Electric-field control of tri-state phase transformation with a selective dual-ion switch

Materials are described here that can change their crystalline phase in response to the electrically controlled insertion or extraction of oxygen and hydrogen ions, giving rise to three distinct phases with different optical, electrical and magnetic properties. Materials that change their phase in response to the electrical injection or extraction of an ionic species are harnessed in a wide range of applications, including batteries that can operate in a range of climates and smart windows that can control the amount of light or heat that passes through them. As Nianpeng Lu et al. report, increasing the number of transferrable ionic species can greatly enhance the functionality of the system. Specifically, they report a material system in which the electrical insertion and extraction of oxygen (O2−) and hydrogen (H+) ions can be independently controlled, giving reversible access to three distinct material phases that have very different optical, electrical and magnetic properties. This finding could further broaden the range of potential applications for phase-changing materials. 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.