Materials can be transformed from one crystalline phase to another by using an electric field to control ion transfer, in a process that can be harnessed in applications such as batteries1, smart windows2 and fuel cells3. Increasing the number of transferrable ion species and of accessible crystalline phases could in principle greatly enrich material functionality. However, studies have so far focused mainly on the evolution and control of single ionic species (for example, oxygen, hydrogen or lithium ions4,5,6,7,8,9,10). Here we describe the reversible and non-volatile electric-field control of dual-ion (oxygen and hydrogen) phase transformations, with associated electrochromic2 and magnetoelectric11 effects. We show that controlling the insertion and extraction of oxygen and hydrogen ions independently of each other can direct reversible phase transformations among three different material phases: the perovskite SrCoO3−δ (ref. 12), the brownmillerite SrCoO2.5 (ref. 13), and a hitherto-unexplored phase, HSrCoO2.5. By analysing the distinct optical absorption properties of these phases, we demonstrate selective manipulation of spectral transparency in the visible-light and infrared regions, revealing a dual-band electrochromic effect that could see application in smart windows2,9. Moreover, the starkly different magnetic and electric properties of the three phases—HSrCoO2.5 is a weakly ferromagnetic insulator, SrCoO3−δ is a ferromagnetic metal12, and SrCoO2.5 is an antiferromagnetic insulator13—enable an unusual form of magnetoelectric coupling, allowing electric-field control of three different magnetic ground states. These findings open up opportunities for the electric-field control of multistate phase transformations with rich functionalities.
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This study was financially supported by the National Basic Research Program of China (grants 2015CB921700, 2016YFA0301004 and 2015CB921002); the National Natural Science Foundation of China (grants 11274194, 51561145005, 51332001 and 11334006); the Initiative Research Projects of Tsinghua University (grant 20141081116); and the Beijing Advanced Innovation Center for Future Chip (ICFC). L.G. acknowledges support from the National Program on Key Basic Research Project (2014CB921002) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB07030200) and National Natural Science Foundation of China (grants 51522212, 51421002 and 51672307). The Advanced Light Source is supported by the US Department of Energy under contract no. DE-AC02-05CH11231.
The authors declare no competing financial interests.
Reviewer Information Nature thanks D. Milliron, S. Ramanathan and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, c, e, Reciprocal space mapping of thin films of a, SrCoO2.5, c, SrCoO3−δ and e, phase A (HSrCoO2.5) in the (103) crystalline plane of the LSAT substrate. Å−1 represents the reciprocal space units, and Q100 and Q001 represent projected directions in the reciprocal space. b, d, f, XRD rocking curves around the peaks of b, SrCoO2.5 (008), d, SrCoO3−δ (002) and f, phase A (HSrCoO2.5) (008).
a–d, In situ XRD results obtained near the LSAT(002) peak during phase transformations from: a, SrCoO2.5 to phase A (HSrCoO2.5); b, phase A (HSrCoO2.5) to SrCoO2.5; c, SrCoO2.5 to SrCoO3−δ; and d, SrCoO3−δ to SrCoO2.5, with fixed gating voltages of +3.5 V, −2.3 V, −2.7 V and +1.8 V, respectively. The dashed horizontal lines indicate when the phase transformation occurred. e–h, Summarized gating durations for transformations from: e, SrCoO2.5 to phase A (HSrCoO2.5); f, phase A (HSrCoO2.5) to SrCoO2.5; g, SrCoO2.5 to SrCoO3−δ; and h, SrCoO3−δ to SrCoO2.5, with different gating voltages. The gating duration is defined as the time required for the phase transformation to complete. All gating durations of less than 2,000 minutes were obtained from the in situ XRD measurements shown in a–d; for extended gating durations (more than 2,000 minutes), the data were obtained from the ex situ gating experiments (e, g). The black squares in e and g indicate that the initial SrCoO2.5 phase remains unchanged even after a gating duration of 2,880 minutes. Red and blue points correspond to negative and positive gating voltages, respectively. The vertical dashed lines indicate the electrolysis voltage of water. i, X-ray photoelectron spectroscopy measurements of the as-grown SrCoO2.5 (green) and ILG-induced SrCoO3−δ (red) and HSrCoO2.5 (blue) phases, grown on LSAT(001) substrate. The peaks represent the electron-binding energy of the specific atomic orbitals in selected elements. LMM and KLL refer to states in Auger electron spectroscopy. j, k, Reversible electric-field-induced, well defined transformations between SrCoO2.5 and phase A (HSrCoO2.5) (j) and between SrCoO2.5 and SrCoO3−δ (k) at selected gating voltages. The films of material were 50 nm thick. The gating experiments were performed in air with the as-received ionic liquid DEME-TFSI.
a, In situ XRD measurements of phase transformations in films with thicknesses of 20 nm, 50 nm and 100 nm, grown on LSAT(001) substrate. The top and bottom panels show ILG with positive and negative voltages, respectively. b, Thickness dependence of gating durations for films grown on the LSAT(001) substrate. c, In situ XRD measurements during phase transformations for films with different strain states. The compressive strain is calculated on the basis of the difference in lattice constant between the substrates (LaAlO3, LSAT and SrTiO3) and the bulk SrCoO2.5 phase. d, Strain dependence of gating durations.
a, HAADF-STEM results with the calculated lattice structures (shown in the insets) of SrCoO2.5 and SrCoO3−δ. The zone axis is along the  direction of the LSAT substrate. b, Statistical analysis of the in-plane strontium–strontium interatomic distances for the three phases, and their comparison with theoretical calculations. c, Chemical expansion during phase transformations. d, Energy-dispersive X-ray spectroscopy results for as-grown SrCoO2.5 (green) as well as ILG-induced SrCoO3−δ (red) and HSrCoO2.5 (blue), grown on LSAT(001) substrate. Error bars represent the standard deviation of the measurements.
a–c, Three possible lattice configurations for SrCoO2, based on first-principle calculations, with: a, parallel oxygen-vacancy channels; b, orthogonal oxygen-vacancy channels; or c, infinite planar lattice structure. Calculated lattice constants and total energies are also given. For reference, the lattice constants and total energy of SrCoO2.5 are: a = 5.65 Å; b = 5.51 Å; c = 15.76 Å; Etotal = −218.30 eV. d–g, Calculated crystalline lattice configurations for HySrCoO2.5 when y equals: d, 0.125; e, 0.25; f, 0.5; and g, 1.0.
a, b, XRD θ–2θ scans for thin films of a, HSrCoO2.5 and b, SrCoO3−δ grown on LSAT(001) substrate, as a function of time after ILG-induced formation followed by storage at room temperature (25 °C) and in a 1 atm air environment with relative humidity of 40%. The diffraction peaks of HSrCoO2.5 remain nearly unchanged even after 11 days, suggesting that this phase is a robust equilibrium state. Meanwhile, for SrCoO3−δ, a negligible shift in the peak position (of about 0.1°) was observed after 11 days, suggesting that this phase still holds the structure of perovskite, with the occurrence of a tiny amount of oxygen vacancies. (Interestingly, with a similar hydrogenated system (HVO2), the structure gradually returns to that of the dehydrogenated VO2 phase after 8 days at room temperature, owing to the slow release of hydrogen18.) c, d, In situ temperature-dependent XRD θ–2θ scans for thin films of c, HSrCoO2.5 and d, SrCoO3−δ around the LSAT(002) peak, annealed in a 1 atm oxygen gas (oxidizing) or a 1 atm argon gas (reducing) environment, respectively. The crystalline structures of both phases remain stable up to 175–195 °C, above which they change into the brownmillerite SrCoO2.5. The SrCoO2.5 phase is a thermodynamic equilibrium state, which is stable at temperatures up to 350 °C in mild oxidizing or reducing environments, owing to the robustness of the Co3+ valence state.
a, Proposed origin of H+ and O2− ions during ILG: the water inside the ionic liquid is decomposed into negatively charged O2− and positively charged H+ ions through electrolysis. b, Gating current (IG) as a function of gating voltage (VG). c, Comparison of SIMS spectra around m/z ≈ 2.0 for samples gated with the as-received ionic liquid DEME-TFSI containing water (H2O), or the developed ionic liquids EMIM-BF4 and DEME-TFSI containing heavy water (D2O). d, Depth profiles of the D+ ions in films gated with the developed ionic liquids containing heavy water.
a–d, XRD θ–2θ scans for typical as-grown SrCoO2.5 samples (green), and after positive (blue) and negative (red) voltage gating using the following hydrophobic ionic liquids: a, EMIM-TFSI, and b, HMIM-TFSI; also with c, the hydrophilic EMIM-BF4; as well as with d, the aqueous electrolyte NaOH (1 mol per litre). e, Residual water concentrations of ionic liquids, determined by the Karl Fischer titration method. Quoted errors represent the standard deviation of the measurements. The results show that the ILG-induced phase transformations occur with all of these ionic liquids, and that even the hydrophobic ionic liquids have enough residual water to achieve the phase transformation, confirming the generic nature of the ILG-induced phase transformations.
a, b, Time evolution of optical transmission spectra from 50-nm-thick SrCoO2.5 films grown on double-sided polished LSAT(001) substrate, with gating voltages of a, +3.5 V and b, −2.7 V. The results show clear phase transformations from SrCoO2.5 to HSrCoO2.5 (a), and from SrCoO2.5 to SrCoO3−δ (b). VIS, visible light; NIR, near-infrared.
Extended Data Figure 10 Comparison of the optical absorptions and calculated densities-of-states for the three phases.
a, Optical absorptions of the three phases, plotting α (the optical absorption) versus ħω (the photon energy). Below a photon energy of 4.0 eV, there are two main absorption features in all three phases, which can be attributed to the intraband d–d transition (lower-energy end; the absorption peaks α, σ and δ) and interband p–d transition (higher-energy end; the absorption peaks β, ε and γ). Consistent with our electrical transport studies, we find that the SrCoO3−δ is metallic, with strong absorption among the whole optical range studied here. However, both SrCoO2.5 and HSrCoO2.5 show insulating behaviour, with strong absorption spectra emerging around the interband transitions (β and ε). Furthermore, the absorption by SrCoO2.5 is even greater than that of the metallic SrCoO3−δ phase at higher energies (greater than 2.5 eV), owing to the large p–d excitation. However, the absorption by HSrCoO2.5 is strongly suppressed compared with that by SrCoO2.5, owing to enhancement of the direct bandgap. b–d, Calculated total and projected density of states (DOS) onto oxygen 2p, CoT (tetragonal layer) and CoO (octahedral layer) orbitals for: b, SrCoO2.5; c, HSrCoO2.5; and d, SrCoO3. In comparison with SrCoO2.5, the bandgap of HSrCoO2.5 becomes larger, owing to the ascension of the unoccupied cobalt 3d state. For SrCoO3, the Fermi energy level cuts into the oxygen 2p and cobalt 3d DOS, consistent with the metallic nature of this phase suggested by our electrical transport studies.
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Lu, N., Zhang, P., Zhang, Q. et al. Electric-field control of tri-state phase transformation with a selective dual-ion switch. Nature 546, 124–128 (2017). https://doi.org/10.1038/nature22389
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