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Phase Stabilities of AgBiS2 and AgCuS
Below its congruent melting point at Tm = (1074 ± 4) K, AgBiS2 exists in two structural forms . The low-temperature phase P-AgBiS2, which corresponds to the mineral matildite, is hexagonal and the high temperature phase a-AgBiS2, which corresponds to the mineral schapbachite, is cubic [17-19]. Experiments on stoichiometric AgBiS2 indicate that the cubic form (a-AgBiS2) is stable above T = (468 ± 5) K [16, 19] and coexists with the hexagonal form (P-AgBiS2) in the temperature range from 468 ± 5 to 455 ± 3 K, and that below T = (455 ± 3) K, only the hexagonal form is the stable phase .
In the ternary Ag-Cu-S system, three equilibrium ternary phases have been identified: Ag155Cu045S (Jalpaite, а-phase), Ag12Cu08S (mckinstryite, P-phase) and Ag1-zCu1+zS (stromeyerite, y-phase) [20-23]. All the ternary phases exist on the Ag2S-Cu2S tie line and are stable only at temperatures below T = 392 K [21, 22]. According to Skinner  and Frueh , stomeyrite (Ag1-zCu1+zS) exhibits extended stoichiometric variation of 0 < z <0.1. Thus, slight differences in the melting temperatures of stromeyerite reported in the literature can be expected. The Ag0 89Cu110S reported by Tokuhara et al.  as the stable phase of stromeyerite is in agreement with the estimated stoichiometric variation of stromeyerite. In this study, we have synthesized stromeyerite in excess of Cu2S which has an approximate composition Ag0.93Cu1.07S. Stromeyerite with a composition AgCuS has been reported to be synthesized through different methods [26, 27]. At T = 363 K, Ag0.93Cu1.07S transforms congruently to the hexagonal solid solution of Cu2S, i.e. (Cu, Ag)2S (hcp) . The Cu-rich solid solutions transforms into cubic solid solutions (Cu, Ag)2S (fcc) [21, 28]. The fcc solid solutions have homogeneity regions on both sides starting from the ideal stoichiometric compositions Cu2S and Ag2S .
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