Neutron diffraction study on the low-temperature phase transitions in Rb2ZnBr4 under hydrostatic pressure

Under hydrostatic pressure, the low-temperature phases of Rb2ZnBr4 was inve stigated by neutron scattering. The pressure coefficient dT/dp at phase III -to-IV was 7 K/kbar, and -45 K/kbar at phase IV-to-V. Three soft phonon mod es which contribute to the phase III-to-IV transition were observed at the S point under the pressure of about 0.7 kbar. The behavior of the modes was similar to that at atmospheric pressure. The extra extinction rule observe d in phase IV was not affected by hydrostatic pressure. At high pressure, t wo high-order pseudo-commensurate phases characterized by wave vectors q(z) similar or equal to 3/11 and 4/15 were observed in addition to the already known stable commensurate ones of q(z) = 2/7 and 1/4.