Evidence for an I2/a to Imab phase transition in the silica polymorph moganite at ∼570 K

Rietveld analysis of synchrotron X-ray powder diffraction data for the silica mineral moganite from 100 K to 1354 K has revealed a reversible phase transition from space group l2/a to Imab at approximately 570 K. The thermal expansion behavior of the lattice parameters alters sharply at the transition point, and the monoclinic beta angle decreases to 90 degrees. The displacive transition from alpha- to beta -moganite is effected by the rotation of apparently rigid tetrahedra about the [010] axis, and the linear temperature dependence of the volume strain and of the non-symmetry-breaking e(11) and e(22) strains indicates that the character of the transition is second-order. The continuous increase in the b axis over the entire temperature range reveals a concomitant rotation of tetrahedra about [100] that does not affect the overall symmetry. In addition, we present a refinement without structural constraints for alpha -moganite at room temperature using time-of-flight neutron diffraction data.