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

Abstract

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 β) every 50 K in this temperature range. We observed that a and b parameters increase with increasing temperature, while c decreases. The β angle also decreases with temperature and approaches 120°. 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 × 10⁻⁶ K⁻¹ at 100 K to 9.3 × 10⁻⁶ K⁻¹ at 600 K and remains nearly constant above this temperature. The Suzuki model based on the zero-pressure Mie–Grüneisen equation of state was implemented to fit the unit cell volume data. The refined parameters are ${V_0}$ = 546.30(2) ų, $Q$ = 7.20(12) × 10⁶ J/mol and ${\theta _{\text{D}}}$ = 1018(43) K, where ${\theta _{\text{D}}}$ is the Debye temperature and ${V_0}$ is the unit cell volume at 0 K with an assumption that ${K^\prime }$ is equal to 1.8. The obtained Debye temperature is consistent with that determined in a previous study for heat capacity measurements.