The compositional variations of the fragility of lithium borate binary glasses have been investigated over a wide composition range up to 64 mol% of Li 2 O. The frequency dependences of the dynamic heat capacity C p (ω) above the dynamic glass transition temperature T g have been observed by Temperature-Modulated DSC. The fragility or steepness index m has been determined. The composition dependence has been discussed on the basis of the fragility model proposed by Vilgis, and it has been revealed that the trends of the fragility and the fluctuation of the coordination number of boron atom have the correlation over a wide composition range. The result indicates the origin of the fragility relates to the degree of the fluctuation of the coordination number. The formation of the 4-coordinated boron atom plays the dominant role in the fragility of the borate glassy system.

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The elastic properties of potassium borate glass, x K 2 O . (100 À x )B 2 O 3 , where x is the molar composition of K 2 O in mol %, have been investigated by Brillouin scattering spectroscopy over a wide composition range of 2 x 42 mol %. From the observed values of longitudinal sound velocity, the elastic constant has been determined and compared with that of lithium borate glass. The sound velocity increases with increasing x below x ¼ 30; however, for a further increase in x , it decreases owing to the softening caused by the formation and increase in the number of nonbridging oxygen atoms. The absorption coefficient also increases markedly above x ¼ 30 owing to the scattering of acoustic waves by nonbridging oxygen atoms. The temperature dependences of both sound velocity and absorption coefficient of 14K 2 O . 86B 2 O 3 show a markedly change at approximately the glass transition temperature T g of 395 C.

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...

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