Anomalous variation of Boson peak and fragility and their correlations with intermediate-range structure in PbO-B2O3 glasses

Abstract

Understanding the microscopic origin of physical properties is fundamental to the study of glasses. In the present study, unique trends were observed in structural and dynamical properties in lead-borate [xPbO:(1-x)B2O3] glasses upon increasing the modifier. Properties driven by short-range structural changes like, glass transition temperature (Tg) and elastic modulii exhibit a broad maxima around x = 0.3, in congruence with the average coordination number <r>. Using Raman spectroscopy, this behaviour is attributed to the gradual conversion of three-coordinate (B3) to four-coordinate (B4) boron species. However, dynamic properties like the Boson peak and fragility behave similar to the above structural properties except an anomalous dip at x = 0.3. The corresponding dynamic correlation length, therefore, exhibits a decrease except a peak at x = 0.3 with increasing x. The static correlation lengths estimated from the first sharp diffraction peak (FSDP), did not follow a linear trend with the dynamic correlation lengths, indicating a different spatial origin. The analysis of the Boson peak and FSDP suggests that while the Boson peak arises from intermediate-range ordering within the network units, the FSDP arises due to ordering of voids between such units. Moreover, the Boson peak maximum showed a linear decrease with increasing <r>, while the Boson peak position and fragility were found to vary linearly. These correlations emerge from the evolution of the network packing and connectivity with the addition of modifier and changing role of lead oxide as a glass former at higher concentrations. The anomalous variation in dynamical properties reflects the critical role of Pb2+ in tuning the intermediate-range structure of the vitreous network.