Topotaxial formation of Mg4Ta2O9 and MgTa2O6 thin films by vapour-solid reactions on MgO (001) crystals

Thin-film solid state reactions in the system MgO–Ta2O5 were experimentally studied, subjecting MgO (001) substrates to Ta–O vapours at different temperatures in a high vacuum system. Thin films mainly containing the phases MgTa2O6 and Mg4Ta2O9 formed by reactions between the Ta–O vapours and the magnesium oxide. The crystallographic orientations of these phases were studied by X-ray diffractometry including pole figure analysis. Mg4Ta2O9 was observed to grow (11.4)-, (11.6)- and (11.9)-oriented, while MgTa2O6 had two preferential orientations, viz. (430) and (214). The crystallographic relationships between the reaction products and the MgO substrate and their possible origins are discussed.