Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 239

Network‐oxide glass formers like SiO₂, GeO₂, P₂O₅, and B₂O₃ [1] are defined by three‐ or fourfold directionally bonded polyhedra comprising hybridized units measuring ~2.5 Å, similar to nearest‐neighbor arrangements in crystalline polymorphs [1]. For low‐density hybrid glasses like a‐ZIF‐4 and a‐ZIF‐8, tetrahedral units are much larger, measuring about 9.5 Å [14]. Generally, SRO polyhedra are comparatively rigid, with variations in bond angle of less than 10%. The IRO is located between 3 and 4 Å for oxides increasing to 13 Å for hybrid glasses, covering correlation distances between SRO polyhedra (Figure 2 [1, 7, 14]). In oxide glasses the interpolyhedral distance is defined by the BO that is also hybridized, with interpolyhedral angles ranging from around 145° for SiO₂, 130° for GeO₂, and P₂O₅ to 120° for B₂O₃ (Figures 1 and 4 [1]). The imidazolate bridge between metal nodes in a‐ZIFs is ~145° [14]. On average, the rigidity of tetrahedral and bridging angles is similar.

Figure 5 The collective atomic vibrations involved in the boson peak observed either dynamically in the reduced density of states g(E)/E² (a, b) or thermodynamically in the non‐Debye excess low‐temperature specific heat C_p/T³ (c, d) for silica (left) and densified silica (right). Similar features occur in crystalline SiO₂ isomorphs of similar density [18]. (e) INS spectra of the collapse of zeolite Y [16], the cage subunits merging into a single peak of lower intensity I_BP as a glass is formed while ν_BP increases – dashed arrow. (f) Boson peak in the metallic glass Zr₅₀Cu₄₀Al₁₀[20] where annealing increases the density, but decreases the trapped enthalpy and the C_p/T³ intensity I_BP falls as ν_BP increases – dashed arrow.

Source: (a–d) Reproduced from [18] © (2014) APS; (e) reproduced from [16] © (2005) AAAS; (f) reproduced from [20] © AIP.

In network glasses LRO begins at around 6 Å – the width of a typical sixfold ring (Figures 1 and 4) – and continues as far as out as features in the RDF can be discerned (Figure 2). Providing a direct link with a multiplicity of rings of corner‐sharing polyhedra with different sizes, LRO is perpetuated through modest variations in bond angles, as illustrated in Figure 6 with the two‐dimensional (2‐D) distributions directly observed for silica [1, 22]. Combinations of experimental RDFs with computer simulations afford 3‐D models of network topology where rings are often puckered in conformations foreign to crystalline geometries through variation and twisting of dihedral angles (Figures 1 and 4). The network statistics in SiO₂ glass include five‐, six‐, and sevenfold rings, as illustrated schematically in Figure 7, in contrast to the sixfold ring topology of crystalline silicates. In addition, three‐ and fourfold rings are also found, but in much smaller proportions [1, 6]. They have been identified with the oxygen “defects” that give rise to breathing modes in Raman spectra [1]. These miniature rings increase in number when pressure is applied, for example, in indentation experiments. The converse applies in B₂O₃ glass where the network is less 3‐D than in SiO₂ and where Raman spectra are instead dominated by the threefold boroxol ring feature [1]. Pressure causes boroxol rings to break up into buckled ribbons, the dimensionality decreasing further.

Another consequence of LRO in network glasses is the quasiperiodic alignment of groups of caged voids associated with the aperiodic network of rings (Figure 7) to which the FSDP at Q_FSDP (Figure 2) is attributed [7]. In glass formers like SiO₂ and B₂O₃, 2π/Q_FSDP distances lie around 4 Å. In chalcogenide glasses, such as As₂S₃ and GeSe_2, SRO polyhedra are larger (~4 Å), leading to larger quasiperiodic void separations (~6 Å). In all cases FSDP distances decrease as pressure is applied but also become more dispersed, the FSDP peak widening and decreasing in intensity with increasing glass density [7].

Figure 6 Direct observation of the atomic structures of glasses. (a) Atomic force microscopy image from freshly fractured silica in ultra‐high vacuum, revealing a 2‐D projection of the near‐surface structure and showing SRO and fragments of rings – solid and dotted lines – contributing to LRO [24]. (b) Atomic resolution transmission electron microscope image of a graphene‐supported silica bilayer, showing SRO and extensive LRO network with a variety of ring structures [22], consistent with Zachariasen's CRN [13] (Figure 7a). (c) Nanobeam electron diffraction patterns of Zr_0.667Ni_0.333 metallic glass [23] (left), with their simulated patterns (right) in terms of the icosohedra shown in (d). The different SRO reflect the variety of icosohedra in Bernal's DRPHS model of liquid metals [25] (Figure 7b).

Source: (a) Reproduced from [24] © (2004) Elsevier; (b) reproduced from [22] © (2012) ACS; (c, d) reproduced from [23] © Nature Publications.

Importantly these changes in the FSDP properties of network glasses with pressure correlate with those of the boson peak [17], where ν_BP increases and I_BP decreases with increasing pressure and therefore density, supporting the view that the BP comprises collective atomic motion of large groups of atoms whose breathing frequency increases as their size shrinks. Furthermore, excess C_p in glasses is attributed to a double‐well vibrational potential, which, in silica, can be modeled through the librational twisting of pairs of tetrahedral units, underscoring how the dynamics of IRO promote buckling of rings across LRO in network structures.