Neutron scattering study of the incommensurate and commensurate phases ofRb2ZnBr4

Measurements are presented of the diffuse scattering corresponding to order-parameter fluctuations in the incommensurate phase ($200\lesssim T<\mathrm{}355\mathrm{}$ K), and the commensurate phase ($T\lesssim 200$ K) of ${\mathrm{Rb}}_2$Zn${\mathrm{Br}}_4$. The diffuse-scattering distribution is ellipsoidal in shape with the principal axis along the modulation wave vector. The order-parameter fluctuations are quasielastic in both the incommensurate and the commensurate phase. Below the lock-in transition the diffuse-scattering distribution narrows in the $b$ direction, which is the direction of spontaneous polarization in the commensurate phase. In addition to this quasielastic diffuse scattering normal acoustic-phonon branches are observed originating from the satellites. A Landau freeenergy model is presented which is in nearly quantitative agreement with the measured temperature dependence of the modulation wave vector. The Landau free energy predicts the existence of a dielectric polarization wave in the incommensurate phase which has the periodicity of the third harmonic of the primary modulation wave. We have calculated the effect of an applied electric field on the diffraction pattern in the presence of the polarization wave. Measurements of the field dependence of the intensity of first- and higher-order satellites agree with the calculation; i.e., the intensity of the first- and third-order satellites is virtually independent of the field whereas the intensity of the second-order satellite is enhanced by the field.