Compositional dependence of the local structure of Se x Te 1-x alloys: Electron energy-loss spectra, real-space multiple-scattering calculations, and first-principles molecular dynamics N. A. Katcho, 1,3 E. Lomba, 3 E. Urones-Garrote, 1 A. R. Landa-Cánovas, 2 and L. C. Otero-Díaz 1 1 Departamento de Química Inorgánica I, Facultad CC. Químicas, UCM, E-28040 Madrid, Spain 2 Instituto Ciencia de Materiales, CSIC, 28049 Cantoblanco, Madrid, Spain 3 Instituto de Química Física Rocasolano, CSIC, Serrano 119, E-28006, Madrid, Spain Received 16 March 2006; published 6 June 2006 In this work we present an investigation on the composition dependence of the local structure in Se x Te 1-x crystalline alloys analyzing their experimental energy-loss spectra with the aid of a real-space multiple- scattering modeling approach and first-principles molecular dynamics. The concourse of this latter technique is essential for a proper modeling of the alloy spectra. From our results, it can be inferred that Se x Te 1-x alloys exhibit a high degree of substitutional disorder ruling out the existence of fully ordered alternating copolymer chains of Se and Te atoms. DOI: 10.1103/PhysRevB.73.214203 PACS numbers: 61.66.Dk, 79.20.Uv, 82.80.Pv, 71.15.Pd I. INTRODUCTION Selenium and tellurium at ambient conditions are isomor- phic direct gap semiconductors whose most stable crystalline structure is a trigonal phase consisting of a parallel arrange- ment of helical chains. 1 In the case of Se, these persist after melting the sample—which remains semiconducting— whereas in Te, melting is accompanied by the formation of an entangled network, with a subsequent increase in the co- ordination numbers and conductivity. 2 Bonding in both cases is predominantly covalent, although intrachain bonding is somewhat stronger in trigonal Se, as can be inferred from a comparison of the vibrational spectra of Se and Te. 3,4 As a consequence of their chemical similarity, Te and Se combine in a solid solution Se x Te 1-x which preserve the trigonal struc- ture of the elemental substances and the semiconducting character, being the width of the band gap tunable by com- positional changes. Moreover, in liquid Se-rich and equimo- lar alloys, a continuous semiconductor-metal transition can also be induced by an increase in the Te ratio. 5–8 This tunable behavior of Se-Te alloys makes them particularly interesting materials for their potential technical applications. There remain, however, still open questions as to the mi- croscopic structure of this solid solution. It seems clear from the x-ray investigations 9–11 that the trigonal structure of the elemental Te and Se is preserved with lattice parameters ex- hibiting minor deviations from Vegard’s law. One might be then tempted to conclude that the corresponding crystal structures present substitutional disorder, with no preferential short range chemical order. However, the Mössbauer study performed by Boolchand and Suranyi 12 seems to suggest that the equimolar SeTe alloy is an ordered copolymer of alter- nating Se and Te atoms. Their study is consistent with certain deviations from Vegard’s law also observed by Grison 11 in the equimolar alloy. In any case, from the Te 125 Mössbauer spectra one might conclude that the Se-Te bonds are some- what stronger than the average of Se-Se and Te-Te bonds. This is in agreement with the results of recent ab initio simulations 13 for molten Se x Te 1-x in which it was found that, in the liquid and amorphous SeTe alloys, nearest neighbors of a given atom belong mostly to a different chemical species suggesting a higher stability of Se-Te bonds. Taking into ac- count that extended x-ray-absorption fine structure EXAFS studies 14 indicate that Se-Se bonds are substantially stronger that Se-Te bonds, this would imply that Te bonding is ex- tremely labile. It seems thus that our knowledge of the bond- ing properties in Se x Te 1-x is still somewhat incomplete. In this regard, the purpose of the paper is twofold. On one hand, we want to explore the ability of electron energy-loss near edge structure ELNES analysis to provide information on the local structure and bonding properties of these crystalline semiconducting alloys. On the other hand, we will resort to real-space multiple-scattering RSMS calculations to pro- vide an interpretation of the ELNES spectra, and this will additionally furnish an assessment on the abilities of these type of theoretical approaches for the modeling of electron energy-loss EEL spectra. We will see that for the RSMS approach to properly reproduce the spectra of the alloys, atomic positions have to be optimized by means of ab initio molecular dynamics. The rest of the paper is organized as follows. In the fol- lowing section we summarize the experimental details of sample preparation and ELNES data acquisition and analyze the influence of the sample preparation on the structure of the alloys. Section III is devoted to the details of the calcu- lations and in Sec. IV we present and discuss our most rel- evant results. II. EXPERIMENTAL DETAILS AND X-RAY STRUCTURE Selenium-tellurium alloys with 25, 50, and 75% of Se were prepared by melting the appropriate quantities of 99.9999% pure elemental Se and Te enclosed in vacuum- sealed quartz tubes, with a length of 15 cm and a diameter of 11 mm. The samples were heated for 24 h at 773 K and annealed for one week at 453 K to facilitate the build up of chemical short and long range order. X-ray powder diffrac- tion XRPD patterns were obtained on a Siemens D-501 diffractometer Cu K 1 radiation  = 0.15406 nm. The EEL spectra were recorded using a Philips CM200 Field Emission Gun FEG microscope coupled to a GIF 200 spectrometer, operating in diffraction mode with a collection PHYSICAL REVIEW B 73, 214203 2006 1098-0121/2006/7321/2142038 ©2006 The American Physical Society 214203-1