Short and Medium Range Order in Se1-xTex Glasses

Complementary XAFS measurements (LURE and NSLS) of Sel,Te, glasses have been canied out on both the Se and Te Kedges at low and room temperatures. Using a multi-shell best fit analysis procedure, we have reconstructed the Se and Te local environment: (i)first shell intrachain nearest neighbors (Se-Sel, Se-Tel, Te-Se~,and Te-Tel); (ii) second shell intrachain (Se-Sez and Se-Tez) and interchain next nearest neighbors (SeoSe3 single scattering in the chains Se-Sel-Se3 and Se-Tel-Se3). For the f i t and second coordination shells we suggest that the intrachain chemical order increases with Te content. On the other hand, we propose a model of random distribution of Se and Te atoms between the chains. Many efforts have been devoted to obtain by XAFS method the intrachain structural parameters of Sel,Te, glasses, amorphous thin films and liquids [I-31. However, relatively little is known about the medium range order in the Sel,Te, glasses. Only the interchain next-nearest-neighbors (SeoSe3 single scattering in the chain Se-Sel-Se3) have been obseved at about 3.64 A in a-Se and a-Sel,Te, glasses [I-31. In this paper we present the XAFS study of intrachain and interchain order of Sel,Te, glasses at low (80 K) and room temperatures. XAFS spectra at the Se K-edge @21 beamline LURE) and Te K-edge (X11A beamline NSLS) were measured in transmission using a standard setup. The details of the experiment and of the data analysis will be published in the forthcoming paper [4]. The obtained local environment built on exclusive chain model (see Figure 1): (i) intrachain nearest neighbors (Se-Sel, Se-Tel, Te-Sel and Te-Tel) and (ii) intrachain (Se-SQ and Se-Te2) and interchain (SeoSe3 single scattering in the chains Se-Sel-Se3 and Se-Tel-Se,) next-nearest neighbors. From the first shell XAFS data we found (see Table 1) that the number of Se-Tel, Te-Sel and Te-Tel bonds increases, but the number of Se-Sel bonds decreases more rapidly with Te content in the glasses, than would suggest the model of completely random distribution of Se and Te atoms (chemical disorder). Therefore we suggest that the intrachain chemical order increases with Te content in the glasses. Detailed analysis of the distances in relation with the glassy composition shows that in the first coordination shell, the distances increase slightly with Te content up to the composition S%7Teo3: SeSel distance elongates from 2.340 A to 2.347 Se-Tel (Te-Sel) distance elongates from 2.540 a to 2.545 hi, and Te-Tel distance elongates from 2.725 A to 2.740 A. Than the distances suddenly increase at the composition Seo6Teo4 up to 2.36 A (Se-Sel), 2.55 A (Se-Tq, Te-Sel) and 2.745 A (Te-Tel). The obtained Debye-Waller OW) factors of Se-Sel, Se-Tel (Te-Sel) and Te-Tel pairs show similar behaviour and in the error bar of measurements are independent on the glass compositions. The DW factors of Se-Sel, Se-Tel (Te-Sel) at low temperature are about 0.0019-0.0022 A2 and increase with the temperature up to 0.0031-0.0038 A2 value as in the trigonal crystalline selenium (1-Se). Therefore, we suggest that thermal disorder gives the main contribution to the DW factors of Se-Sel, Se-Tel (Te-Sel) pairs as at room temperature as at low temperature (80 K). While, the changes of the DW factor values for the Te-Tel bond remain inside the error bar of measurements. The interchain (Se-Se2, Se-Te2) and intrachain (Se-Sel-Se3; Se-Tel-Se3) next-nearest neighbors give main XAFS signal (single scattering contributions), respectively, at small and great wave vectors values, due to very different DW factors and give significant contributions to the second peak of (2.8-4.1 A) for the studied glasses. In comparison with the room temperature crystallographic data, our XAFS results (see Figure 2) on t-Se crystal at low and room temperatures show little shorter interchain distances 3.40 and 3.47 A and the intrachain distances 3.70 and 3.71 A, respectively. For interchain pair the DW factor increases significantly more rapidly from 0.006 up to 0.028 hi2 with temperature than the DW factor for interchain pair from 0.007 up to 0.013 A. For the glasses at room temperature the obtained interand intrachain XAFS signals are very small due to the high values of DW factors and only low temperature (80 K) results are presented. The interchain distances (Se-Se2) and DW factors are in the same range as those obtained for the t-Se crystal at room temperature and we suggest that structural disorder is predominant in glasses for Se-Se2 pair. The distance for this pair increases a little with the Te content in the glasses from 3.40 up to 3.45 A. The intrachain distance (Se-Sel-Se,) and DW factors (0.008 up to 0.010 A2) are a little shorter than the distance and DW factor in the t-Se crystal at room temperature. The intrachain distance increases with variation of Te composition from 3.65 up to 3.70 hi. The intrachain (Se-Tel-Se3) contribution has been detected at highest Te content in the glasses and corresponds to the distance Se--Se3 at 3.97 A and small DW factor 0.008 A2. The obtained next-nearest neighbor intrachain distances Se--Se3 are in good agreement with the obtained intrachain Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19972114 C2-1000 JOURNAL DE PHYSIQUE IV distances Se-Se,, Se-Tel in the first coordination shell of the glasses. Moreover, the analysis of the Te K-edge second shell shows the interchain Te-Se2 pair at 3.43-3.48 A and the DW factor about 0.03 A2. From the second shell XAFS data we found that the number of intrachain Se-Tel-Se3 bonds increases, but the number of Se-Sel-Ses bonds decreases more rapidly with Te content in the glasses that would suggest the model of completely random distribution of Se and Te atoms inside chains, and is in good agreement with the obtained model of chemical ordering in the first coordination shell. Therefore, as for the first coordination shell, we suggest that the intrachain chemical order increases with Te content in the glasses. On the other hand, the number of interchain Se-Tez bonds increases, but the number of Se-Sez bonds decreases in agreement with Te content in the glasses, that would suggest the model of completely random distribution of Se and Te atoms between the chains. Table I: Structural data obtained from the best-fit analysis of the Se and Te Kedge EXAFS in Sel,T&. N is the number of Se (f 0.1) or Te (f 0.1) atoms located in the frst shell at distance R from the Se (f 0.005 A) or Te (* 0.007 A); 2 is the DW factors (f 0.0005 A'); E is the fitting error [4]. Compound Se-Sel and Te-Se! Se-Tel and Te-Tel (X lo4)