No abstract

It follows from our experimental data that the measurements of the time modulation of a neutron beam gives a real possibility of determining the dynamical effects together with a very small velocity of motion of crystallographic lattice planes of the specimen under investigation. This method can be also applied for the measurements of amplitudes of vibrations of single crystals, excited piezoelectrically or by magnetostriction. The velocities of motion of crystallographic lattice planes are of the order of several tens of cm.sec -1, and the corresponding amplitudes in the range of 10 -4 cm. A qualitative description of the X-ray diffuse scattering from NaNbO3 single crystals as a function of temperature up to 800°C is given. Two types of distinct diffuse scattering where observed in {I00} reciprocal planes and on (100)reciprocal axes. While the diffuse scattering in reciprocal planes shows no critical behaviour at the different phase transitions and can be attributed to a 'linear disorder' similar to that suggested earlier for KNbO3, the diffuse scattering on reciprocal axes is critical in the vicinity of the 641 °C phase transition. The atomic displacements involved with the linear disorder that persists up to 800°C are attributed to the niobium atoms; the critical planar disorder in the cubic phase is attributed to rotations of oxygen octahedra similar to those suggested for SrTiO3 and KMnF3. Both types of disorder exist in the cubic paraelectric phase.

No abstract

No abstract