Neutron scattering studies of structural phase transitions at Brookhaven

Systematic neutron scattering studies on phase transitions have been carried out at Brookhaven in recent years. A consistent picture has now emerged from these investigations of "how" certain structural phase transitions take place. These are caused by a lattice dynamical instability; namely, the frequency of a particular lattice vibrational mode tends toward zero as the transition temperature is approached from above. At the transition these displacements are incorporated into the new atomic positions of the low-temperature phase. Modes of this type have become known as "soft" modes. This concept of soft modes was first emphasized by Cochran in 1960 for the special case of the BaTi${\mathrm{O}}_3$-type ferroelectrics. We have demonstrated that the generalized concept applies to many other types of structural phase transitions such as SrTi${\mathrm{O}}_3$(110°K), ${\mathrm{Nb}}_3$Sn, Si${\mathrm{O}}_2$ and N${\mathrm{D}}_4$Br. The neutron scattering technique has been particularly useful in obtaining detailed knowledge of dynamical properties near the phase transition. This is due to a fortunate combination of the wavelength of typical thermal neutrons (2.5Å) and their energy (13 meV). The phase transition studies, in turn, have contributed to considerable improvement of the inelastic neutron scattering technique, which may offer many future applications.