Structure of copper halide melts, rare earth chalcogenide glasses and glassy germania at high pressure

Abstract

The structure of molten CuCl, CuI and (CuCl)x(CuI)1−x (0 ≤ x ≤ 1) mixtures was investigated at the total structure level by using neutron diffraction. The results are compared with those obtained for CuCl and CuI from other experiments, theory and computer simulation. It is found that existing models of CuCl and CuI must first be improved before a realistic attempt can be made to account for the structure of their mixtures.

The structure and thermal properties of (R2X3)0.07(Ga2X3)0.33(GeX2)0.60 glasses, where R denotes a rare earth element and X denotes a chalcogenide element S or Se, were studied using a combination isomorphic substitution in neutron and x-ray diffraction, 71Ga magic angle spinning (MAS) nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) methods. The results show that Ge and Ga are four-fold coordinated by the chalcogen element in these glasses. Difference function methods were applied to the diffraction results and for the large rare earth ions provided an R-S coordination number of 8.1(2) and nearest neighbour R-S bond distance of 3.03(3) ˚A in the sulphide glass and an R-Se coordination number of 8.0(2) and R-Se bond distance of 3.05(3) ˚A in the selenide glass. For the small rare earth ions in the selenide glass a smaller R-Se coordination number of 5.0(2) and R-Se bond distance of 2.93(3) ˚A were obtained.

In situ high pressure neutron diffraction experiments were performed on GeO2 glass using the Paris-Edinburgh press on the D4C diffractometer at the Institut Laue-Langevin (ILL), France. An analysis procedure was developed to account for the gasket and background scattering at each pressure point and improved neutron shielding led to the extraction of high quality total structure factors at pressures up to 8 GPa. Experiments were also performed using the PEARL instrument at the pulsed neutron source ISIS, UK, where improvements were made to the incident neutron beam collimation, pressure cell shielding and experimental methods to obtain the correctly normalised total structure factors for GeO2 glass at pressures up to 14(1) GPa. The results show that Ge is four-fold coordinated by oxygen from ambient pressure to 5 GPa. A gradual increase in the Ge-O coordination number is then observed, reaching 4.9(1) at 9.0(5) GPa and 5.5(1) at 14(1) GPa, which is accompanied by an A.increase in the Ge-O bond length from 1.73(2) to 1.81(2) ˚ With pressure increasing from ambient, there is a smooth increase in the position and decrease in height of the first sharp diffraction peak which is associated with the intermediate range order. The overall results are therefore consistent with two densification mechanisms, one associated with a collapse of the network structure and the other with an increase in the local coordination number and Ge-O bond length.

Date of Award	1 Jun 2009
Original language	English
Awarding Institution	University of Bath
Supervisor	Philip Salmon (Supervisor)