Thin-Film Stabilization of Metastable Phases in the Sm1-xSrxCuOy System
Abstract
The pulsed laser deposition (PLD) technique has been utilized for thin film synthesis of new metastable phases in the Sm1-xSrxCuOy (y = 2.5-0.5x+ δ) system. The films are grown epitaxially on (100)-oriented SrTiO3 substrates in a low-pressure oxygen ambient with in situ monitoring using reflection high-energy electron diffraction (RHEED). X-ray diffraction and transmission electron microscopy are used for structural characterization of the films. Although the synthesis of SmCuO3-δ—the structural analogue of the defect perovskites LaCuO3-δ(0 ≤ δ ≤ 0.5)—have not been successful, perovskite-type phases have been stabilized with Sr substitution over a relatively wide composition range (0.4 ≤ x ≤ 0.66). With further increase in the Sr content, an ordered 2√2ap × 2√2ap × ap structure with mixed copper valence has been synthesized for x ~ 0.75. Finally, for terminal Sr concentrations (x ≥ 0.9), the doped infinite-layer-type SrCuO2 phases are obtained. The transport properties of the Sm1-xSrxCuOy films are sensitive to the oxygen concentration used during deposition and subsequent cooldown. For films cooled in 760 Torr of O2, a systematic decrease in the resistivity is observed as a function of Sr content, with the lowest resistivity and metallic temperature dependence being observed for the ordered structure. However, films with higher Sr concentrations, with the infinite-layer structure, exhibit relatively high resistivity under these annealing conditions. A significant decrease in the resistivity, along with semimetallic temperature dependence, is observed for these (Sm-doped) films when they are vacuum annealed prior to cooldown. No evidence of superconductivity has been detected in any of the Sm1-xSrxCuOy films down to 5 K. © 1994, American Chemical Society. All rights reserved.