Hard X-rays as pump and probe of atomic motion in oxide glasses

Abstract

Nowadays powerful X-ray sources like synchrotrons and free-electron lasers are considered as ultimate tools for probing microscopic properties in materials. However, the correct interpretation of such experiments requires a good understanding on how the beam affects the properties of the sample, knowledge that is currently lacking for intense X-rays. Here we use X-ray photon correlation spectroscopy to probe static and dynamic properties of oxide and metallic glasses. We find that although the structure does not depend on the flux, strong fluxes do induce a non-trivial microscopic motion in oxide glasses, whereas no such dependence is found for metallic glasses. These results show that high fluxes can alter dynamical properties in hard materials, an effect that needs to be considered in the analysis of X-ray data but which also gives novel possibilities to study materials properties since the beam can not only be used to probe the dynamics but also to pump it.

X-rays induced atomic dynamics in a lithium-borate glass

“…Not all oxide glasses have the same resistance to X-rays as the canonical SiO 2 glass discussed in [10] and some of them, like the (Li 2 O) 0.5 (B 2 O 3 ) 0.5 glass here studied, show modifications in their local structure already at relatively low absorbed doses as can be seen in figure 1. There, the scattered intensity, I(q), profiles measured for different values of the total absorbed dose are reported together with a more detailed information concerning the position, amplitude and width of the main peak at different doses.…”

“…There is, however, an interesting difference between the induced dynamics in silica and in the lithium borate glass here reported since in the former case the correlation functions are compressed, with β ∼ 1.5, while in the latter case we observe a stretched exponential decay (see the inset of figure 3). [10]. The values of N units relative to silica are nearly a factor 10 smaller than those corresponding to the lithium borate.…”

“…In their work a complete decorrelation of the intermediate scattering function was observed even at small fractions of T g indicating an atomic dynamics several orders of magnitude faster than the one observed with macroscopic probes like PCS or viscosity measurements. This unusually fast relaxation was still described by a Kohlrausch-Williams-Watts [9] function Φ q (t) = A q exp[−(t/τ) β ], with a shape parameter β sometimes smaller than 1 [8], corresponding to the usual "stretched" shape, and sometimes larger than 1 [10], signalling instead a "compressed" shape. This fast relaxation process was then recognized to be beam-induced [10], with the observed decorrelation time strictly dependent on the intensity of the X-ray beam impinging on the sample.…”

“…This unusually fast relaxation was still described by a Kohlrausch-Williams-Watts [9] function Φ q (t) = A q exp[−(t/τ) β ], with a shape parameter β sometimes smaller than 1 [8], corresponding to the usual "stretched" shape, and sometimes larger than 1 [10], signalling instead a "compressed" shape. This fast relaxation process was then recognized to be beam-induced [10], with the observed decorrelation time strictly dependent on the intensity of the X-ray beam impinging on the sample. The results found in [10] can be summarised as follows.…”

“…This fast relaxation process was then recognized to be beam-induced [10], with the observed decorrelation time strictly dependent on the intensity of the X-ray beam impinging on the sample. The results found in [10] can be summarised as follows. a) The characteristic time of this process is inversely proportional to the intensity of the X-ray beam impinging on the sample.…”