Hot Science Under Pressure

Abstract

The structure of glasses that cover a large range of compositions in the (MgO)x(Al2O3)y-(SiO2)(1−x−y) system was investigated by neutron diffraction and 27Al magic angle spinning nuclear magnetic resonance (NMR) spectroscopy. Site-specific information was thereby gained on the composition-dependent local structure of the Si, Al and Mg atoms. The results were interpreted with the aid of a structural model developed for M-aluminosilicate glasses (M = alkali or alkaline Earth metal), which assumes that Al atoms can take on more than one structural role. For compositions with R = x/y >1, the glass structure consists primarily of SiO4 and AlO4 tetrahedral units, which are linked through bridging oxygen (BO) atoms to form an aluminosilicate network. The Mg2+ ions either (i) associate with non-bridging oxygen (NBO) atoms, that is, an oxygen atom that is connected to only one Si- or Al-centred tetrahedral unit; or (ii) stabilise the formation of Al-centred tetrahedral units by balancing the negative charge associated with an AlO4 unit. For compositions with R < 1, there are an insufficient number of Mg2+ ions to stabilise all the Al3+ ions in tetrahedral units. In the R < 1 regime, the formation of Al-centred tetrahedral units, therefore, requires some fraction of the Al3+ ions to behave in a similar way to Mg2+ ions. These Al3+ ions are not part of the aluminosilicate network and reside in sites that have five or six nearest-neighbour oxygen atoms.

In situ high-pressure neutron diffraction experiments were performed on calcium aluminosilicate (CAS) glasses at pressures up to 17.5(5)GPa. In their as-prepared form, the CAS glasses primarily consist of a tetrahedral aluminosilicate network, with Al centred tetrahedral units stabilised by Ca2+ ions. A pair-distribution function (PDF) analysis of the diffraction data revealed the complete or almost complete conversion of Al-centred units from tetrahedral to octahedral, accompanied by a shift in the average Al-O distance from rAl-O ≈ 1.760(10)Å at ambient conditions to rAl-O = 1.894(10)Å at 14.4(5)GPa. The Al-O coordination environment shows little change upon further compression to 17.5(5)GPa. In comparison, the average Si-O distance remains 1.611(10) ≤ rSi-O ≤ 1.634(10)Å for pressures P ≤ 14.4(5)GPa, reflecting the persistence of SiO4 tetrahedra. For the highest pressure of 17.5(5)GPa, The average Si-O distance increases to rSi-O = 1.643(10)Å, which could mark the onset of the conversion of Si-centred units from tetrahedral to octahedral. An investigation of the CAS glasses recovered from 8.2(5) and 17.5(5)GPa showed that their structure did not recover to its as-prepared form and highlighted substantial differences between the structure of CAS glasses before, during and after compression. A relationship between the average Al-O coordination number and the densification of aluminosilicate glasses was established by comparing the results from this study with those available in the literature.

The structure of crystalline and amorphous materials from the Na1+aAlaGe2−a(PO4)3 (NAGP) system with a = 0, 0.4 and 0.8 was investigated by neutron diffraction PDF analysis. The results for the crystalline materials conform to the expected crystal structure, wherein corner-sharing PO4 and GeO6 or AlO6 polyhedral units are linked to form a 3-dimensional network with Na+ ions residing in interstitial cavities. The corresponding glasses contain a significant proportion of sub-octahedral Ge- and Al centred units, which are not present in the crystal structure. The differences between the glass and crystal structure means that substantial structural reorganisation must be undertaken during the crystallisation process. The measured Al-O and Ge-O coordination numbers were used to assess the network connectivity in NAGP materials and how this evolves during the early stages of crystallisation. A structural model for the alumina-free glass was proposed, which centres on the formation of Na2P6GeO18 super-structural units within a tetrahedral GeO2 network.

New assemblies were developed for the Paris-Edinburgh press to access high pressure- high temperature (high P-T) conditions. These high P-T assemblies were designed for either (i) in situ neutron diffraction on the PEARL diffractometer at ISIS; (ii) in situ X-ray diffraction on the I15 beamline at the Diamond Light Source (DLS); or (iii) laboratory-based experiments at the University of Bath. The materials used to construct the high P-T assemblies were tailored to suit their application. Calibration experiments were used to assess their performance. The high P-T assemblies developed for PEARL were used to study the structure of crystalline GeO2 up to a temperature of 2166(173)K at 9.9(5)GPa. The assemblies developed for X-ray diffraction experiments were used to demonstrate the functionality of the high pressure apparatus for I15. The high P-T assemblies developed for laboratory based experiments were used to characterise the behaviour of different furnace materials. A high P-T processing technique was established with a priority placed on the accurate determination of the sample conditions. The technique was used to produce a set of permanently densified GeO2 glasses.

Date of Award	17 Nov 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Philip Salmon (Supervisor), Anita Zeidler (Supervisor), Craig Bull (Supervisor) & Christine M. Beavers (Supervisor)