Structure and low energy vibrations in silver borate glasses with low silver content

Structural changes of (Ag2O)x(B2O3)1-x glasses, over the range between X = 0.0 and X = 0.14, have been monitored by Raman spectroscopy. Low temperature heat capacity measurements have been also performed between 0.4 K and 25 K to investigate along with low frequency Raman scattering the behavior of the low-energy vibrational dynamics. A progressively decreasing intensity of the main band at 808 cm−1 and the appearance of a band at about 775 cm−1 with increasing Ag2O content have been observed and analyzed in detail. The strongly polarized band at 808 cm−1 is ascribed to the breathing vibration of boroxol rings and the decrease of its intensity indicates the reduction of the number of these units in the network. The band at 775 cm−1 is assigned to vibrations of pentaborate units, formed from two boroxol rings linked by a tetrahedral BO4 group. The rate of formation of BO4 groups appears to increase as X/(1-X). Raman investigation has been extended to very low frequencies, where a broad intense band, the Boson peak (BP), dominates the spectrum of all the samples below 100 cm−1. Addition of Ag2O shifts the BP position from 26 cm−1 up to 33 cm−1 and gives rise to a decrease of its intensity up to X = 0.09 and a sudden increase for X = 0.14, which has been explained as due to rattling motion of silver ions whose contribution is added to that of localized vibrations. The formation of tetrahedral BO4 groups stiffens the network and leads to a decrease of the excess heat capacity over the Debye prediction below 20 K, which is not taken into account by the simple hardening of the elastic continuum but seems to follow the reduction of boroxol rings. This result contrasts the observations on borate glasses with high Ag content and emphasizes the crucial role of the network continuity on the vibrational dynamics of a glass.