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Results and Discussion
The DTA-TGA curve for the analyzed sample Ag0.93Cux.07S in air up to 1173 K is given in Fig. 1. An endothermic and two major exothermic peaks can be noticed in the heating DTA curve. Even if the analyzer was not calibrated to determine enthalpy values accurately the relative values associated with each peak can be compared. The first endothermic onset temperature T = (360.9 ± 2) K corresponds to the phase transition (reaction (1)) temperature determined in an argon gas atmosphere, Ttr = (361.2 ± 1) K . However, the enthalpy value 5.89 kJ mol-1 associated with the endothermic peak is higher than the AtrH° = (4.832 ± 0.055) kJ mol-1 value determined for reaction (1) , after calorimetric measurements in argon gas atmosphere. In addition to the inaccuracy in the setup of the DTA analyzer employed in this study for enthalpy measurements, the * 1 kJ mol-1 difference may also be partly attributed to the different gas atmospheres used in the current measurements.
As shown in Fig. 1, after the endothermic peak, two exothermic peaks accompanied by significant gains in mass have appeared. These are indications for oxidation of the solid solution (Cu, Ag)2S(hcp), which is the stable phase above T = (361.2 ± 1) K . The SEM-EDS analyses of the oxidized sample, for which the Backscattered Scanning Electron micrographs is shown in Fig. 2, indicate formations of CuO, Ag2SO4, and Ag (with 3.88 wt% Cu dissolved). Based on the standard Gibbs energies of reactions (2)-(6) illustrated in Fig. 3, and the DTA-TGA
Fig. 1 The DTA-TGA curves of the oxidation processes of the synthesized sample Ag0 93Cui.07S. Dashed lines indicate cooling curves
Fig. 2 Backscattered Scanning Electron micrographs of the oxidized Ag0.93Cu1.07S sample after the simultaneous DTA-TGA analyses. They are composed of CuO, Ag2SO4, and Ag (with 3.88 wt% Cu dissolved)
curves as a function of temperature shown in Fig. 1, the oxidation process above T = (614.2 ± 2) K is defined as follows:
The first exothermic peak above the onset temperature T = 615.1 K is due to reaction (2), and the second exothermic peak above the onset temperature T = 830.4 K is due to reaction (6). The corresponding standard Gibbs energies for reactions (2)-(6) are calculated with the thermodynamic database of HSC Chemistry 6  and illustrated in Fig. 3.
In Fig. 1 the mass gain above T = (614.2 ± 2) K is due to the oxidation of Cu2S and Ag2S in the solid solution (Cu, Ag)2S according to reactions (2) and (5), respectively. This temperature of oxidation reaction is in agreement with the peak temperature T = 605 K reported by Sceney et al.  while oxidizing Cu2S. The decrease in mass observed above T = 780 K is due to the increased loss of sulfur in the form of SO2(g), according to reaction (3). These observed temperature value and the effect on TGA curve is in agreement with the results reported by Zivkovic et al.  for oxidation processes of Ag2S sample. The second exothermic peak was accompanied with a sharp increase in mass. This sharp increase in mass can be due to the formation of Ag2SO4, according to reaction (6), which also has relatively higher enthalpy of reaction. As the Gibbs energies of reactions in Fig. 3 shows, the
Fig. 3 Standard Gibbs energies of reactions (2)-(6) calculated using the databases of thermodynamical and process calculation software HSC Chemistry 6 
formation of Ag2O according to reaction (4) is not thermodynamically favored over reaction (3). This is also observed in the analysis of our oxidized sample that contains only silver instead of Ag2O. This is in agreement with the observation of Zivkovic et al. . The endothermic peak on the cooling curve at T = (932.76 ± 2) K corresponds well with the melting temperature of Ag2SO4, T =931 K, determined by Singh et al. . The endothermic peak on the cooling curve at T = (659.9 ± 2) K could be due to the phase transition of the produced a-Ag2SO4 to P-Ag2SO4, which is close to the phase transition temperature determined by Kumari and Secco  in a protected atmosphere, Ttra = 689 K.
An attempt to independently oxidize AgBiS2 sample and measure DTA-TGA values failed due to the reaction of the products with Al2O3 crucible. The SEM-EDS analyses of the analyzed sample indicate formations of Al-based phases including Al2BiO4, Backscattered Scanning Electron micrographs of the oxidized AgBiS2 sample is shown in Fig. 4. According to the high-temperature calorimetric
Fig. 4 Backscattered scanning electron micrographs of the oxidized AgBiS2 sample after the simultaneous DTA-TGA analyses. They constitute different alloys and compounds of Al-O-Bi-Ag studies of Oudich et al. , В12Оз readily reacts with AI2O3 in air to form two ternary phases. Thus, the formations of Al-Bi-O phases indicate that Bi2O3 has been produced in the high temperature oxidation of AgBiS2.Thus, the possible oxidation process of the sample AgBiS2 could be:
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