Simultaneous analysis of changes in long-range and short-range structural order at the displacive phase transition in quartz

A new look at the displacive phase transition in quartz is reported, using neutron total diffraction experiments and a new implementation of the Reverse Monte Carlo method in which explicit account is taken of the Bragg peak intensities. This approach yields information about short-range and long-range details of the phase transition simultaneously, and reproduces both aspects of the structure correctly and self-consistently. This study gives, for the first time, a visualization of what actually happens at the phase transition. The picture which emerges is that the symmetry-change associated with the phase transition allows the excitation of many low-frequency high-amplitude modes of vibration which create considerable orientational disorder of the SiO₄ tetrahedra.