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

Rietveld analysis of synchrotron X-ray powder diffraction data for the sili ca mineral moganite from 100 K to 1354 K has revealed a reversible phase tr ansition from space group l2/a to Imab at approximately 570 K. The thermal expansion behavior of the lattice parameters alters sharply at the transiti on point, and the monoclinic beta angle decreases to 90 degrees. The displa cive transition from alpha- to beta -moganite is effected by the rotation o f apparently rigid tetrahedra about the [010] axis, and the linear temperat ure 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 ra nge reveals a concomitant rotation of tetrahedra about [100] that does not affect the overall symmetry. In addition, we present a refinement without s tructural constraints for alpha -moganite at room temperature using time-of -flight neutron diffraction data.