Successive structural phase transitions in Na/sub x/WO3

Three successive phase transitions, cubic I to tetragonal I to tetragonal II to cubic II, in the perovskite Na/sub x/WO3 have been studied by neutron scattering. The softening of the M3 phonon modes which correspond to a rotation of the WO6 octahedra is observed in the cubic I phase, and the tetragonal I phase is confirmed to be a state resulting from a single M3-mode condensation. Powder diffraction studies have shown that the other two low-temperature phases also result from M3-mode condensations, confirming the structural model proposed by Clarke. Although at the second and the third transitions the axes of the resultant rotations change abruptly, the measured order parameter, which is proportional to the absolute rotation angle, does not have any appreciable discontinuity. This fact and the three successive M3-mode condensations are unique compared with other successive phase transitions in insulating perovskite materials