Trans. Indian Inst. Met. Vol.57, No. 2, April 2004, pp. 201-203 T NOTE 1. INTRODUCTION Considerable attention has been given in the recent times, to understand the relation between the electrical switching and network topological thresholds 1-3 exhibited by chalcogenide glasses, namely, the rigidity percolation threshold (RPT) and chemical threshold (CT). The rigidity percolation deals with dimensionality and rigidity of the glassy network and is closely associated with the average coordination number (<r>) of the system. The average coordination number of a chalcogenide glass is calculated from <r> = ( ) ∑ = = n i i i i Z X 1 100 Where, x i = atomic percentage of the element; z i = coordination number of i th element. The constraint theory of Phillips and Thorpe 4 proposes that in chalcogenide network glasses, at a critical average coordination number (<r c >), a mechanical equilibrium is established, wherein the degrees of freedom per atom and the number of constraints acting on it become equal. 5,6 Glasses with average coordination numbers less than the critical average coordination are under-constrained and are elastically in floppy state, whereas glasses with average coordination numbers greater than the critical average coordination are over-constrained and rigid. The composition corresponding to the critical average coordination is named as the rigidity percolation threshold (RPT) of the glass. For a glassy network with purely covalent bonding, rigidity percolation occurs at the critical average coordination <r c > = 2.4; however the critical average coordination may shift to values higher than 2.4 7,8 , due to ionic interactions between constituent atoms. It has also been pointed out recently that in certain glassy systems the rigidity percolation may occur over a range of average coordination numbers, with a distinct onset and completion 9 . ELECTRICAL SWITCHING BEHAVIOUR OF BULK As-Te-Si GLASSES M. Anbarasu, N. Manikandan and S. Asokan Department of Instrumentation, Indian Institute of Science, Bangalore-560012. E-mail : sasokan@isu.iisc.ernet.in (Received 5 October 2003 ; in revised form 7 December 2003) ABSTRACT The current-voltage characteristics and electrical switching behavior of bulk As 40 Te 60-x Si x have been investigated over a wide range of compositions (2 w x w 14). Most of the glasses have been found to exhibit a current controlled negative resistance behavior and memory switching. The overall features of the I-V characteristics of As 40 Te 60-x Si x are not altered with the sample thickness. However, the switching voltage V t is found to increase linearly with increasing sample thickness in the range of 0.15 mm to 0.45 mm. It is observed that the switching voltage (V t ) decreases with increasing Si content up to x = 3 (average coordination number <r> = 2.46). Above x = 3, a turn around is seen in the composition dependence of V t and it increases with “x” thereafter. The increase in V t culminates in a maximum at x = 8 (<r> = 2.56). As per the information available in literature, the composition dependence of switching voltages in chalcogenide glasses are influenced by two topological thresholds namely rigidity percolation threshold (RPT) and chemical threshold (CT) respectively. It is likely that the observed turn around and maximum in the composition dependence of V t are associated with rigidity percolation in the system.