**7. Conclusions**

Borate glass is most contemporary glasses and optical materials for technological and environmental applications. Borate glass is one of the typical network oxide glasses with covalent bonds and belongs to the strong type of glass formers. Alkali metal ions are well known modifiers of the borate glass network and control various properties. Basic physical properties such as elastic constants, density, and vibration modes are reviewed in relation with the variation of structural units in modified borate glass network.

The boson peak in the terahertz range is the low-energy excitations in glasses and disordered crystals. It is related to the excess part of vibrational density of states. The alkali metal effects on the boson peak are discussed on the basis of experimental results of neutron inelastic scattering, neutron diffraction, Raman scattering, lowtemperature heat capacity, and ultrasonic measurements. For all the alkali borate glasses, the universal nature of the master plots in boson peak spectra and *Cp/T*<sup>3</sup> vs. *T* curve are observed. This fact indicates that the distribution of the low-energy excess VDoS remains the same for all the alkali modification.

The good correlation is found between the peak temperature of *C*p/*T*<sup>3</sup> , boson peak frequency, and shear modulus. this fact indicates that the origin of a boson peak is closely related to the Ioffe-Regal limit for transverse acoustic waves. The static and dynamical correlation lengths show also the good correlation. As the ionic radius of alkali ions increase, both correlation lengths also increase. This fact suggests that the boson peak vibration is related to the medium range order in the boron-oxygen network near the voids filled by alkali ions.
