Low-Energy Vibration Excess in Silica Xerogels

Abstract

Specific heat (1.5 -25 K) and Raman scattering (below 100 cm 21 ) in silica xerogels permit one to assess the frequency dependence of the coupling coefficient C͑v͒ and the low-energy density of states g͑v͒. Different network connectivity does not change the shape of the quasielastic scattering, but alters the spectral shape of C͑v͒. The results permit one to ascribe the frequency shifts of the boson peak to the contribution of C͑v͒ and to the Debye density of states g D ͑v͒. Furthermore, the different magnitudes of g͑v͒ appear to be a manifestation of changes in the Debye-like phonon contributions arising from connectivity changes. [S0031-9007 (96)02257-0]

Dynamics of a hydrogenated silica xerogel: A neutron scattering study

“…We were able to decompose the scattering into a smaller interfacial water component and a larger fraction of protons bonded to the silica network. The g͑ ͒ of the latter is very similar to that of amorphous silica in the boson peak frequency range, 5,14,23 but the amplitude of the protons in these modes is more than twice as large as the one of the network atoms.…”

“…From the heat capacity measurements, 5,14 we know that the density of states of the xerogel is approximately equal to the one of pure silica, so the number of vibrational modes must be the same. Obviously, the amplitude of the protons in an OH bound to the silica network, either within the network or as silanol on a pore surface, must be considerably larger than the amplitudes of the oxygen and the silicon atoms.…”

“…The present paper supplies such information from an inelastic neutron scattering experiment on a xerogel sample, studied earlier by some of us with Raman scattering and low-temperature heat capacity measurements. 5 The silica gels were prepared by hydrolysis of mixtures of triethoxylane and tetraethoxilane dissolved in ethanol. Details concerning the preparation of xerogels are given elsewhere.…”

Low-frequency Raman scattering in model disordered solids: percolators above threshold

“…The investigation of the frequency dependence of C(ω) for model disordered solids is becoming more and more attractive also in view of the recent demonstration, by Fontana and coworkers [20,21], that a combination of Raman and specific heat experiments allows for the determination of the density of vibrational states with an accuracy comparable to neutron scattering experiments, and therefore provides the functional form of C(ω).…”

The Raman coupling function in permanently densified GeO2glasses

“…Therefore, it is still possible to define a vibrational density of states g(ω), which describes the vibrational characteristics of a disordered system and determines its thermal properties, as in the case of the crystal. An excess of modes over the Debye law in the low frequency regime, called Boson Peak (BP), has been ubiquitously observed in a wide range of materials both in experiments [1][2][3][4] and in numeric simulations [5][6][7]. Modifications of the Boson Peak take place when a hardening of the elastic medium, due to different varying thermodynamic parameters, occurs.…”