NEUTRON POWDER DIFFRACTION STUDY OF THE FERROELASTIC PHASE-TRANSITIONAND LATTICE MELTING IN SODIUM-CARBONATE, NA2CO3

Detailed neutron powder diffraction data are presented which show that the phase transition (P6(3)/mmc-C2/m) at 754 K in sodium carbonate, N a2CO3, is a second-order proper ferroelastic m = 2 transition. The ord er parameter is characterized by the shear strain epsilon(5), with a c oupled rotation of the carbonate groups. The diffraction peaks show cl ear evidence for the divergence of the temperature factors at the tran sition temperature due to the softening of the acoustic modes at small wave vectors. This results in the appearance of broad diffuse scatter ing at Bragg positions at the expense of the intensities of the Bragg peaks. We call this process 'lattice melting'.