Neutron and X-ray scattering studies of structural phase transitions and soft modes in Rb_2ZnBr_4

Creators Shigematsu H.

Creators Mashiyama H.

Creators Oohara Y.

Creators Ohsima K.

The low-temperature phase transitions of rubidium tetrabromozincate have been investigated with the use of neutron and X-ray scattering techniques to clarify the mechanism of the transitions and to determine the space groups of the low-temperature phases. The low-frequency excitations which soften at the phase transition T_3 ＝ 112 K have been measured by the inelastic neutron scattering technique. On the other hand, no soft phonon mode which makes a contribution to the phase Ⅳ-to-Ⅴ transition at T_4 ＝ 76 K has been observed. Both in phases Ⅳ and Ⅴ, superlatticereflections appear at the position (h)/(2) (k)/(2) (l)/(3), where the Miller indices h, k and l are referred to the lattice constants of phase Ⅰ. In phase Ⅴ, new Bragg reflections emerge at the positon hk0: h + k ＝ 2n + 1 and increase continuously with decreasing temperature. The space groups are considered to be P11b with Z ＝ 24 for phase Ⅳ and C1c1 with Z ＝ 48 for phase Ⅴ according to general group theoretical considerations. Furthermore, structural models for phase Ⅳ are proposed to explain the extra extinction rule, the antiferroelectricity and other observed character of the crystal.

[ISSN]0953-8984

[NCID]AA10672168

https://doi.org/10.1088/0953-8984/10/26/014

[isPartOf] [URI]http://www.iop.org/EJ/journal/JPhysCM