Nuclear quadrupole resonance study of local structure of the Ge–As–Se ternary chalcogenide glasses E. Mammadov a,b, , P.C. Taylor b a Institute of Physics, National Academy of Sciences of Azerbaijan, Baku, AZ1143, Azerbaijan b Physics Department, Colorado School of Mines, Golden, CO 80401, USA Available online 31 January 2008 Abstract The local bonding structure in the Ge–As–Se ternary glassy system has been investigated by 75 As nuclear quadrupole resonance (NQR) at 77 K. For the compositions with average coordination number  r ¼ 2:2; 2:4; 2:54 the NQR measurements yield no evidence for the presence of As–As homopolar bonds. For the composition Ge 2 As 2 Se 5 (  r ¼ 2:67) the NQR lineshape exhibits two peaks corre- sponding to arsenic sites with zero (or three) and two As–As bonds. High Ge and As content leads to formation of microclusters, where the predominant structural units are, possibly, As 4 Se 4 molecules or partial molecules with As–As bonds. The NQR spectrum for the composition Ge 0.33 As 0.12 Se 0.55 (  r ¼ 2:78) shows a weak peak corresponding to an arsenic site with zero (or three) homobonds and two resolved peaks in the high frequency region. Such behavior is consistent with an increase in inhomogenity in the local structure and perhaps nanoscale phase separation. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.43.Dq; 61.43.Fs Keywords: Chalcogenides; Nuclear magnetic (and quadrupole) resonance 1. Introduction Chalcogenide glasses have some unique properties, such as low phonon energy, high values of refractive indexes and transparency in middle and far infrared parts of the electro- magnetic spectrum, that make them attractive for different applications in optoelectronics and photonics. Glasses in the Ge–As–Se ternary system (undoped and doped with rear-earth elements) are under intensive investigation as promising materials for active media in optoelectronics [1–4] and ultrafast all-optical switches in telecommunica- tion systems [5,6]. However, the local structure of these amorphous materials is not well understood. It is believed that the local structure of the Ge x As y Se 1xy glasses contains Ge–Se and As–Se covalent bonds. Each element has a coordination number corre- sponding to eight minus its group number (‘8  N’ rule [7]) in the periodic table of elements, while the average coordination number for the entire glass is expressed as  r ¼ r A x þ r B y þ r C ð1  x  y Þ; ð1Þ where A, B, and C are the constituent elements, respec- tively. Glasses in this system occur over a broad glass- forming region with average coordination number ranging from  r ¼ 2 for pure selenium to 3.3 for compositions with high concentrations of As and Ge. Since selenium is the main glass former in these glasses, it is convenient to con- sider local structure of the ternary system on the basis of structural units of the corresponding binary systems, Ge– Se and As–Se. In this work we investigate the local structural ordering and its dependence on the average coordination number. 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.051 Corresponding author. Address: Institute of Physics, National Acad- emy of Sciences of Azerbaijan, Baku AZ1143, Azerbaijan. Tel.: +994 12 439 3528. E-mail address: mammadov_eldar@hotmail.com (E. Mammadov). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 354 (2008) 2732–2735