Confined acoustic modes and spectral determination of network connectivity: Raman signatures of nanometric structure in g-Ge x Se 1Kx D. Sharma a , Alka Ingale b , A.M. Awasthi a, * a Thermodynamics Laboratory, UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India b Centre for Advanced Technology, Indore 452 013, India Received 18 February 2005; accepted 10 March 2005 by A.K. Sood Available online 24 March 2005 Abstract Low-temperature Raman scattering results are presented on three glass compositions of Ge x Se 1Kx (15, 20, and 27 At. Ge%), and conciled with their reported structural data. The acoustic range is marked by the absence of Boson peak, and features discrete modes corresponding to nanometric dynamic aggregates. First sharp diffraction peak (FSDP) in structure factor S(k), due to Ge–Ge correlations brought about by shared Ge[Se 1/2 ] 4 tetrahedra medium-range structures, signifies 3D networking of Se-chains. A measure of volume-fraction of these entities determined from the FSDP area, and their number density r MRS f(k 3 Area) FSDP is found to relate directly to the network connectivity, defined from optical Raman spectra in terms of the degree of cross-linking of Se-chains. q 2005 Elsevier Ltd. All rights reserved. PACS: 61.10Ki; 61.43KFs Keywords: A. Glasses; D. Confined acoustic modes; D. Network connectivity; D. Rigidity phases and transitions; E. Raman spectroscopy 1. Introduction There has been considerable work on the bulk glasses via light scattering experiments. Disorder in amorphous and heterogeneous systems has a strong effect on vibrational dynamics in the acoustic range. Network glass system Ge x Se 1Kx has received a great deal of interest, in terms of the changes in its structure with increasing Ge content. Knowledge about the structure of network glasses has improved steadily, since the initial work of Zachariasen [1] introduced the idea of continuous random network (CRN). Despite the successes in understanding the structure, some concerns remain; most serious being that the network cannot be truly random [2]. Three key compositions of Ge x Se 1Kx were selected to bring out representative variation in structural and vibrational properties across the series; xZ0.15 just ensuing FSDP [3], xZ0.20 being the ‘rigidity percolation’ threshold [2,4], while xZ0.27 onsets the ‘stressed-rigid’ phase [2,4]. In constraints-counting terminology outlined below, they typify as under, critical, and over-constrained networks, respectively. Mechanical constraints (essentially bond- bending and stretching) on an r-fold coordinated atom work out to be [N c Zr/2C(2rK3)] in number [5].A covalent system is network classified depending upon the atomic-averaged difference h(N c K3)i between constraints and atomic degrees of freedom. An under-constrained system admits the displacement of groups of atoms eliciting little elastic response. These Solid State Communications 134 (2005) 653–658 www.elsevier.com/locate/ssc 0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.03.020 * Corresponding author. Tel.: C91 731 2463913; fax: C91 731 2465437. E-mail address: amawasthi@udc.ernet.in (A.M. Awasthi).