Structure and ferroelectric properties of thin heteroepitaxial NaNbO$_{3}$ films obtained by RF cathode sputtering

Thin films of sodium niobate (NaNbO3) on a MgO(001) substrate with a predeposited SrRuO$_3$ layer were obtained for the first time by the method of RF cathode sputtering in an oxygen atmosphere. X-ray diffraction data showed the obtained films to be single-phase and single-crystalline. The parameters of the unit cells of NaNbO$_3$ and SrRuO$_3$ layers in the tetragonal approximation were found to be $c_{\mathrm{NaNbO}_3}$ = 0.3940 (1) nm, $a_{\mathrm{NaNbO}_3}$ = 0.389 (1) nm; $c_{\mathrm{SrRuO_3}}$ = 0.4004 (1) nm, and $a_{\mathrm{SrRuO}_3}$ = 0.392 (3) nm. Misfit strain of the unit cell of NaNbO$_3$ amounted to $\varepsilon_{33}$ = 0.007 and $\varepsilon_{11}$ = 0.002. The results of dielectric and piezoelectric measurements showed that the films occurred in a ferroelectric state.