5464 in agreement with [10] . Since low valency atom like Ag increases the axial ratio of β-Sn tetragonal unit cell and high valency atom decreases it. Figure 2(b) shows the variation of the volume of the

unit cell v of the β-Sn matrix with Ag concentration. The volume of unit cell has a maximum value 112.15 ų at 2 at % Ag. Figure 2(c) shows the variation of Fermi energy E_F with Ag concentration. It is evident that Fermi- energy decreases by increasing Ag concentration from 10.21 eV to 10.16 eV. This decrease is due to that Ag

atom with valency +1 replaces Sn atom with valency +4. Figure 2(d) shows the variation of the diameter of Brillouin zone K_B211 in the (211) direction with the diameter of Fermi sphere 2k_F. It is evident that the Hume-Rothery condition for phase stability is not satisfied for this alloy.

Figure 3 shows the DSC curves for as-quenched melt-spun pure Sn-5Bi-x Ag (x = 0, 1, 2, 3, and 4 in at %) alloys. A transition occurs at about 413 K for all alloys; however the endothermic peak for this transition decreases by increasing Ag concentration. This transition may be due to the eutectic reaction which starts at 413 K. Finally, the endothermic peak due to melting was obtained, from which the melting temperature T_m and the enthalpy of fusion DH_m have been determined. DH_m was obtained from the integral under the DSC peak of melting as given by Equation (1);

where; C_p is the heat capacity at constant pressure, T₀ and T_f are known as onset melting temperature and final melting temperature of the specimen, respectively. Table 2 shows the detail of the DSC results. It is evident that the melting temperature decreases by increasing Ag concentration from 492.29 K for Sn-5Bi to 483.58 K for Sn-5Bi-4Ag. The enthalpy of fusion DH_m decreases by increasing Ag concentration from 57.27 kJ∙kg⁻¹ for Sn-5Bi to 47.5 kJkg⁻¹ for Sn-5Bi-3Ag.

Figure 4(a) shows the temperature dependence of resistivity obtained for as-quenched melt-spun Sn-5Bi alloy in the temperature range from 300 to 500 K. The behavior is metallic i.e. the resistivity increases by increasing temperature. The resistivity at room temperature r is found to be 4.56 × 10⁻⁶ Ωm.

The resistivity at room temperature for pure Sn rapidly solidified was found to be 0.166 × 10⁻⁶ Ωm [5] . This means that the addition of Bi causes large increase in the resistivity. This increase in the resistivity is attributed to two reasons; the first is the presence of Bi atoms in Sn lattice which acts as a scattering center for the conduction electrons. The second is due to the precipitation of Bi as a distinct phase which has high resistance (2.09 x 10⁻⁶ Ωm) according to [3] . The temperature coefficient of resistivity TCR was calculated to be 9.15 × 10⁻³ K⁻¹ (see Table 3). The TCR is increased due to the addition of Bi since TCR for pure Sn rapidly solidified was found to be 3.23 × 10⁻³ K⁻¹ [9] .

Figure 4(b) shows the temperature dependence of resistivity (TDR) obtained for as-quenched melt-spun Sn-5Bi-xAg (x = 1, 2, 3, and 4 in at %) alloys. It is evident that the behavior is semiconducting for all alloys containing Ag i.e. the resistivity decreases by increasing temperature. The variation of resistivity at room temperature r with Ag concentration is shown in Figure 4(c). The resistivity at room temperature r decreases by increasing Ag concentration. This decrease in resistivity can be attributed to the increase in the axial ratio c/a due the addition of Ag as shown in Figure 2(a). The energy gap E_g was calculated for the semiconducting alloys and the result is shown in Figure 4(d). The energy gap E_g decreases by increasing Ag concentration from 203 meV for Sn-5Bi-1Ag to 97.5 meV for Sn-5Bi-4Ag (see Table 3). The semiconducting behavior may be ex- plained as the following; both Bi and Ag atoms are dissolved in Sn lattice, the Ag atoms provide a donor level near the conduction band. Electrons from this level are raised to the conduction band by thermal agitation. Therefore the total number of carriers increases with temperature which in turn decreases the resistivity.