Growth behaviors of epitaxial barium molybdate (BaMoO<sub>3</sub>, BaMoO<sub>4</sub>) film

Abstract

Transition metal oxides have been a research hotspot for basic scientific research and frontier applications. Owing to the presence of d<italic/> electrons and strong electron correlation, a wealth of physical phenomena emerges in the transition metal oxide family. In particular, extremely fruitful research progress is achieved in a 3d orbital elemental system. In comparison, the 4d transition metal oxides need more attention. Molybdate has excellent optical and electrical properties. Among <i>A</i>MoO<sub>3</sub> (<i>A</i> = Ca, Sr, Ba), only BaMoO<sub>3</sub> has not been reported for epitaxial films to date. In this work, high-quality epitaxial films of BaMoO<sub>3</sub> and BaMoO<sub>4</sub> are prepared by using the pulsed laser deposition. We conduct the oxygen partial pressure modulation experiments and the results show that the growth of BaMoO<sub>3</sub> is sensitive to oxygen partial pressure. Also, BaMoO<sub>3</sub> has a geometrically similar lattice structure to BaMoO<sub>4</sub>, and there exists epitaxial competition between BaMoO<sub>3</sub> and BaMoO<sub>4</sub>. These two points make the preparation of epitaxial BaMoO<sub>3</sub> films more challenging. The key to the preparation of epitaxial BaMoO<sub>3</sub> thin films is the reduced laser target material, high vacuum environment, and ultra-low oxygen partial pressure. The epitaxy competition can be avoided by using the SrTiO<sub>3</sub> (111) substrate. We conduct oxygen partial pressure modulation experiments on a narrow scale and reveal a self-assembled superlattice of epitaxial BaMoO<sub>3</sub> film on a SrTiO<sub>3</sub>(111) substrate. Both the satellite peaks in the XRD pattern and the HRTEM results indicate the superlattice period of about 7.04 Å. The oxygen partial pressure is the only parameter that regulates this phenomenon, so we presume that the essence of the self-assembled superlattice is periodic oxygen-induced lattice defects. Finally, electrical transport characterization experiments are conducted on representative BaMoO<sub>3</sub> films. The <inline-formula><tex-math id="M1">\begin{document}$\rho \text{-} T$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220736_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220736_M1.png"/></alternatives></inline-formula> curve measurements and fitting results show that the epitaxial BaMoO<sub>3</sub> films on SrTiO<sub>3</sub>(001) substrates have better conductivities. The electrical transport properties of BaMoO<sub>3</sub> films grown on SrTiO<sub>3</sub>(111) substrates are dominated by electron-phonon scattering, and BaMoO<sub>3</sub> films grown on SrTiO<sub>3</sub>(001) substrate have stronger electron-electron scattering interactions. The resistivity of the self-assembled superlattice BaMoO<sub>3</sub> films is relatively high and electron-electron scattering plays an important role in determining the electrical transport property.