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D. GHOSH AND D.D.L. CHUNG Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania, 15213, USA

ABSTRACT observation of an incommensurReported here is the first ate-commensurate phase transition at 316 ± 1 K upon heating stage-2 graphite-bromine prepared in bromine liquid. The transition during heating involved the movement of the 300 reflection toward a lower q value and the sharpening of the 010 reflection, thereby resulting in the commensurate structure characteristic of stage-2 prepared in bromine vapor. The previously reported comThe transition was reversible. tmensurate-incommensurate transition occurred at 326, 329, 332, 338 and 348 K for stage 2 (in bromine liquid), stage 2 (in bromine vapor), stage 3, stage 4, and desorbed materDisordering was observed upon completion ial, respectively. of this transition. INTRODUCTION The phase transitions in graphite-bromine include the melting transition and the commensurate-incommensurate (C-I) transition. The melting transition was observed by electron diffraction [1], calorimetry [2] and x-ray diffraction [3]; it occurred at 373.7 K [3] in dilute desorbed compounds. It was also observed by Raman scattering from 345 to 380 K in stage 3 [4]. The C-I transition was first observed by x-ray diffraction at 'b340 K in dilute desorbed compounds [3], and was confirmed by Raman scattering at 335 + 10 K for stage 3 and 353 ± 10 K for stage 4 [4]. In this paper, we have made the first observation of a third phase transition, namely an incommensurate-commensurate (I-C) transition, which occurred only in stage 2 prepared in liquid bromine (called Type B [5]) and was observed by x-ray diffraction at 316 + 1 K. Type B is incommensurate at room temperature, whereas Type A (stage 2 prepared in bromine vapor) is commensurate [5]. Upon heating Type B to 316 K, some diffraction spots moved toward a lower q value, resulting in the commensurate pattern characteristic of Type A at room temperature. The I-C transition was reversible. Raman scattering results suggested that the C-I transition is an orderdisorder transition [4]. By using x-ray diffraction, we have confirmed that disordering occurs upon completion of the C-I transition. Raman scattering showed that the C-I transition occurred at a higher temBy using x-ray diffraction, we have perature for stage 4 than stage 3 [4]. shown that this transition temperature increased with decreasing intercalate concentration, such that the trend was Type A