Solid State Communications 151 (2011) 1459–1462 Contents lists available at SciVerse ScienceDirect Solid State Communications journal homepage: www.elsevier.com/locate/ssc Magnetic resonance study of arsenic bonding sites in ternary chalcogenide glasses E. Mammadov a,* , D. Bobela b , A. Reyes c , S. Mehdiyeva a , P.C. Taylor b a Institute of Physics, National Academy of Sciences, Baku, AZ 1143, Azerbaijan b Physics Department, Colorado School of Mines, Golden, CO 80401, USA c National High Magnetic Field Laboratory, Tallahassee, FL 32310, USA article info Article history: Received 21 February 2011 Received in revised form 12 May 2011 Accepted 28 June 2011 by R. Phillips Available online 5 July 2011 Keywords: A. Disordered systems A. Semiconductors E. Nuclear resonances abstract 75 As NQR and high-field NMR experiments have been performed on Ge x As y Se 1-x-y glasses. Evolution of As bonding structure from arsenic sites with axially symmetric distribution of the electric field gradient (EFG) to highly asymmetric As surroundings has been revealed. Arsenic atoms form pyramidal structural units in Ge 2 As 2 Se 7 with no evidence of significant concentration of homopolar bonds. In Ge 2 As 2 Se 5 most of arsenic atoms form structural units with two As–As bonds per atom and asymmetric EFG distribution. Arsenic bondings become more complicated in Ge 0.33 As 0.12 Se 0.55 where all arsenic sites are highly distorted. The combination of NQR and NMR data provide valuable information on arsenic bonding dynamics in these glasses. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Ternary chalcogenide glasses are of interest for several im- portant applications such as phase-change memory devices, ac- tive and passive media for optoelectronics, waveguides etc [1,2]. Glasses of Ge–As–Se (GAS) system have extensively been studied for rare-earth-doped optical fiber amplifiers [3,4]. However, due to lack of understanding of their local structure improvement of de- vice parameters and development of new materials is mainly based on a trial-and-error approach. Application of scattering techniques to reveal information on the local structural arrangement in these materials is hindered due to absence of long-range atomic order. The problem is further complicated in ternary systems as a com- positional disorder is introduced in addition to the structural dis- order. Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) techniques provide direct information on asym- metry of nearest-neighbor surrounding of an atom absent from scattering techniques and have been proved as a powerful tool for study of the local structure in disordered solids [5,6]. The local structure of chalcogenide glasses is often described using the 8-N rule when coordination of atoms is determined by number of covalent bonds formed to maintain a closed shell of * Corresponding author. E-mail addresses: mammadov_eldar@hotmail.com, emammadov@physics.ab.az (E. Mammadov). eight electrons with N being the corresponding group number of each atom [7]. On the other hand, bondings in these glasses are formed with preference of heteropolar bonds with highest bond energy being formed first [8]. Thus, in the GAS system Ge–Se and As–Se bonds are the most probable. To satisfy all their bonding requirements four-fold coordinated Ge and three-fold coordinated As atoms form tetrahedral and pyramidal structural units, respectively, cross-linking Se-chains [7]. We present results of combined NQR and high-field NMR studies of arsenic local bonding structure in GAS glasses. 2. Experimental procedure Glasses for NMR and NQR experiments have been synthesized from high purity (6N ) constituent elements by rocking the melts at 900 °C overnight and quenching in ice water. Powdered samples of different compositions as Ge 0.05 As 0.1 Se 0.85 , Ge 0.15 As 0.1 Se 0.75 , Ge 0.18 As 0.18 Se 0.63 (Ge 2 As 2 Se 7 ), Ge 0.22 As 0.22 Se 0.55 (Ge 2 As 2 Se 5 ), and Ge 0.33 As 0.12 Se 0.55 were prepared. NQR and NMR measurements were performed at liquid nitro- gen temperature. The spin-echo technique with 90°–180° pulse se- quence was utilized for both experiments using 90° pulse widths as 5 and 4 μs, respectively. Intensity of the NQR echo signal was obtained by averaging over 2000 scans for each data point. A stan- dard NMR probe with a coil made of copper wire has been utilized to obtain spin-echo signal in NMR measurements. Since the echo amplitude for the samples was very small a special pulse setting 0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.06.040