Network-Forming Nanoclusters in Binary As–S/Se Glasses: From Ab Initio Quantum Chemical Modeling to Experimental Evidences

Hyla Nanoscale Research Letters (2017) 12:45 DOI 10.1186/s11671-016-1788-8 NANO EXPRESS Open Access Network-Forming Nanoclusters in Binary As–S/Se Glasses: From Ab Initio Quantum Chemical Modeling to Experimental Evidences M. Hyla1,2 Abstract Network-forming As2(S/Se)m nanoclusters are employed to recognize expected variations in a vicinity of some remarkable compositions in binary As–Se/S glassy systems accepted as signatures of optimally constrained intermediate topological phases in earlier temperature-modulated differential scanning calorimetry experiments. The ab initio quantum chemical calculations performed using the cation-interlinking network cluster approach show similar oscillating character in tendency to local chemical decomposition but obvious step-like behavior in preference to global phase separation on boundary chemical compounds (pure chalcogen and stoichiometric arsenic chalcogenides). The onsets of stability are defined for chalcogen-rich glasses, these being connected with As2Se5 (Z = 2.29) and As2S6 (Z = 2.25) nanoclusters for As–Se and As–S glasses, respectively. The physical aging effects result preferentially from global phase separation in As–S glass system due to high localization of covalent bonding and local demixing on neighboring As2Sem+1 and As2Sem−1 nanoclusters in As–Se system. These nanoclusters well explain the lower limits of reversibility windows in temperature-modulated differential scanning calorimetry, but they cannot be accepted as signatures of topological phase transitions in respect to the rigidity theory. Keywords: Chalcogenide glasses, Nanocluster, Reversibility window, Ab initio calculation Background Chalcogenide glasses (ChG) of binary As–Ch system (Ch = S, Se) are representatives of disordered covalent network solids, which clearly demonstrate glass-forming tendencies predicted in terminology of rigidity theory initially developed by Phillips and Thorpe [1, 2]. Within this approach, the strongest glass-forming ability is a character for ChG possessing structural network with the number of degrees of freedom equals to the number of Lagrangian constraints per atom nc associated with nearest-neighbor bond-bending and stretching forces (so in this case, the short-range configuration entropy and network strain energy tend to zero). In such a way, an average coordination number of glass network should be Correspondence: 1 Institute of Physics of Jan Dlugosz University of Czestochowa, Al. Armii Krajowej 13/15, 42-200 Czestochowa, Poland 2 Lviv Scientific-Research Institute of Materials of Scientific Research Company “Carat”, Stryjska str. 202, Lviv 79031, Ukraine close to Z = 2.40 for best glass-forming compounds (i.e., stoichiometric As2S3 and As2Se3), which are optimally constrained (nc = 3) and thus, not affected by physical aging. The optimally constrained (rigid but unstressed) intermediate phase (IPh) in ChG is expected in narrow compositional domain between under-constrained floppy phase (FPh, nc < 3) and over-constrained stressed rigid phase (SRPh, nc > 3) [3, 4]. In respect to theoretical calculations [4], the typical width of such IPh is expected to be rather narrow in ChG, since the saturated covalentbonded glassy network exists at a cost of very low entropy as topologically self-organized phase (nc = 3). Since the earliest 2000s, Boolchand et al. [5–8] tried to prove experimentally the IPh employing the method of temperature-modulated differential scanning calorimetry (TM-DSC) as probe for ChG with nearly vanishing nonreversing enthalpy (ΔHnr) forming the so-called reversibility window (RW). Nevertheless, the experimentally detectable RW in ChG of binary As–S (Z = 2.225 ÷ 2.29) © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Hyla Nanoscale Research Letters (2017) 12:45 Page 2 of 7 [6] and As–Se (Z = 2.29 ÷ 2.37) [6–8] systems occurs essentially shifted towards Ch side and too compositionally stretched to be accepted as realistic IPh signatures. Additionally, the compositional boundaries for RW in TMDSC experiments were instable, showing an obvious trend to physical aging with prolonged duration [8–12] and changed storage conditions [8, 13, 14]. Furthermore, the notable dependence of aging time scales on the distance from glass transition region [15], which plays a decisive role in view of the known Williams–Landel–Ferry relation [16], was also ignored in these measurements. Despite this argumentation, testifying that compositional boundaries of IPh in As–S/Se ChG determined as TM-DSC-probed RW [5–8] are rather artifacts of measuring procedure (revealing essential variation in sensitivities to different atomic entities [15, 17, 18]), origin of these compositional anomalies in a vicinity of Z = 2.225 (As–S system) and Z = 2.29 (As–Se system) remains still controversial. In this work, this specificity for As–S/Se ChG systems will be traced using ab initio quantum chemical modeling known as cation-interlinking network cluster approach (CINCA) [19–22] applied to As2(S/Se)m nanoclusters. Although As–S and As–Se are isotypical ChG systems, their network-forming tendencies differ essentially. Thus, the region of glass formation stretches from Z ≅ 2.00 (elemental Se) to Z ≅ 2.60 (As3Se2 glassy alloy) in As–Se system [23–26], whereas it is distinctly narrower in As–S system being in the range of ~2.05 < Z < (2.44–2.46) [23–26]. Myers and Felty [26] explained this by different melting behaviors of these systems. The region of stable homogeneous glasses depends on melt-quenching glass preparation technological route. For example, Hruby [27] pointed out that a second glass-forming region in As–S system exists at Z = 2.51 ÷ 2.66, when the melt was held for several hours at 300 ÷ 400 °C above the liquidus temperature. Taking into account the phase diagrams of As–Ch systems (Ch = Se, S) [26, 28, 29], two intrinsic decomposition processes are to be expected for Ch-rich glass compositions (Z ≤ 2.40). The first process can be attributed to instability in a glassy network composed by As2Chm atomic clusters interlinked by Ch chains due to local demixing on compositionally close As2Chm+1 and As2Chm−1 clusters [12], this local chemical decomposition obeying scheme: 2
As2 Chm ↔As2 Chmþ1 þ As2 Chm−1 ð1Þ The second decomposition process is connected with global possibility of glassy network to be separated on two distinct phases, the stoichiometric As2Ch3 and “pure” chalcogen Ch. Noteworthy, this global phase separation in As2Chm ChG results in two corner-shared AsCh3/2 pyramids (i.e., As2Ch3 cluster) and Chm−3 remainder according to the reaction [12]: (...truncated)