Interplay between Cation and Charge Ordering in La1/3Sr2/3FeO3 Superlattices

DOI: 10.1002/aelm.201500372 superlattices, in particular the (LaMnO3)m/ (SrMnO3)n system, has yielded important insights into the relationships between charge transfer, electronic structure, cation disorder, and magnetic ordering.[11–15] Ferroelectric superlattices have also received considerable attention,[16–18] with short-period superlattices emerging as a platform in which to realize improper ferroelectrics.[19,20] In contrast, superlattice analogs of materials that undergo first-order electronic phase transitions, such as charge ordering, remain largely unexplored. The iron-based perovskite La1/3Sr2/3FeO3 (LSFO) is an intriguing material system in which to explore the relationship between cation ordering and charge ordering. Bulk LSFO exhibits an abrupt increase in resistivity at T* ≈ 195 K, due to the onset of a nominal charge-ordered state with an ordering wavevector of q = [1/3 1/3 1/3].[21–23] Previous experimental and theoretical work has provided evidence that the charge ordering occurs on both Fe and O sites due to the hybridization of Fe 3d and O 2p bands in La1−xSrxFeO3. Concurrent with the charge-ordering transition is the onset of antiferromagnetism, in which a relatively long-period ↑↑↑↓↓↓ spin structure is stabilized along the [111] direction.[27] The spin configuration has been shown to play a key role in stabilizing the charge-ordered state.[28] While these transitions have been studied in both bulk and thin film materials,[29–34] cation-ordered equivalents of LSFO have yet to be reported, even though the 1:2 La:Sr cation ratio lends itself to short period superlattices, such as (LaFeO3 (LFO))n/( SrFeO3 (SFO))2n. Using a combined experimental and computational approach, we have investigated the electronic properties of cation-ordered analogs of LSFO. We have synthesized shortperiod superlattices via oxide molecular beam epitaxy (MBE) in which the arrangement of LFO and SFO layers is systematically varied. While LFO and SFO films exhibit insulating and metallic behavior, respectively, the superlattices display a charge-ordering transition as indicated by a discontinuity in the temperature-dependent resistivity, the magnitude of which decreases as the interfacial density is reduced. Density functional theory (DFT) calculations provide a model to interpret the charge-ordering transition, and the layer-resolved oxygen The electronic properties of digital superlattices are reported, which are cation-ordered analogs of the perovskite La1/3Sr2/3FeO3, a material that undergoes a charge-ordering transition. Superlattices of LaFeO3 (LFO), an antiferromagnetic insulator, and SrFeO3 (SFO), a conductor with a helical magnetic ground state, are fabricated via oxide molecular beam epitaxy. Three isocompositional superlattices with repeat structures of SSLSSL (S2), SSSLSL (S3), and SSSSLL (S4) (S = SFO, L = LFO) are studied with cation orderings along the [001] and [111] directions for experimental and computational work, respectively. The experimental superlattice structures are confirmed via synchrotron X-ray diffraction and corresponding simulations of (00L) crystal truncation rods. The S2 and S3 superlattices are found to undergo an electronic phase transition as measured by a discontinuity in the temperature-dependent resistivity similar to the random alloy, indicating that the superlattices do not behave as a simple combination of LFO and SFO. The charge-ordering transition is not observed in the S4 sample. The electronic structure calculations using density functional theory, confirming the energetic favorability of charge ordering in the S2 and S3 structures compared to the S4 structure, are consistent with experimental trends.