Pre-transitional behavior in cubic potassium niobate

Pre-transitional behavior prior to the onset of the cubic-to-tetragonal phase transformation in KNbO₃ has been studied with synchrotron x-ray scattering at NSLS, Brookhaven National Lab. Measurements of the Thermal Diffuse Scattering (TDS), Grazing Incidence X-ray Scattering (GIXS), and precursor Bragg scattering have been performed at a series of temperatures on a single crystal sample in the cubic phase (T_c = 405 °C). The TDS was measured in the (001), (011) and (111) cubic planes. The GIXS was measured above, at and below the critical scattering angle of the sample. Based on localized soft mode theory, dynamic tetragonal fluctuations may be expected prior to the onset of the phase transition initiated by defects or surfaces. The GIXS data indicate that the tetragonal phase is apparent prior to the phase transition of the bulk -- consistent with the notion that nucleation originates from potent defect sites during a martensitic-like displacive transformation. The TDS intensity distribution is disk-shaped; arising from a localized, low energy transverse optic mode in the [100] directions extending out to the Brillouin zone boundary (reduced wave vector, q = 0.5). Scans made in the (011) planes show TDS intensity extending out to a reduced wave vector of q - 0.2. Preliminary analysis indicates that a low energy transverse optic mode extends in the [011] direction as well. The TDS intensity decreases as the transition temperature is approached from above. Comparison of the TDS results with inelastic neutron scattering data will be made.