Density functional theory Raman spectroscopy Medium-range order Short-range order Calorimetry Enthalpy relaxation Structural relaxation a b s t r a c tWe review intermediate phases (IPs) in chalcogenide glasses and provide a structural interpretation of these phases. In binary group IV selenides, IPs reside in the 2.40 < r < 2.54 range, and in binary group V selenides they shift to a lower r, in the 2.29 < r < 2.40 range. Here, r represents the mean coordination number of glasses. In ternary alloys containing equal proportions of group IV and V selenides, IPs are wider and encompass ranges of respective binary glasses. These data suggest that the local structural variance contributing to IP widths largely derives from four isostatic local structures of varying connectivity r, two include group V based quasi-tetrahedral (r = 2.29) and pyramidal (r = 2.40) units, and the other two are group IV based corner-sharing (r = 2.40) and edge-sharing (r = 2.67) tetrahedral units. Remarkably, binary group V (P, As) sulfides exhibit IPs that are shifted to even a lower r than their selenide counterparts; a result that we trace to excess S n chains either partially (As-S) or completely (P-S) demixing from network backbone, in contrast to excess Se n chains forming part of the backbone in corresponding selenide glasses. In ternary chalcogenides of Ge with the group V elements (As, P), IPs of the sulfides are similar to their selenide counterparts, suggesting that presence of Ge serves to reign in the excess S n chain fragments back in the backbone as in their selenide counterparts.Ó 2009 Elsevier B.V. All rights reserved. Three types of glass transitionsGlasses are intrinsically non-equilibrium solids and their physical properties generally evolve over long times, i.e., these solids slowly age. The aging of glasses is itself a fascinating subject and has been debated since the early work of Kohlrausch [1-3]. There is now evidence to suggest that the stretched exponential relaxation observed in glasses may well be characterized by specific exponents, which are determined [2] largely by the nature (of long or short range) of forces that control how traps or defects diffuse as networks relax. For a long time it was widely believed that glass transitions are also hysteretic and age [3] as observed in a traditional differential scanning calorimetry. There are new findings showing that under select conditions [4-6] aging of glasses may not occur.New insights into the nature of the glass transition [4,7] have now emerged using modulated-DSC (m-DSC). A significant advantage of m-DSC over traditional DSC is that the method permits deconvoluting the total heat flow into a reversing heat flow term (which captures the local equilibrium specific heat) and a nonreversing heat flow term (which captures non-equilibrium effects of the changing structure). The thermally reversing heat flow term usually reveals a rounded step-like jump. One defines the glass transition temperature, T g , from the inflexion point of the step, and the specific hea...

GexPxS1−2x glasses in the compositional range have been synthesized and examined in temperature modulated differential scanning calorimetry (MDSC) and Raman scattering experiments. Trends in the non-reversing enthalpy ΔHnr(x) near Tg show the term to almost vanish in the 0.090(5)0.135. In analogy to previous results on chalcogenide glasses, we identify compositions at x<0.09 to be elastically floppy, those in the 0.0900.135 to be stressed rigid. MDSC results also show that the ΔHnr term ages in the stressed-rigid and floppy phases but not in the intermediate phase. The intermediate phase is viewed to be a self-organized phase of a disordered network. It consists of at least four isostatically rigid local structures: corner-sharing GeS4, edge-sharing GeS2, pyramidal P(S1/2)3 and quasi-tetrahedral S = P(S1/2)3 units for which evidence comes from Raman scattering. The latter method also shows the existence of P4S7 and P4S10 molecules in the glasses segregated from the backbone. These aspects of structure contribute to an intermediate phase that is significantly narrower in width than in the corresponding selenide glasses.

No abstract

No abstract

No abstract