Experimental study of silver-palladium sellenides

Vymazalová A.¹, Chareev D.A.², Kristavchuk A.V.², Laufek F., Drábek M., Voronin M.V.² Osadchi E.G.²

¹Czech Geological Survey, Prague, Czech Republic, ²Institute of Experimental Mineralogy RAS, Chernogolovka, Russia

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The Ag-Pd-Se system comprises four minerals (chrisstanleyite, palladseite, verbeekite, naumanite) and ten binary phases are known from the Pd-Se and Ag-Se systems.

In order to better understand the formation of minerals belonging to this system at natural conditions, predict possible new minerals and determine stable phase associations, the Ag-Pd-Se ternary system has been investigated at three temperature intervals. The Ag-Pd-Se system has been studied at the temperatures of 350, 427 and 527˚C, the evacuated silica glass tube technique was used for the purpose of this study. The experimental products were investigated in terms of reflected light, electron microprobe and X-ray diffraction techniques.

The binary systems have been summarized by Okamoto [1] (Pd-Se); Karakaya and Thompson [2] (Ag-Pd); Karakaya and Thompson [3] (Ag-Se).

The experimental results proved that the thermal stability of binary phases is in agreement with the data proposed by Okamoto [1] for the Pd-Se system, and Karakaya and Thompson [3] for the Ag-Se system. All the binary phases have been investigated in terms of solubility of the third component. The Pd₁₇Se₁₅ phase (at 350, 430, 530˚C), analogue of mineral palladseite, and Pd₉Se₂ (at 430, 530˚C) dissolve the ternary component, up to 6 at. % Ag, and up to 3 at.% Ag, respectively.

The experimental study revealed the existence of four ternary phases in the Ag-Pd-Se system, the Ag₂Pd₃Se₄ phase and the three new AgPd₃Se, Ag₁₁Pd₁₁Se₆, and Ag₇Pd₇₃Se₂₀ phases.

Mineral chrisstanleyite Ag₂Pd₃Se₄ is known from nature occurrences however its synthetic analogue has not been studied so far. The X-ray diffraction data for the synthetic Ag₂Pd₃Se₄ phase are in an agreement with the crystal structure data determined by Topa et al. [4] for natural Ag₂Pd₃Se₄. Chrisstanleyite forms stable associations with phases PdSe₂, Ag₂Se and Pd₁₇Se₁₅ ss (solid solution) at 350ºC. It dissociates at 427±10ºС. Apparently at 350ºC, the Ag₂Se phase is not a mineral naumanite but its cubic high temperature polymorph, whereas naumanite is orthorhombic. It is slightly confusing, as the phase Ag₂Se appears orthorhombic at 350ºC however it is caused due to reverse process of phase transition that is induced while cooling [5]. It is important to note that the synthetic PdSe₂ is orthorhombic at 350ºC whereas mineral vebekeeite was described as being monoclinic [6].

The AgPd₃Se ternary phase is cubic, space group Pa , a = 8.6288(1) Å [7]. The phase forms a narrow solid solution Ag_1-x Pd_3+xSe, x = 0 to x = 0.15 (from 17.3 to 20.3 at.% Ag) at 350ºC, the homogeneity range slightly increases with increasing temperate (430 and 530ºC). The AgPd₃Se phase coexists with Ag₂Pd₂Se, Pd₁₇Se₁₅ (6 at.% Ag), Ag-Pd alloy, Pd₃₄Se₁₁, Pd₇Se₂ at 350ºC. At 430ºC the phase also coexists with Pd-Se eutectic melt (69 at.% Pd) that appears in the system, the binary Pd₃₄Se₁₁ phase is not stable above 430ºC. At 530ºC the range of the binary eutectic solution increases.

The Ag₁₁Pd₁₁Se₆ ternary phase forms an extensive solid solution Ag_11±xPd_11±xSe₆, with compositional range at 350ºC: 38.3-46.3 at.% Ag, 31.9-41.2 at.% Pd and 20.5-22.0 at.% Se. The homogeneity range decreases with increasing temperature (430 and 530ºC). The phase melts congruently at 575±5ºС. The phase is cubic, space group Fm 3 m, a = 12.3168(2) Å. At 350, 430, 530ºC the Ag₁₁Pd₁₁Se₆ phase coexists with Ag₂Se, Pd₁₇Se₁₅ ss, AgPd₃Se and Ag-Pd alloy. The composition of Ag-Pd alloy slightly differs with the increasing temperature.

The Ag₇Pd₇₃Se₂₀ is not stable at 350ºC. At 430ºC the phase coexists with AgPd₃Se, Pd₇Se₂, Pd₄Se and Pd₉Se₂ ss, and at 530ºC the phase also coexists with Ag-Pd alloy. Its crystal structure is unknown so far.

To conclude, two binary palladium sellenides dissolve silver, the Pd₁₇Se₁₅ phase, analogue of palladseite, up to 6 at.% Ag, and the Pd₉Se₂ phase up to 3 at.% Ag. Stable associations determined in this study can help to explain the formation, natural occurrences and conditions of formation of silver-palladium sellenides. Three (AgPd₃Se, Ag₁₁Pd₁₁Se₆, Ag₇Pd₇₃Se₂₀) newly determined ternary phases can be expected to be found in nature, likely in association with palladium sellenides or other PGM.

This research was funded through the grant P210/11/P744 from the Grant Agency of the Czech Republic, the project LA 11125 / KONTAKT II from the Ministry of Education, Youth and Sports of the Czech Republic, and the grant of the President of the Russian Federation for State Support of Young Russian Scientists (MK-1557.2011.5).

References:

1. Okamoto, H. (1992): The Pd-Se (palladium-sellenium) system. J. Phase Equilibria and Diffusion,13, 1, 69-72.

2. Karakaya, I. and Thompson, W.T. (1988) The Ag-Pd (Silver – Palladium) system. J. Phase Equilibria, 9(3), 237-243.

3. Karakaya, I. and Thompson, W-T. (1990) The Ag-Se (Silver – Sellenium) system. J. Phase Equilibria, 11(3), 266-271.

4. Topa, D., Makovicky, E. and Balic-Žunic, T. (2006) The crystal strucrures of jagueite, Cu₂Pd₃Se₄, and chrisstanleyite, Ag₂Pd₃Se₄. Can. Mineralogist 44, 497-505.

5. Billetter, H. and Tuschewitz, U. (2008) Structural phase transitions in Ag₂Se (naumannite). Z. Anor. Allg. Chemie 634 (2), 241-246.

6. Roberts, A.C., Paar, W.H., Cooper, M.A., Topa, D., Criddle, A.J. and Jedwab, J. (2002) Verbeekite, monoclinic PdSe₂, a new mineral from the Musonoi Cu-Co-Mn-U mine, near Kolwezi, Shaba Province, Democratic Republic of Congo. Mineralogical Magazine 66(1), 173-179.

7. Laufek, F., Vymazalová, A., Chareev, D.A., Kristavchuk, A.V., Lin, Q., Drahokoupil, J. and Vasilchikova, T.M. (2011) Crystal and electronic structure study of AgPd₃Se. J. Solid State Chemistry 184(10), 2794-2798.

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