Physical ageing in Se–Te–Sb glasses M.K. Vanitha a , M.V. Hanumantha Rao a,b , S. Asokan c , K. Ramesh d,n a Department of Physics, Christ University, Bangalore 560029, India b LEOS, Indian Space Research Organization, Bangalore 560058, India c Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India d Department of Physics, Indian Institute of Science, Bangalore 560012, India article info Article history: Received 4 October 2011 Received in revised form 31 December 2012 Accepted 12 January 2013 Available online 22 January 2013 Keywords: A. Chalcogenides A. Glasses C. Differential Scanning Calorimetry (DSC) D. Specific heat D. Thermodynamic properties abstract Bulk Se 60x Te 40 Sb x glasses in the composition range 0 rx r14 were prepared by the melt quenching method. Differential Scanning Calorimetric (DSC) and thermal crystallization studies were performed to understand the thermodynamic property like glass transition and structural transformations. These glasses exhibit sharp endothermic peak at the glass transition (T g ). Disappearance of the endothermic peak at T g in the rejuvenated samples clearly indicates the ageing effect in these glasses. Addition of Sb to Se–Te increases the connectivity of the structural network which is evidenced from the increase in T g . A distinct change in the slope of the T g at x ¼6, indicates a major change in the way the network is connected. The glass forming ability and the thermal stability also exhibit a maximum at x ¼6. T g increases with the ageing time and the corresponding fictive temperature (T f ) calculated from the specific heat curves shows a decreasing trend. The molecular movements along the polymeric Se chains might cause the structural relaxation and the physical ageing. The physical ageing effect has been understood on the basis of the Bond Free Solid Angle (BFSA) model proposed by Kastner. Thermally crystallized samples show the formation of rhombohedral Sb 2 Te 3 , rhombohedral Sb 2 Se 3 and hexagonal Te phases. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Tellurium based chalcogenide glasses find wide range of applica- tions in many fields like thermal imaging, ultra-high density phase change storage and memory, integrated fiber optics, infrared photo detectors and photo voltaic and bio sensors [1–9]. For example, Ge– Ga–Te glasses transmit infrared light even up to 25 mm [10]. Ge–Sb– Te is known for its phase change and memory applications [11,12]. Ge–As–Te glasses are found to have good sensitivity for toxic gasses at room temperature [13]. a-Se is well known for its application in xerography and recently it has been used as X-ray image detectors in medical imaging [14]. The chemical durability and corrosion resis- tance of Te can be increased to a larger extent by the addition of Se [15,16]. Tellurium polymerizes the Se network by entering into the Se chains. Te also opens up the Se 8 rings and forms mixed rings of Se and Te (rings of Se 2 Te 6 ) [17]. The increase of Te increases the number of chains linearly and reduces the length of the chains logarithmi- cally in Se network. When the length of the chains becomes very less, the glass formation becomes very difficult. Thus the glass formation in systems with higher Te content is difficult [17–19]. To improve the glass forming ability and alter the properties of a binary glass, a third element can be added or the cooling rate has to be increased [20, 21]. For example, the addition of Group V element Sb to binary Se–Te modifies the structural and electrical properties considerably [22–24]. The increase of Sb in Se–Te increases the electrical con- ductivity by decreasing the conductivity activation energy. Even with the addition of the third element Sb to Se 60 Te 40 , the glass formation is found to be difficult in this work as the Te content is high. Hence, obtaining glasses with higher Te and Sb is a difficult task. Though, chalcogenide glasses find applications in many fields, their performance can be affected by the ageing effect since the glasses are not in thermodynamic equilibrium state [25,26]. It means the properties can vary over a period of time which can result in the poor performance of the device made out these glasses. Since the glasses are generally prepared by the melt quenching method, they deviate from their metastable equili- brium state. Hence, they have excess enthalpy, excess free volume and excess configurational entropy compared to their crystalline counterparts and extrapolated equilibrium states of the super- cooled liquid at a fixed temperature [25,26]. Exposure of the chalcogenide glasses to light can accelerate the ageing process [27–29]. When a glass is heated in a Differential Scanning Calorimeter (DSC), it undergoes an endothermic reaction at the glass transition region. This can be observed as an endothermic step (base line shift) in the DSC scan if the sample is not aged and as an endothermic peak in an aged glass. Reheating (rejuvenation) Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jpcs Journal of Physics and Chemistry of Solids 0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.01.010 n Corresponding author. Tel.: þ91 80 2293 2716; fax: þ91 80 2360 2602. E-mail address: kramesh@physics.iisc.ernet.in (K. Ramesh). Journal of Physics and Chemistry of Solids 74 (2013) 804–810