A new polymorph of eucryptite (LiAlSi[O.sub.4]), [epsilon]-eucryptite, and thermal expansion of [alpha]- and [epsilon]-eucryptite at high pressure

X-ray diffraction experiments have been carried out on [beta]-eucryptite (LiAlSi[O.sub.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 [epsilon]-eucryptite, can be indexed according to an orthorhombic unit cell with a = 10.217(4) [Angstrom], b = 8.487(4), [Angstrom] c = 5.751(3) [Angstrom], and V = 498.7(4) [[Angstrom].sup.3] for XRD data at 2.2 GPa and 298 K. On heating at 2.2 GPa, [epsilon]-eucryptite and [beta]-eucryptite were metastable over the temperature interval 298-873 K; at higher temperatures they underwent an irreversible phase transition to [alpha]-eucryptite. Both hexagonal [alpha]-eucryptite and [epsilon]-eucryptite show anisotropic thermal expansion. For [alpha]-eucryptite, we obtained [[alpha].sub.a] = 6.71([+ or -]0.25) x [10.sup.-6] [K.sup.-1], [[alpha].sub.c] = 1.07([+ or -]0.05) x [10.sup.-5] [K.sup.-1], and [[alpha].sub.v] = 2.42([+ or -]0.1) x [10.sup.-5] [K.sup.-1] at 1.94(2) GPa over the temperature range 298-1073 K, For [epsilon]-eucryptite at 2.32(8) GPa, we find larger thermal expansion in a smaller temperature range 298-773 K, with [[alpha].sub.a] = 1.47([+ or -]0.15) x [10.sup.-5] [K.sup.-1], [[alpha].sub.b] = 6.65([+ or -]1.33) x [10.sup.-6] [K.sup.-1], [[alpha].sub.c] = 7.83([+ or -]0.88) x [10.sup.-6] [K.sup.-1], and [[alpha].sub.v] = 2.99([+ or -]0.15) x [10.sup.-5] [K.sup.-1]. In combination with a previous determination of thermal expansion at ambient pressure, the pressure effect on volume thermal expansion of [alpha]-eucryptite is determined to be -2.68 x [10.sup.-6] [GPa.sup.-1] [K.sup.-l], and the temperature derivative of the bulk modulus is estimated to be -0.015 GPa/K.