Neutron powder diffraction study of the ferroelastic phase transition and 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, Na 2 CO 3 , is a second-order proper ferroelastic m=2 transition. The order parameter is characterized by the shear strain epsilon 5 , with a coupled rotation of the carbonate groups. The diffraction peaks show clear evidence for the divergence of the temperature factors at the transition temperature due to the softening of the acoustic modes at small wave vectors. This results in the appearance of broad diffuse scattering at Bragg positions at the expense of the intensities of the Bragg peaks. We call this process 'lattice melting'.