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Electronic switches based on the migration of high-density point defects, or memristors, are poised to revolutionize post-digital electronics. Despite significant research, key mechanisms for filament formation and oxygen transport remain unresolved, thus hindering our ability to predict and design crucial device properties. For example, predicted retention times based on current models can be 10 orders of magnitude lower than ones experimentally realized. Here, using electrical measurements, chemical spectroscopy, and first-principles calculations on tantalum oxide memristors, we reveal that the formation and stability of conductive filaments crucially depend on the stability of the amorphous oxygen-rich and oxygen-poor compounds, which undergo composition phase separation. Including the previously neglected effects of this amorphous phase separation reconciles unexplained discrepancies and enables predictive design of key performance indicators such as retention stability. This result emphasizes non-ideal thermodynamic interactions as key design criteria in post-digital devices with defect densities substantially exceeding those of today’s covalent semiconductors.

The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, plays a key role in many symmetry-lowering crystal deformations1. Lattices of Jahn-Teller ions can induce a cooperative distortion, as exemplified by LaMnO3 (refs.2–4). While many examples occur in octahedrally5 or tetrahedrally6 coordinated transition metal oxides due to their high orbital degeneracy, this effect has yet to be manifested for square-planar coordination, as found in infinite-layer Cu (refs.7-9), Ni (refs.10–12), Fe (refs.13–15), and Mn oxides16. Here we synthesize single-crystal CaCoO2 thin films by topotactic reduction of the Brownmillerite CaCoO2.5 phase. We observe a dramatically distorted infinite-layer structure, with Angstrom-scale displacements of the cations from their high-symmetry positions. This can be understood to originate from the Jahn-Teller degeneracy of the dxz and dyz-orbitals in the d7 electronic configuration along with substantial ligand-transition metal mixing. A complex pattern of distortions arises in a 2√2× 2√2×1 tetragonal supercell, reflecting the competition between an ordered Jahn-Teller effect of the CoO2 sublattice, and geometric frustration of the associated displacements of the Ca sublattice. As a result of this competition, the CaCoO2 structure forms an extended two-in/two-out type of Co distortion following “ice rules”17.