Neutron and X-ray Scattering Studies of Rb2CoCl4 and Successive Phase Transition in A2BX4-type Crystals

Soft phonon modes, which are related to the normal–incommensurate phase transition at Ti = 294 K and the lowest-temperature phase transition at T3 = 66 K in Rb2CoCl4, have been measured by inelastic neutron scattering. For the normal–incommensurate phase transition, phonon dispersion curves have been determined in the low-temperature commensurate phases, and the phase mode has been observed far below Ti. The soft mode is fully overdamped above Ti. Moreover, another soft phonon mode, which contributes to the phase transition at T3, has been observed. Furthermore, the space group of the lowest phase is directly confirmed to be C1c1 by verifying extinction rules of X-ray diffraction. It has been recognized that the ratio of the ionic radius of A+ to the average bond length B–X in the tetrahedral BX42− ion can control the phase transition sequence. Rb2CoCl4 belongs to the group in which this ratio is in the range from 0.69 to 0.85, and the transition temperature decreases loosely with increasing the lattice p...