Understanding structure-property relations in β-eucryptite under pressure and at elevated temperature

Abstract

β-eucryptite (LiAlSiO4) has received widespread attention from both industry and academia due to its negative coefficient of thermal expansion (CTE) and one-dimensional Li ionic conductivity. Additionally, β-eucryptite undergoes a pressure-induced phase transformation at relatively low pressures. These various behaviors arise because the crystal structure is open and highly anisotropic. The present study uses several experimental methods to better understand the relation between the structure and the electrical and mechanical behavior of β-eucryptite. Synthesis and processing methods were developed to make pure β-eucryptite and β-eucryptite doped with Mg of varying particle sizes. In-situ diamond anvil cell - x-ray diffraction was performed to study the pressure induced phase transformation from β-eucryptite to the high pressure phase β-eucrypite. With the assistance of Rietveld refinement and atomistic modeling, the crystal structure of the β-eucrypite was determined to be an orthorhombic with space group Pna21. This is the first time that both space group and atomic positions of the high pressure phase have been reported. It is also observed that Mg-doped β-eucryptite undergoes the pressure induced transformation at slightly higher pressures than pure β-eucryptite (2.47 GPa compared with 1.8 GPa hydrostatic stress), implying that Mg stabilizes β-eucryptite. Furthermore, the presence of Mg leads to a state in which two high pressure phases coexist. It was observed that the critical pressure for transformation to the high pressure phase decreases with increasing β-eucryptite grain size, up to a critical grain size beyond which grain size does not play a role. The experimental results were described by a nucleation and growth model for transformation. The effect of structural changes in β-eucryptite from low to high temperature was examined by measuring the Li ionic conductivity with electrochemical impedance spectroscopy (EIS). The conductivity is strongly influenced by the Li order-disorder transition at ~500 ºC where the activation energy dramatically increases. It is shown that the conductivity depends on whether or not the Li ions motion is correlated or uncorrelated, leading to three activation energies: at low temperatures it is correlated, at ~500 ºC it is uncorrelated and at high temperatures it is correlated again. The present study focuses on measuring the material properties under pressure and at high temperature, separately. These measurements provide the first step towards elucidating the relation between mechanical and electrical properties. The order-disorder transition is caused by the rearrangement of Li ions at evaluated temperature, an event which is expected to influence the pressure-induced phase transformation. Specifically, a change in the size of the Li channels resulting from Li redistribution, is expected to modify the phase transformation pressure.