Crystallography, microstructure and morphology of Mg4Nb2O9/MgO and Mg4Ta2O9/MgO interfaces formed by topotaxial solid state reactions

Mg4Nb2O9/MgO and Mg4Ta2O9/MgO interfaces of definite crystallography were formed by topotaxial thin film solid state reactions in the systems MgO–Nb2O5 and MgO–Ta2O5. MgO (0 0 1) single crystal substrates, heated to different temperatures, were subjected to Nb–O and Ta–O vapors generated by e-beam evaporation in high vacuum. Thin films mainly containing the phases Mg4Nb2O9 and MgNb2O6, respectively Mg4Ta2O9 and MgTa2O6, were formed by gas–solid reactions. The crystallographic relationships between the product phases and the MgO substrate were systematically studied by X-ray diffractometry and transmission electron microscopy (TEM). Surprisingly pole figure analysis revealed more than one orientation relationship for some of the phases: Mg4Nb2O9 and Mg4Ta2O9 grew with (1 1 .4), (1 1 .6) and (1 1 .9) orientations, depending on temperature. Selected area diffraction patterns and high resolution TEM images show that these three orientations have a common Mg4(Nb/Ta)2O9 [ 1   1 ¯   .0 ] /MgO [ 1   1 ¯   0 ] axis and differ by the angle between the Mg4(Nb/Ta)2O9 (0 0 .1) and MgO (1 1 1) planes. Crystallographic illustrations of this phenomenon are given, and possible origins and consequences for the solid state reaction are discussed. Indications for two different interfacial reaction mechanisms are found.