1 5 stage 2 and 2 8 8 2 2 k f or all concentrations x

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Unformatted text preview: . a l . (52) for a two dimensional triangular lattice. Good agreement between the two sets of values was obtained X-ray diffraction data revealed that t h e A g x T i S 2 p owders with x 2 0 . 2 5 that were quenched from 800 C c onsisted of a m ixture of stage 1 a nd stage 2 w hile similar samples quenched from 1000 C w ere nominally pure stage 1 . T h i s result is also reflected in the calorimetric d a t a where a double peak is observed in the DSC scans of the batch A s amples with x 2 0.25. T h e double peak is postulated t o occur because t h e lower density (stage 2 ) part of the samples have a transition temperature that is 2 0 K l ower than the stage 1 p ortion. T h e s e samples thus yield two distinct transitions. These results are also consistent with a recently developed s t a g i n g phase diagram (Bardhan e t . al. 68): P a g e - 61T wo additional transitions were observed in the course of this work. One was observed in both the pristine and intercalated samples and occurred a t T and 4 42 2 2 K u pon cooling. = 449 2 2 2 o n h eating A t ransition in this approximate temperature range had been observed previously ( 3 9 ) b ut its behaviour- was more characteristic of a second order transition. Another transition at 2 0 8 2 . 2 K a ppeared in the calorimetric scans of the intercalated samples. There was no evidence of either of the above transitions in X-ray diffraction patterns and the origin of both transitions is unclear. The nature and physical origin of the transitions at 208 K a nd 4 4 9 K c ould constitute the subject of future investigations. It would also be of interest to carry out calorimetric measurements on large good quality single crystals of A g T i S x 2' T h e results of such experiments would e nable a m ore detailed comparison with theoretical predictions. F i n a l l y , the acquisition of apparatus with high temperature capabilities would enable one to study staging t r a n s i t i o n s , a subject that is of significant current interest. Page -62- APPENDIX D S C PROGRAM I c o o l the s a m p l e to 2 5 0 K . 1: - 2: -- h old t h e s a m p l e a t 2 5 0 K i sothermally for 3 m i n u t e s t...
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This note was uploaded on 10/08/2009 for the course CME MAT E 630 taught by Professor Dr. during the Fall '09 term at University of Alberta.

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