Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol Kinetics based evidence for intermediate phase in Ge 15 Te 85 − x In x chalcogenide glasses G. Sreevidya Varma a , Abhishek Chaturvedi b , U. Ramamurty b , S. Asokan a,⁎ a Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560 012, India b Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India ARTICLE INFO Keywords: Chalcogenides Glass transition Intermediate phase Activation energy ABSTRACT This paper presents the thermal studies carried out using Differential Scanning Calorimetry on Ge 15 Te 85 − x In x glasses (1 ≤ x ≤ 11) to find glass transition temperature, T g and crystallization temperature, T x . Further, the variation of T g and T x for different heating rates have been studied. The composition dependence of T x , T g and stability factor, ΔT have been investigated. The crystallization kinetics has been studied by a non-isothermal method; the activation energy of crystallization, E x has been determined using Kissinger method whereas glass transition activation energy, E g using Kissinger as well as Tool-Narayanaswami-Moynihan (TNM) methods for all the compositions. The results obtained reveal the opening of an intermediate phase, IP in the composition range 2 ≤ x ≤ 7. T g , T x and ΔT increases continuously across the IP region. The E x exhibits a plateau and E g follow a bell shaped curve in the IP region. 1. Introduction Chalcogenide glasses find applications in optical and electrically addressed data storage devices, which work on the principle of large change in reflectivity and resistivity when the material undergoes amorphous to crystalline transition and vice-versa [1]. The activation energies of crystallization, melting and glass transition are key para- meters to evaluate the archival life and recording ability of amorphous chalcogenides [2,3]. As the process of data storage includes amorphous to crystalline conversion which consists of nucleation and growth processes, the knowledge about activation energy is important to con- trol crystallization. The effective working limit of a chalcogenide glassy memory device is determined by stability against crystallization and speed of re-crystallization. The most common material used in chalcogenide-based phase change memory devices is Ge-Te-Sb, commonly known as GST. The recent studies show that in memory operations, In-Ge-Te (IGT) thin films have much more thermal stability than Ge-Te-Sb (GST) because of their higher melting points due to the strong ionic IneTe bond [4]. The crystallization temperature of IGT is 150 K higher than commercially used GST, so it can be used in automotive applications also [5]. Chalcogenide glasses are made up of covalent networks and their degree of connectivity can be changed by varying the composition [6]. The degree of connectivity depends on the mean co-ordination number of the system. At the critical average coordination number 〈r c 〉 = 2.4, the system yields maximum mechanical stability and optimum glass forming ability. Rigidity theory [7] has predicted that flexible networks would spontaneously become stressed-rigid, as their mean coordination number, 〈r c 〉 becomes ≥2.4. The mean coordination number, 〈r c 〉 which marks such a transition, is called the Rigidity Percolation Threshold (RPT). Thorpe invented an integer algorithm (Pebble game) [8] and found two hierarchical elastic transitions by self-organizing networks, the rigidity transition followed by the stress transition at slightly higher connectivity, namely from a floppy to an isostatically rigid state occurring at 〈r c1 〉 and from an isostatically rigid to a stressed rigid state occurring at 〈r c2 〉. For random covalent networks, the ri- gidity percolation occurs at a single point, at a mean coordination 〈r c 〉 = 2.4. In several families of covalent and ionically modified glass and oxide systems, Boolchand et al. [8–10] have observed a new elastically percolative phase of metastable matter in between the two rigidity transitions and it is known commonly known as Boolchand's inter- mediate phase (IP). In the IP region, the network is rigid, but un- stressed. The new cross-links in the IP, find way to the flexible part of the network, thereby self-organizing it and avoiding forming redundant bonds [11]. The intermediate phase (IP) glass compositions possess remarkable physical properties; the melt to glass transition is thermally reversible and compositions in IP are apparently stress free, self-organized, show minimal aging, fill space efficiently and are good glass formers [8–12]. http://dx.doi.org/10.1016/j.jnoncrysol.2017.06.006 Received 6 March 2017; Received in revised form 24 May 2017; Accepted 5 June 2017 ⁎ Corresponding author. E-mail address: sasokan@iap.iisc.ernet.in (S. Asokan). Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx 0022-3093/ © 2017 Elsevier B.V. All rights reserved. Please cite this article as: Sreevidya Varma, G., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.06.006