Composition Dependence of the Boson Peak and Universality in Lithium Borate Binary Glasses: Inelastic Neutron and Raman Scattering Studies

Abstract

We study the low-energy vibrational dynamics, called the boson peak (BP), of binary lithium borate glasses as a function of the composition of these glasses by inelastic neutron and Raman scattering. Firstly, we analyze the variations in the properties of the BP with the composition. The position and intensity of the BP linearly change with increasing x owing to the changes in the intermediate glass structures. Secondly, we demonstrate that all spectral shapes with different compositions can be scaled by a single master curve, and are the same for all spectra. The results suggests that there is a universal distribution of the vibrational density of states, which does not change with the composition, even though the structures of lithium borate glasses markedly change. The effect of Li 2 O doping can be understood to be a chemical structure-induced densification.KEYWORDS: boson peak, low energy excitation, lithium borate glasses, vibrational density of states, inelastic neutron scattering, dynamic structure factor, Raman scattering DOI: 10.1143/JPSJ.79.033801A structurally disordered material shows physical properties different from those of its ordered crystalline counterpart. One of the most significant differences is the low frequency part of the vibrational spectra. A broad excitation peak, called a boson peak (BP), can be observed in the Raman and inelastic neutron spectra obtained at energies of approximately 2 -10 meV corresponding to the THz frequency range. The observation of the BP indicates the existence of an excess low-energy vibrational density of states (VDoS) beyond the prediction of the Debye model for crystals. Since the BP is commonly observed in many different types of disordered materials, it is considered that it must be the key to gaining a fundamental understanding of disordered materials. Despite constructive effort, the microscopic origin of the BP still remains a serious open question in condensed matter physics. 1-3)Boron oxide (B 2 O 3 ) glass is a typical network glass former, and classified into one of the ''strongest'' type of glass like SiO 2 glass, on the basis of its ''strong-fragile'' classification.4) The BP of pure B 2 O 3 glass was studied by inelastic neutron scattering (INS) 5) suggesting that its origin is related to the in-and random-phase motions of boroxol rings, and by hyper-Raman scattering 6) suggesting that the coherent librations of several boroxol rings are related to the hyper-Raman BP. Both results indicate that the BP of B 2 O 3 glass originates from the some type of vibrational motion of boroxol rings, which are considered an intermediate structural unit. The addition of lithium oxide (Li 2 O) to B 2 O 3 glass induces a change in the coordination number of boron atoms from 3 to 4, resulting in an increase in network connectivity with increasing Li 2 O content. 7,8) Therefore, the physical properties of binary lithium borate glasses,, where x is the Li 2 O content in mol %, markedly vary with x. We have shown that the physical properties such as sound velocity...

Non-Debye excess heat capacity and boson peak of binary lithium borate glasses

“…2, the peak temperatures increase the higher with increasing x, while the magnitudes of the excess C p (T) become much less intense with increasing x. These general trends are very similar to the trends of the boson peaks obtained by Raman scattering [14,15] and INS [15], both of which relates to the excess VDoS, while the Raman intensity is weighted by the light-vibration coupling constant. Such similar behaviors are reasonable, because the non-Debye C p (T) results from the excess VDoS, as has been shown above.…”

“…Recently, we reported the raw dynamic structure factor S(Q ave , ω) of LiB with the different compositions [15], where Q ave is the mean momentum transfer (1.7 Å − 1 ) and ω is the energy transfer in cm − 1 . Reduced Molar heat capacity Fig.…”

“…B and Li in the samples were isotopically enriched by 11 B (N99 atom%) and 7 Li (N97 atom%), respectively, to avoid neutron absorption by 10 B and 6 Li. The samples were previously used for INS experiments by a direct geometry chopper-type ToF spectrometer, AGNES, belonging to the Institute for Solid State Physics, University of Tokyo [15]. The same samples were used for the heat capacity measurements to ensure experimental consistency.…”

“…It has been revealed that the physical properties of LiB such as density [11], sound velocity [11,12], stretched exponentiality [13], and Raman [14] and INS spectra [15] drastically vary with the composition, in contrast with CsB, and they show maxima or minima in their composition dependences owing to changes in the intermediate structure units. The Angell's fragility of LiB also markedly varies from strong to fragile [16], indicating that the LiB system can cover a wide variety of glass-forming materials from the viewpoint of strong-fragile classification.…”

Boson peaks of lithium borate glasses studied by inelastic neutron and light scattering

“…Detailed methods of the high homogeneity glass preparation are described elsewhere [18,19]. In order to avoid the strong neutron absorption by 6 Li and 10 B, all samples for neutron experiments were isotopically substituted from 10 B to 11 B and 6 Li to 7 Li using the starting materials of 7 LiOH·OH (97 atom% 7 Li) and H 3 11 BO 3 (99 atom% 11 B) [20]. Neutron inelastic scattering measurements of LiB glasses, x 7 Li 2 O· (1 − x) 11 B 2 O 3 with x = 0.08, 0.14, and 0.22 were carried out at 25°C (far below T g ) using a direct geometry chopper-type ToF spectrometer AGNES belonging to Institute for Solid State Physics, University of Tokyo.…”