Topotaxy of Corundum-Type Tetramagnesium Diniobate and Ditantalate Layers on Rock-Salt-Type Magnesium Oxide Substrates

In-plane alignments were found to occur in the product layers of corundum-type Mg4Nb2O9 (MNO) and Mg4Ta2O9 (MTO) on rock-salt-type MgO(001) substrates as a result of a topotaxial interface reaction between a Nb2O5 (or Ta2O5) precursor and MgO at 800°C and higher. Such alignments were attributed to a topotaxial formation of (114)-oriented MNO (or MTO) layers. This (114) orientation is hidden in the conventional XRD theta-2theta scans due to an overlap of the film (114) peak with the substrate peak, MgO(002). Its presence was revealed by XRD pole-figure measurements. The crystallographic relationship between MNO(114) [or MTO(114)] and MgO(001) is also discussed.