Rigidity transitions in binary Ge-Se glasses and the intermediate phase (original) (raw)
Related papers
Composition and temperature dependence of the low-frequency Raman scattering in Ge–As–S glasses
Journal of Non-crystalline Solids, 2004
The Raman spectra of ÔstoichiometricÕ (GeS 2 ) x (As 2 S 3 ) 1Àx glasses are presented for various compositions (x = 0.40, 0.60, 0.80, 0.83, 0.90) and compared to that of the Ônon-stoichiometricÕ Ge 0.30 As 0.10 S 0.60 glass (in which there is excess of Ge with regard to the stoichiometric system). The study is focused on the variation of the low-frequency Boson peak with glass composition and temperature. The ratio of intensity of the as-recorded Boson peak to that of the high frequency main molecular band remains almost composition-independent for all glasses, except for the non-stoichiometric one. This ratio is considerably larger in the latter glass, implying a higher degree of disorder which is attributed to an increased number of defects. In the (GeS 2 ) x (As 2 S 3 ) 1Àx glasses, there is evidence of a shift of the Boson peak towards the lower frequencies with increasing Ge-content, indicating an increase of the correlation length of the medium range structure. On the other hand, the non-stoichiometric glass displays a lower Boson peak frequency than any (GeS 2 ) x (As 2 S 3 ) 1Àx glass, implying, again, a higher correlation length of the medium range structure. Both these results, i.e., the shifts towards the lower frequencies, are interpreted in terms of the higher free volume displayed by the Ge-rich glasses. In the case of the stoichiometric (GeS 2 ) 0.83 (As 2 S 3 ) 0.17 (or Ge 0.25 As 0.10 S 0.65 ) glass and the non-stoichiometric Ge 0.30 As 0.10 S 0.60 one, Raman spectra have also been measured over the temperature range 20-295 K and shown significantly different dependence. Comparison of the intensity variation of the BP with temperature further confirms the higher degree of disorder of the non-stoichiometric glass. Finally, the reduced BP intensities of the glasses show almost the same spectral profiles over the entire temperature range, thus displaying universal lineshape behavior with temperature.
Raman spectra of Ge[sub x]As[sub y]Se[sub 1−x−y] glasses
Journal of Applied Physics, 2009
Various Ge-As-Se glasses spanning a mean coordination number ͑MCN͒ from 2.2 to 2.94 have been investigated using differential scanning calorimetry and Raman spectroscopy. The glass transition temperature T g was found to increase with increasing MCN, except for those glasses located within the nanoscale phase-separated region of the phase diagram. The evolution of Raman features at wavenumbers from 150 to 350 cm −1 exhibits two transitionlike features. Merging of the 225 and 250 cm −1 modes at MCN= 2.5 is a symbol of the extinction of Se-Se bonds. Additionally, the appearance of two modes at 280-290 and 170 cm −1 at MCNϾ 2.7 come from the defect modes of ethanelike Ge 2 Se 6/2. The increase in the scattering from these defects is an important factor leading to enhanced optical loss in the glasses with high MCN.
Raman signatures of intermediate phase in quaternary Ge15Te80−xIn5Agx glasses
Journal of Non-Crystalline Solids, 2014
Micro-Raman studies are conducted on as-quenched and annealed Ge 15 Te 80 − x In 5 Ag x glasses to probe the structural network and its evolution with composition. These studies reveal the presence of tetrahedral GeTe 4 structural units in as-quenched samples. Specific signatures of the intermediate phase (IP) are observed in the composition dependence of Raman frequencies and corresponding intensities of different modes in the composition range, 8 ≤ x ≤ 16. In addition, the Raman peak positions are found to shift with silver doping. Apart from the Raman results, the compositional dependence of density, molar volume and thermal diffusivity, observed in the present study, confirms the presence of the intermediate phase. In thermally annealed samples, a unique variation of Raman wave-numbers in the intermediate region is observed due to the retention of some of the local structure even after the sample is crystallized. The observed Raman peaks are attributed to crystalline tellurium and silver lattice vibrational modes. Based on our present and earlier studies, we propose the occurrence of three thresholds in Ge 15 Te 80 − x In 5 Ag x glasses, namely percolation of rigidity, percolation of stress and the onset of chemical phase separation on a nanoscale at 8%, 16% and 20% of silver concentration respectively.
Intermediate Phase in Germanium-Selenide glasses: A Theoretical and Experimental Study
2008
frequency of CS tetrahedra increases linearly below the IP window and shows a power law behavior above IP, while it saturates within the IP range . Raman measurements under pressure by Wang et. al.[10] provide further insight into the nature of IP. Above and below the IP range, the CS Raman mode shows a threshold P c , below which the variation in the mode frequency with composition is negligible. In the IP window, the mode frequency increases monotonically with the applied pressure suppressing a threshold. The absence of a pressure threshold suggests the stress-free nature of the IP. The evolution of the topology of the network through the IP range has been thoroughly studied in Si x Se 1-x glasses [11], whose topology is believed to be similar to that of Ge x Se 1-x . The variation in the Raman frequencies of CS, ES and Se chain mode (CM) with Si composition x provide some understanding of the evolution of the connectivity between Si tetrahedra and Se chains. It is observed that for x > 0.19, the CS, ES and CM modes become increasingly asymmetric and require more than one Gaussian to deconvolve the spectrum. Asymmetric nature of the signal suggests increase in the coupling among Si tetrahedra due to the decrease in Se chain lengths as x increases above 0.19. At x = 0.19, the CM splits into two modes. The intensity of the higher mode increases to a maximum at x = 0.26 (second rigidity threshold) and decreases to zero as x increases to 0.33. The lower mode decreases linearly above x = 0.19. This peculiar behavior of CM suggests the appearance of short Se chain segments in the IP window, most probably Se dimers or trimers as suggested by the authors, which link the CS and ES tetrahedra. It is interesting to note that the total constraint count for Se dimer connecting two tetrahedra (Ge-Se-Se-Ge) is equal to 3.0, the number of degrees of freedom of the network. The presence of such structural segments may explain the disappearance of pressure threshold mentioned above. The thermal response of the A 1 (tetrahedral breathing mode) of CS tetrahedra and Se CM mode also shows some interesting features. Wang et. al. have shown that below the IP window, the frequency shift rate, between 100K and 300K, for A 1 mode remains higher than that of CM mode. Between x = 0.2 and x = 0.28, the A 1 shift rate decreases and the shift rate for CM mode increases and coincides with that of A 1 mode. The decrease of the shift rate of A 1 mode in the IP window also suggests the appearance of shorter Se chain segments, which would increase the coupling between tetrahedra thus making A 1 mode less sensitive to the temperature.
The Journal of Physical Chemistry B, 2014
Binary Ge x S 100−x glasses reveal a richness of elastic and chemical phase transitions driven by network topology. With increasing Ge content (x), well-defined rigidity at x c (1) = 19.3(5)% and a stress transition at x c (2) = 24.9(5)% are observed in Raman scattering. In modulated DSC measurements, the nonreversing enthalpy of relaxation at T g reveals a squarewell-like minimum (reversibility window) with window walls that coincide with the two elastic phase transitions. Molar volumes show a trapezoidal-like minimum (volumetric window) with edges that nearly coincide with the reversibility window. These optical, thermal, and volumetric results are consistent with an isostatically rigid elastic phase (intermediate phase, IP) present between the rigidity (x c (1)) and stress (x c (2)) transitions. Complex C p measurements show melt fragility index, m(x) to also show a global minimum in the reversibility window with m < 20, underscoring that melt dynamics encode the elastic behavior of the glass formed at T g. The strong nature of melts formed in the IP has an important practical consequence; they lead to slow homogenization (over days not hours) of nonstoichiometric Ge−S batch compositions reacted at high temperatures. Homogenization of chalcogenide melts/glasses over a scale of a few micrometers is a prerequisite to observe the intrinsic physical properties of these materials.
2008
Raman and calorimetric studies on GexSe1−x glasses have provided evidence for the existence of the intermediate phase (IP) in chalcogenide and other glasses. Here, we present X-Ray Absorption Near Edge Structure (XANES) measurements on germanium selenide glasses in the IP composition range, and detect an electronic signature of the IP. Ab initio molecular dynamics (MD) based models of these glasses are discussed, and an atomistic picture of the IP, based upon the models and available experiments is presented. We show that these models reproduce detailed composition-dependent structure in the XANES measurements, and otherwise appear to properly represent the structure of the GeSe glasses near the IP.
Elastic Phases of Ge x Sb x Se 100–2 x Ternary Glasses Driven by Topology
The Journal of Physical Chemistry B, 2013
Topology offers a practical set of computational tools to accurately predict certain physical and chemical properties of materials including transformations under deformation. In network glasses with increased cross-linking three generic elastic phases are observed. We examine ternary Ge x Sb x Se 100−2x glasses in Raman scattering, modulated DSC and volumetric measurements, and observe the rigidity transition, x = x c (1) = 14.9% that separates the flexible phase from the Intermediate phase, and the stress transition, x = x c (2) = 17.5% that separate the intermediate phase from the stressed rigid one. Raman scattering provides evidence of the structural motifs populated in these networks. Using size increasing cluster agglomeration, we have calculated the rigidity and stress transitions to occur near x c (1) t = 15.2% and x c (2) t = 17.5%, respectively. Theory predicts and experiments confirm that these two transitions will coalesce if edge−sharing Ge-tetrahedral motifs were absent in the structure, a circumstance that prevails in the Ge-deficient Ge 7 Sb x Se 93-x ternary, underscoring the central role played by topology in network glasses. We have constructed a global elastic phase diagram of the Ge−Sb−Se ternary that provides a roadmap to network functionality. In this diagram, regions labeled A, B, and C comprise networks that are flexible, rigid but unstressed, and stressed-rigid, respectively.