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Working under pressure with neutron diffraction: GeO2, B2O3 and SiO2 at pressures up to 18 GPa

Dean A. J. Whittaker*, Philip S. Salmon, James E. W. Drewitt, Kamil Wezka, University of Bath, UK. Matt G. Tucker, Craig L. Bull, Science and Technology Facilities Council, UK. Martin Wilding, Aberystwyth University, UK.

The structure of many glasses can be understood in terms of structural motifs (e.g. tetrahedra) that connect together, typically sharing corners, to form a network exhibiting intermediate range order. These glasses can undergo many interesting structural changes when pressure is applied, including changes in the intermediate range order (i.e. a reorganisation of the network) and the local coordination environment (i.e. an alteration of the basic structural motifs). For instance, the motifs in SiO2 change gradually from 4-fold coordinated tetrahedra to 6-fold coordinated octahedra as the pressure increases from 10 to 40 GPa [1]. GeO2 exhibits similar transformations at lower pressures more readily amenable to in situ experiments; with the 4-fold to 6-fold change occurring as the pressure increases from 5 to 20 GPa [2]. Meanwhile, B2O3 is thought to exhibit a 3-fold to 4-fold coordination change as the pressure increases from 0 to 20 GPa [3]. All three of these glasses are archetypal network forming oxide materials and are of particular interest to the glass community for their fundamental properties. A technique to obtain accurate total pair distribution functions, which describe the atomic structure of materials, using in situ neutron diffraction has been demonstrated for pressures up to ~9 GPa by Drewitt et al. [4]. In these experiments pressure was applied to a sample by using boron nitride anvils in a Paris Edinburgh press. To apply higher pressures to samples (up to 20 GPa) while maintaining a large enough sample volume for neutron diffraction experiments, sintered diamond anvils must be used. So far, attempts to perform such experiments on amorphous materials have resulted in diffraction patterns of limited quality [5]. The main limitation is that the pressure cell contributes a significant amount towards the measured diffraction patterns. We present a simple but novel technique that builds on that used by Drewitt et al., accompanied by new data collected using the time-of-flight diffractometer PEARL at the ISIS pulsed neutron source for three samples (GeO2, SiO2 and B2O3). These measurements were made in situ at pressures up to 18 GPa. Total pair distribution functions and coordination numbers were obtained at these pressures using neutron diffraction for the first time. The information provided is complimentary to that obtained by using x-ray diffraction (e.g. [1]) and the work adds to the body of techniques that can be used to probe the structural changes of glasses under high pressures. References: [1] C. J. Benmore et al., Phys. Rev. B, 81, 054105 (2010). [2] M. Guthrie et al., Phys. Rev. Lett., 93, 115502 (2004). [3] V. V. Brazhkin et al., Phys. Rev. Lett., 101, 035702 (2008). [4] J. E. W. Drewitt et al., Phys. Rev. B, 81, 014202 (2010). [5] M. Wilding et al., J. Phys. Condens. Matt., 20, 244122 (2008).