Determining Activation Volume for the Pressure‐Induced Phase Transformation in β‐Eucryptite Through Nanoindentation

Abstract

β‐eucryptite (LiAlSiO4) has received widespread attention, both industrially and academically because of its low negative average coefficient of thermal expansion (CTE) and one dimensional Li‐ion conductivity. It undergoes a reversible pressure‐induced phase transformation at ~1 GPa to a metastable polymorph, ε‐eucryptite. In the present work, low load (~30 μN) nanoindentation tests were performed on polycrystalline and single crystal β‐eucryptite to characterize this phase transformation. Hundreds of tests at several loading rates revealed a rate‐dependence, with higher loading rates suppressing the deviation from isotropic elastic behavior—a signature of a thermally‐activated process. The occurrence of reversible hysteretic loops in the load‐displacement curves is consistent with a reversible process during nanoindentation, viz. the phase transformation. Calculations of the activation volume suggest that the nucleation event that is believed to mark the onset of the phase transformation is approximately the size of the silica and alumina tetrahedra comprising the β‐eucryptite structure. This study provides fundamental insights into the mechanism of pressure‐induced phase transformation in β‐eucryptite.

Thermal regimes of Li‐ion conductivity in β‐eucryptite

“…The superior Li ion conduction makes β‐eucryptite a promising candidate for electrolyte applications, including thermal batteries and high‐temperature solid electrodes . Furthermore, applications such as electrothermal devices as well as thermal shock resistant structures can benefit from the overall negative coefficient of thermal expansion (CTE) of β‐eucryptite . The crystal structure of β‐eucryptite (space group P6 4 22 or P6 2 22) is a Li‐stuffed derivative of β‐quartz in which half of the Si‐centered tetrahedra, [SiO 4 ] 4− , are replaced with Al‐centered tetrahedra, [AlO 4 ] 5− .…”

“…[16][17][18] Furthermore, applications such as electrothermal devices as well as thermal shock resistant structures can benefit from the overall negative coefficient of thermal expansion (CTE) of b-eucryptite. 8,9,16,17,[19][20][21][22][23][24] The crystal structure of b-eucryptite (space group P6 4 22 or P6 2 22) 1,25-29 is a Li-stuffed derivative of b-quartz in which half of the Si-centered tetrahedra, [SiO 4 ] 4À , are replaced with Al-centered tetrahedra, [AlO 4 ] 5À . 30,31 In ordered b-eucryptite, [SiO 4 ] 4À and [AlO 4 ] 5À tetrahedra are arranged in a spiral along the 6 4 (or 6 2 ) screw axis, forming open channels along which the Li ions reside at welldefined locations.…”

Synthesis and Thermal Expansion of β‐Eucryptite Powders Produced by the Inorganic–Organic Steric Entrapment Method

“…The low/negative coefficient of thermal expansion arises from the cancellation of a crystallographic expansion along the a-and b-axes (a a = 8.6 9 10 À6 /°C) with a contraction along the c-axis (a c = -18.4 9 10 À6 /°C). 4,5 The low/negative coefficient of thermal expansion, in tandem with the pressureinduced phase transformation have made b-eucryptite an ideal candidate for use in materials for tailorable thermal expansion and improved mechanical properties from transformation toughening or weakening. 3 The e to b transformation coincides with particle ejection from the surface, which has been attributed to elastic strain energy from the transformation, in conjunction with a 7.7 vol% expansion in the lattice.…”

“…3 The e to b transformation coincides with particle ejection from the surface, which has been attributed to elastic strain energy from the transformation, in conjunction with a 7.7 vol% expansion in the lattice. 4,5 The low/negative coefficient of thermal expansion, in tandem with the pressureinduced phase transformation have made b-eucryptite an ideal candidate for use in materials for tailorable thermal expansion and improved mechanical properties from transformation toughening or weakening. [6][7][8] Like several other ceramic materials which have low coefficients of thermal expansion (Mg 2 Al 4 Si 5 O 18 , Al 2 TiO 5 , and Nb 2 O 5 ), b-eucryptite spontaneously microcracks as a result of the residual stress developed from the thermal expansion anisotropy.…”

In situ Diamond Anvil Cell–Raman Spectroscopy and Nanoindentation Study of the Pressure‐Induced Phase Transformation in Pure and Zinc‐Doped β‐Eucryptite

“…When the pressure is released before 17 GPa, β‐eucryptite recrystallizes, indicating presence of structural memory in the material . Macroindentation and nanoindentation studies have also yielded valuable information in understanding the mechanism of the pressure‐induced phase transformation in β‐eucryptite. Jochum et al .…”