Incommensurate phases in barium sodium niobate

The x-ray-diffraction spectra of ${\mathrm{Ba}}_2$Na${\mathrm{Nb}}_5$ ${\mathrm{O}}_{15}$ have been investigated by means of a precession camera between room temperature and 500°C, across the orthorhombic-tetragonal transition near $T_{\mathrm{I}}=300\mathrm{}$°C. Long exposures have revealed reflections, onsetting with the orthorhombic phase, which are incommensurate with respect to the periodicity of the basic crystal lattice. Referred to the pseudotetragonal lattice vectors, the modulation vector is $\overrightarrow{\mathrm{k}}=\frac{(1+\delta )({\overrightarrow{\mathrm{a}}}^{*}+{\overrightarrow{\mathrm{b}}}^{*})}{4}+\frac{c^{*}}{2}$ where $\delta$ increases, on heating, from a value of 0.01 at room temperature, up to 0.115 at $T_{\mathrm{I}}$. Its variations exhibit an upward jump at $T_{\mathrm{II}}=275\mathrm{}$°C from 0.035 to 0.085. These results show for the first time, the existence of two incommensurate phases in barium sodium niobate. These incommensurate phases have the same modulation direction. They both have an average orthorhombic symmetry and are therefore ferroelastic on symmetry grounds. Above $T_{\mathrm{I}}$, precursor effects are disclosed in the form of strong diffuse scattering rods which condense at $T_{\mathrm{I}}$ into the satellite reflections. Birefringence and DTA (differential thermal analysis) measurements are consistent with the x-ray results.