PHYSICAL REVIEW B VOLUME 36, NUMBER 15 15 NOVEMBER 1987-II Coordination-number-induced morphological structural transition in a network glass B. Norban, D. Pershing, R. N. Enzweiler, and P. Boolchand University of Cincinnati, Cincinnati, Ohio 45221-0030 J. E. CxriSths and J. C. Phillips AT&T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 14 November 1986; revised manuscript received 9 July 1987) Mossbauer electric field gradients, Raman vibrational modes, and crystallization temperatures exhibit threshold behavior near the composition x =0.20 in binary Si Tel „glasses. This thresh- old is evidence of a morphological structural change that may be driven by network connectivity or average coordination number. At x (0. 20, the network largely consists of Si-cross-linked Te„ chain segments. At x ) 0. 20, Te„chains reconstruct with tetrahedral Si(Telq2)4 units and nucleate a defect-ridden Si&Te3-like layered molecular fragment. These fragments represent the elastically rigid domains that percolate above the threshold. I. INTRODUCTION Current interest in elastic properties of glassy net- works has been stimulated by the idea of rigidity per- colation. ' Undercoordinated (or underconstrained) net- works corresponding to a mean coordination number (m ) &2. 40 are pictured as elastically Aoppy because these networks are easily deformed under a shearing force. On the other hand, overcoordinated (or overcon- trained) networks corresponding to (m ) & 2. 40 are visu- alized as elastically rigid because such networks resist a change in shape under a shearing force. These ideas due to Phillips' and Thorpe have led to the realization that at the threshold coordination number of (m ) =2. 40, a glass network constrained by bond-stretching and -bend- ing forces sits at a mechanical critical point in which rig- id regions are thought to percolate in a mean-field sense. In a glassy A B, network composed of cations ( 3 =Cre, Si) and anions (B =S, Se, and Te) that are, re- spectively, fourfold and twofold coordinated, the critical composition x =x, corresponding to vector percolation is given by 2(x, + 1) =2. 40 yielding x, =0. 20. Experimental evidence in support of mechanical thresholds in binary Cse-Se (Ref. 3) glasses and ternary Ge-As-Se glasses was recently provided us- ing a macroscopic probe (ultrasonic elastic constant) of network rigidity. The structural consequences of mechanical thresholds were elucidated by Bresser et al. using Mossbauer site spectroscopy which was also correlated with Raman band spectroscopy. In this paper we present experimental evidence for thresholds in ' Te electric field gradients, Raman vibra- tional modes, and crystallization temperatures each occurring very near the composition x =0. 20 in binary Si„Tel glasses. These results appear to be consistent with the notion that molecular fragments based on the c-Si2Te3 structure are populated in the bulk glasses above the vector percolation threshold at x &0. 20. At x (0. 20 Te„chain segments and tetrahedral Si(Te&&2)4 units predominantly occur in the glassy networks. The present experiments, along with those reported re- cently, appear to demonstrate that the so-called mechan- ically rigid regions present in overcoordinated glasses largely consist of compacted molecular fragments de- rived from corresponding crystals. II. EXPERIMENTAL PROCEDURES Binary Si„Te& glasses were prepared by alloying the elements in vacuum followed by a water quench. 99. 9999%-pure Si chunks and 99. 9999%-pure Te chunks from Cerac Inc. , Milwaukee, Wisconsin, were used as the starting materials. The elements in the desired pro- portion were sealed in 5-mrn-inner-diameter fused-quartz ampules at a pressure of 5 &( 10 Torr using a CTI cryopump-based vacuum system. The melts, typically 0.5 g in size, were held at 1000'C for 5 d in a vertical configuration and periodically shaken to homogenize be- fore quenching in water. This procedure yielded homo- geneous glassy samples in the composition range 0. 10&x &0. 25 that displayed reproducible glass transi- tion Tg(x) and crystallization temperatures T, (x). Our differential-scanning-calorimetry traces taken with a Perkin-Elmer model 2C instrument at selected compositions x are displayed in Fig. 1. These results show the presence of two distinct crystallization temper- atures, T, & and T, z at x &0. 20 but only one crystalliza- tion temperature at x &0. 21. Figure 2 displays the com- positional dependence of Tg(x), T„(x), and T, 2(x) taken at a 20-K/min scan rate. Raman scattering from the bulk glass chips was ob- tained using the 7525-A exciting line from a Kr-ion laser. Spectra were recorded as a function of laser power to understand the various steps leading to photo- crystallization both at x &0. 20 and at x &0. 20. Details of the experimental setup are described elsewhere. Fig- ures 3(a) and 3(b) reproduce spectra observed at x =0. 18 36 8109 1987 The American Physical Society