X-ray studies of structural changes in melanophlogite with varying temperature

Abstract

The a and c unit cell dimensions of melanophlogite (MEP) have been determined in the temperature range −50 to 700 °C, showing a different expansion behavior for the low temperature α-phase. The c-axis length, 2c, which is smaller than a-axis length in the α-phase, shows a steep rise reaching the value of a at the tetragonal (α)-cubic (β) transition temperature at about 65 °C, and then remains nearly constant until about 500 °C, after which contraction occurs. The crystal structures of α-MEP (space group = P42/nbc) and β-MEP (space group = Pm3n) were refined using a least-squares refinement of a harmonic structure factor expression, using single crystal X-ray diffraction data measured at seven temperatures from −50 to 200 °C (four points for the α-phase, and three points for the β-phase). The average Si-O distance decreased from 1.593 A at −50 °C down to 1.573 A at 63 °C (the estimated transition point). It then remained nearly constant in the β-phase. The Si-O bond distance corrected using a simple rigid body motion model remained nearly constant at 1.611 A in the temperature range −50 to 200 °C, indicating that the negative temperature dependence is due to strong distortions of the probability density functions of the O atoms. The atomic mean-square displacement, , of the O atoms increased steeply with increasing temperature up to the α-β transition point. The low-high (α-β) transformation in MEP is driven by a mechanism involving atom disorder beginning in the low-temperature phase.