Thermal expansion of coesite determined by synchrotron powder X-ray diffraction

Thermal expansion of synthetic coesite was studied with synchrotron powder X-ray diffraction in the temperature range of 100-1000 K. We determined the unit cell parameters of monoclinic coesite (a, b, c, and beta) every 50 K in this temperature range. We observed that a and b parameters increase with increasing temperature, while c decreases. The beta angle also decreases with temperature and approaches 120 degrees. As a result, the unit cell volume expands by only 0.7% in this temperature range. Our measurements provide thermal expansion coefficients of coesite as a function of temperature: it increases from 3.4 x 10(-6) K-1 at 100 K to 9.3 x 10(-6) K-1 at 600 K and remains nearly constant above this temperature. The Suzuki model based on the zero-pressure Mie-Gruneisen equation of state was implemented to fit the unit cell volume data. The refined parameters are V-0 = 546.30(2) angstrom(3), Q = 7.20(12) x 10(6) J/mol and theta(D) = 1018(43) K, where theta(D) is the Debye temperature and V-0 is the unit cell volume at 0 K with an assumption that K' is equal to 1.8. The obtained Debye temperature is consistent with that determined in a previous study for heat capacity measurements.