A new polymorph of eucryptite (LiAlSiO 4 ), ε-eucryptite, and thermal expansion of α- and ε-eucryptite at high pressure

X-ray diffraction experiments have been carried out on b-eucryptite (LiAlSiO 4 ) 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 e-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) Å 3 for XRD data at 2.2 GPa and 298 K. On heating at 2.2 GPa, e-eucryptite and beucryptite were metastable over the temperature interval 298-873 K; at higher temperatures they underwent an irreversible phase transition to a-eucryptite. Both hexagonal α-eucryptite and ε-eucryptite show anisotropic thermal expansion. For α-eucryptite, we obtained α a = 6.71(±0.25) × 10 -6 K -1 , α c = 1.07(±0.05) × 10 -5 K -1 , and α v = 2.42(±0.1) x 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 α a = 1.47(±0.15) × 10 -5 K -1 , α b = 6.65(±1.33) x 10 -6 K -1 , α c = 7.83(±0.88) × 10 -6 K -1 , and α v = 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 a-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.