Double-cell superstructure and vacancy ordering in tensile-strained metallic thin films of Pr0.50Ca0.50CoO3 on LaAlO3

Abstract

The Pr-based cobaltate $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3}$ (PCCO) presents in bulk form a singular simultaneous valence and spin-state transition that turns the metallic state into insulator, and displays a large and ultrafast photoresponse in the insulating phase. Epitaxial thin films of PCCO have been grown by deposition on $\mathrm{LaAl}{\mathrm{O}}_{3}$(001) (LAO) substrate, chosen to minimize the mismatch with the film. The films grow epitaxially with two times the substrate periodicity $(2{a}_{0}\ifmmode\times\else\texttimes\fi{}2{a}_{0}\ifmmode\times\else\texttimes\fi{}2{a}_{0})$ and the long perovskite axis perpendicular to the surface. We report a reduction of the structural symmetry from Pnma (bulk) to $P{2}_{1}{2}_{1}{2}_{1}$ (film). The $P{2}_{1}{2}_{1}{2}_{1}$ symmetry revealed by synchrotron x ray remains at low temperatures. These PCCO films are metallic, and ferromagnetic below ${T}_{\mathrm{C}}=170\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, confirming the stabilization of excited $\mathrm{C}{\mathrm{o}}^{3+}$ spin states and the suppression of the concurrent Co spin-state, valence, and metal-insulator transitions. $Z$-contrast imaging and electron-energy-loss spectroscopy elemental maps reveal long-range ordered oxygen vacancy planes unexpectedly stacking parallel to the interface, in spite of the tensile character of the PCCO/LAO heterostructure. In contrast to the general tendency reported for strained $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ (LSCO) films, we show that a nominal tensile strain can be also compatible with the presence of alternating O vacancy planes parallel to the interface, instead of perpendicular to it. That occurs thanks to the double cell of the film and the formation of the (1/2, 1/2, 1/2) superstructure studied in this work. These results expand the possibilities of controlling interfacial physical properties via engineering of ordered oxygen-defect structures in strongly correlated oxides.