Abstract

X-ray diffraction experiments have been carried out on β-eucryptite (LiAlSiO4) at pressures up to 2.5 GPa and temperatures up to 1073 K in a large-volume apparatus. With room-temperature compression, we observed a phase transition to a new polymorph between 0.83 and 1.12 GPa. This transition is reversible in character. The new phase, referred to here as ε-eucryptite, can be indexed according to an orthorhombic unit cell with a = 10.217(4) Å, b = 8.487(4), Å, c = 5.751(3) Å, and V = 498.7(4) Å3 for XRD data at 2.2 GPa and 298 K. On heating at 2.2 GPa, ε-eucryptite and β-eucryptite were metastable over the temperature interval 298–873 K; at higher temperatures they underwent an irreversible phase transition to α-eucryptite.

Both hexagonal α-eucryptite and ε-eucryptite show anisotropic thermal expansion. For α-eucryptite, we obtained αa = 6.71(±0.25) × 10−6 K−1, αc = 1.07(±0.05) × 10−5 K−1, and αv = 2.42(±0.1) × 10−5 K−1 at 1.94(2) GPa over the temperature range 298–1073 K. For ε-eucryptite at 2.32(8) GPa, we find larger thermal expansion in a smaller temperature range 298–773 K, with αa = 1.47(±0.15) × 10−5 K−1, αb = 6.65(±1.33) × 10−6 K−1, αc = 7.83(±0.88) × 10−6 K−1, and αv = 2.99(±0.15) × 10−5 K−1. In combination with a previous determination of thermal expansion at ambient pressure, the pressure effect on volume thermal expansion of α-eucryptite is determined to be −2.68 × 10−6 GPa−1 K−1, and the temperature derivative of the bulk modulus is estimated to be −0.015 GPa/K.

You do not currently have access to this article.