Thermal expansion and heat capacity of vitreousB2O3

Abstract

The linear expansion coefficient e of vitreous 8203 was measured from 2 to 90 K. a(T) is negative below 3.3 K but is not fitted uniquely by a simple polynomial. The heat capacity Cz of the same sample was measured from 2 to 20 K and is consistent with earlier data. For T (3.5 K, C&=562T +44T yJ/molK giving 0~a=258. 5 K compared with O~&' --271 K. The "anomalous" thermal properties of vitreous solids in the range 2 to 20 K are discussed,

Anomalous temperature dependence of the first diffraction peak in vitreous boron trioxide

“…However, the most systematic deviation occurs below 50 K, a temperature region where anharmonic effects as reflected by the macroscopic Grüneisen function become significantly large. 12 Surprisingly, a simple high-temperature estimate for these quantities, that is W D (Q)ϭ3ប 2 T/M av k B D 2 calculated using the constants given above follows the data better than the complete expression given by Eq. ͑3͒ ͑see Fig.…”

“…The origin of such counterintuitive result may be ascribed to the significant changes in the dynamics brought forward by the anharmonic parts of the interparticle potential, which lead to a significant volumic contraction below some 3 K as well as to a strong decrease of the Grüneisen parameter, which plummets from some ␥ g Ϸ1 at Tϭ20 K down to Ϫ0.3 at Tϭ2 K. 12 More specifically, and as discussed in some detail in our previous communication, 16 lowering the temperature down to a few kelvins leads to a redistribution of frequencies in the Z(E) generalized frequency spectrum ͑i.e., vibrational density of states͒, where the spectral power is shifted towards low frequencies. The net effect of the decrease in temperature is to shift the higher lobe of Z(E) ͑above some 120 meV͒ towards frequencies below 25 meV as shown in Ref.…”

“…Such findings seem to lend additional support to the hypothesis of assigning to 2/Q p the meaning of a characteristic distance which, simply due to a density effect, increases with increasing temperature. Such values for the temperature coefficient can be compared with those for the macroscopic thermal-expansion coefficient which above 4 K is always positive 12 with values of ϭ 4.6ϫ10…”

“…͑4͒ can be readily made from thermal-expansion and specific-heat data due to White et al. 12 Taking as an example the higher-temperature datum ͑283 K͒ measured by those authors one finds an anharmonic contribution as given by 2␥ G T 283 K ϭ of some 7.2ϫ10 Ϫ4 , a number which can safely be discarded when compared with W h (Q), although such a term has to be taken into account if data close to the glass-transition region ͑above some 500 K͒ are considered. To provide an independent test of the deviation from ideality of W(Q), the departure of the atomic displacements from Gaussian behavior was estimated from the calculated moments of the van Hove G s (r,t), selfcorrelation function.…”

Importance of rings on the microscopic properties of a strong glass

“…17͒ at temperatures within 300-500 K. From there, a value for the derivative ‫͗ץ‬ln͘/‫ץ‬T͉ P ϭϪ7.2ϫ 10 Ϫ4 is found, which would represent an increase in heat capacity of some 0.07 per cent in this high temperature limit due to phonon interactions. The volume expansion term T␥ G ␣ V calculated from expansion data of White et al 16 gives some 4.8 ϫ 10 Ϫ3 so that the total quasiharmonic correction would result in a value for the heat capacity smaller than the harmonic one by some 0.4 percent. In consequence the introduction of the four-body term seems to point in the correct direction since leads to a value for the harmonic heat capacity slightly above the experimental one which will be reduced somewhat by the anharmonic interactions.…”