Phase stability of epitaxial KTa_xNb_1-xO₃ thin films deposited by metalorganic chemical vapor deposition

The phase stability of epitaxial KTa_xNb_1-xO₃ (0 ≤ x ≤ 1) thin films, with compositions over the entire solid solution range, was investigated. KTa_xNb_1-xO₃ thin films were deposited on (100) MgAl₂O₄ substrates by metalorganic chemical vapor deposition. Films with compositions x ≤ 0.30 were orthorhombic, as determined by x-ray diffraction. Dielectric measurements at room temperature indicated the presence of morphotropic phase boundaries at x = 0.30 and at x = 0.74. Temperature-dependent measurements of the dielectric constant for KNbO₃ from 80 to 800 K indicated three structural phase transitions at 710, 520, and 240 K. For intermediate compositions, a decrease in the Curie and tetragonal-orthorhombic transition temperatures was observed with increasing Ta atomic percent, similar to the bulk phase equilibrium. In contrast to bulk materials, an increase in the orthorhombic-rhombohedral transition temperature with increasing x was observed for the films, resulting in the stabilization of a rhombohedral phase at room temperature for compositions 0.45 ≤ x ≤0.73. Differences between the phase stability for the thin films and bulk were attributed to lattice misfit strain.