The first international applied geological congress , 2010-04-26

Title : ( Characterization of temperature-induced phase transitions in AgSbS2 phases by HTXRD, DSC, EXAFS and TEM )

Authors: Morteza Razmara ,

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Abstract Depending on the heat treatment conditions and kinetics, AgSbS2 can be synthesised in different modifications. The cubic disordered form (β-miargyrite) and ordered monoclinic α-miargyrite. Another (third) phase which is different from a and β- forms was synthesised by annealing AgSbS2. This new phase is called the \\\\\\\"intermediate phase\\\\\\\" or γ-miargyrite. EPMA, analytical SEM and Ag and Sb X - ray images confirmed the homogeneity all of these phases. High temperature XRD experiments for β-AgSbS2 showed that only the cubic form is present from room temperature up to 210 °C. From 220 °C the 111 peak shows slightly broadening while the 311 peak starts to split. Above 220 °C, the 200 and 220 peaks show clear splitting. At 240 °C a very small extra peak can just be distinguished at ~ 26.5° 2θ. As the temperature rises, this peak becomes progressively more intense and further extra peaks can be identified at about 45° and 53.5° 2θ. It is clear that extra reflections appear and therefore there is change in the translational symmetry. Between 280 °C and 300 °C there is a dramatic change in the pattern, particularly in the region 26° - 28° 2θ. The small peak at ~ 26.5 becomes very intense and sharp while the peak at 27.5° (111 cubic) has changed. The peak splitting and appearance of extra peaks must reflect the occurrence of a phase transition (β-miargyrite to a more stable, less symmetrical polymorph). This phase transition is effectively complete when peaks become sharp at ~300 °C. It is possible to index the 310 °C pattern: α-miargyrite, the stable low temperature form. However, the trace of the 111 peak vanishes at above 310 °C. Thus, it appears that only β-miargyrite is present to 220°C and that α-miargyrite is present at over 310 °C. At intermediate temperatures the phase transition takes place progressively as temperature is increased, with both more-symmetrical (β) and less-symmetrical (α) polymorphs coexisting, the former presumably decreasing in amount and the latter increasing. DSC scans of synthetic α-miargyrite showed three endothermic reactions. An endothermic reaction beginning below 385 °C is observed and the endothermic reaction starting at 460 °C is likely to be related to the appearance of another phase which is not quenchable and only can exist between 462 °C and the melting point of AgSbS2. The EXAFS spectra were extracted from the raw data for α and β-miargyrite, along with model fits for the Ag-S distances. Ag K-edge EXAFS spectra for β-miargyrite gave a very much shorter mean Ag-S bond length (2.54 Å) than the XRD bond length (2.8282 A) and only showed 3 nearest-neighbour S atoms rather than 6 as expected for the fcc structure. EXAFS results for α-miargyrite gave 3 S atoms at a mean distance from of 2.47 A. To investigate the phase transition in AgSbS2, in situ heating experiments were performed in the TEM. The changes in the diffraction pattern of α-AgSbS, were observed during heating of the sample up to 390 °C. During heating of the specimen to 300 °C, there was no obvious change in the bright-field image or diffraction pattern from the grain. As the temperature was increased above 300 °C, some of the spots in the [010] pattern of the monoclinic phase became more intense than they were initially. These strong spots are close to a [110] cubic fcc pattern with a cell parameter ~ 5.56 Å. Thus there is evidence for an emerging pseudo-cubic symmetry, suggesting an orientation relationship between the structures of the monoclinic and pseudo-cubic cells. The possible orientation relationship between the monoclinic structure and the pseudo cubic cell is: [010]mono // [110]cubic and (001)mono // (111)cubic. The equivalent d-spacings are: d(020)mono = 2.21 Å; d(220)c =2.00 Å; d(001)m = 3.19 Å and d (111)c = 3.26 Å. Further heating of the specimen above 337 °C, resulted in a further change in the intensities of the reflections, while the diffraction patterns from other grains suggested orientation relationships between the monoclinic and cubic cells of [211]cubic // [010]mono and (111)cubic // (101)mono. This orientation is different from the first one and it is not clear if there is any simple relationship between the monoclinic and cubic cells. Thus, the γ-phase structure is distinct from that of both α- and β-miargyrite. The intermediate phase appears to be monoclinic with a large cell.


, Key words: EXAFS spectra, TEM, analytical SEM, HTXRD, γ-miargyrite
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author = {Razmara, Morteza},
title = {Characterization of temperature-induced phase transitions in AgSbS2 phases by HTXRD, DSC, EXAFS and TEM},
booktitle = {The first international applied geological congress},
year = {2010},
location = {مشهد, IRAN},
keywords = {Key words: EXAFS spectra; TEM; analytical SEM; HTXRD; γ-miargyrite},


%0 Conference Proceedings
%T Characterization of temperature-induced phase transitions in AgSbS2 phases by HTXRD, DSC, EXAFS and TEM
%A Razmara, Morteza
%J The first international applied geological congress
%D 2010