Crystal/Glass Phase Change in KSb 5 S 8 Studied through Thermal Analysis Techniques K. Chrissafis, ‡ Theodora Kyratsi, †,‡ K. M. Paraskevopoulos, ‡ and Mercouri G. Kanatzidis* ,† Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, and Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Received October 24, 2003. Revised Manuscript Received February 25, 2004 The reversible crystal-glass and glass-crystal transitions discovered in KSb 5 S 8 were studied in detail with nonisothermal scanning calorimetry techniques. When cooled, molten KSb 5 S 8 becomes a metastable glass, which can quantitatively revert to the crystalline form if heated above 277 °C. Crystalline KSb 5 S 8 is a red semiconductor with a band gap of 1.82 eV, whereas the glass (also red) shows a lower but equally well-defined band gap of 1.67 eV. Two approaches have been used to analyze the glass transition. The activation energy of crystallization E c was calculated using the Kissinger (∼167 ( 3.1 kJ/mol) and Flynn-Wall- Ozawa methods. The kinetic parameters and energy band gaps determined for KSb 5 S 8 suggest possible utility of this system for phase-change, high-density optical data storage applications. Introduction Reports of reversible phase changes such as crystal- glass-crystal transformations in a single compound are relatively rare. If sufficiently rapid, a phase change can be the basis for data storage applications. The primary system used currently in such applications is thin films of Ge 2 Sb 2 Te 5 that exhibit glass to crystal transition on the nanosecond time scale. 1 In addition, materials that exhibit this property can act as rewritable storage media for polarized holograms, 2 opto-mechanical actuators, 3 nonvolatile memory, 4 and infrared optical waveguides. 5 In past reports, we have already described several stoichiometric systems that form glasses upon slow cooling from the melt including (Ph 4 P)InSe 12 , 6 KSbP 2 - Se 6 , 7 and Cs 2 Hg 3 Ge 2 S 8 . 8 The glassy forms exothermi- cally crystallize upon heating shortly after the glass transition temperature T g is reached. Recently, we reported the phase change properties of KSb 5 S 8 9 and pointed out its potential for information storage ap- plications. 10 The observation that crystallization pro- ceeds nearly immediately after passing through T g indicates that the amorphous material is in a high energy state with a low barrier to crystallization. Although the class of chalcogenide glasses is very large, most have continuous, nonstoichiometric compositions, and are made entirely of covalently bonded atoms (e.g., Ge 1-x Se x , 11 Ge-As-Se, 12 and (Ag 2 S) x (GeS 2 ) y , 13 etc). In contrast, glassy KSb 5 S 8 stands out, because, in addition to being stoichiometric, it features two types of bonding in its structure, covalent bonding in the anionic [Sb 5 S 8 ] - framework, and ionic bonding associated with K- - -S interactions. We surmise that this mixed bonding character plays a role in the exceptionally clean and fast phase-change behavior it exhibits. Kinetic phase-change studies in stoichiometric com- pounds are rare in the literature, yet such studies may yield mechanistic insight into such processes. Here we have studied in detail the glass transition and crystal- lization kinetics of KSb 5 S 8 , using differential scanning calorimetry (DSC). We also discuss the crystallization rate and activation energies with respect to those of Ge 2 - Sb 2 Te 5 1 as well as possible utility in optical data storage systems. * To whom correspondence should be addressed. E-mail: Kanatzid@ cem.msu.edu. † Michigan State University. ‡ Aristotle University of Thessaloniki. (1) (a) Yamada, N.; Ohno, E.; Nishiuchi, K.; Akahira, N.; Takao, M. J. Appl. Phys. 1991, 69, 2849-2856. (b) Rubin, K. A.; Birnie, D. P.; Chen, M. J. Appl. Phys. 1992, 71, 3680-3687. (c) Ohta, T. J. Optoelectron. Adv. Mater. 2001, 3 (3), 609-626. (d) Yamada, N.; Matsunaga, T. J. Appl. Phys. 2000, 88 (12), 7020-7028. (2) (a) Mateev, V.; Petkova, T.; Markovsky, P.; Mitkova, M. 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