Electrically Driven Phase Changes in the Superionic Conductor Cu16Rb4Cl13I7

Abstract

The highest room temperature conductivity of any solid electrolyte, 0.34 S/cm, was reported for the superionic conductor Cu16Rb4Cl13I7 [1]. The crystal structure of this material has been determined using single-crystal X-ray diffraction [2] and neutron powder diffraction [3, 4], allowing models to be developed for the conduction pathways within the structure. Due to its high conductivity and insensitivity to air and humidity, Cu16Rb4Cl13I7 is particularly interesting as a solid electrolyte for synaptic cells targeted for neuromorphic computing applications [5] and three-terminal, fast-switching memory elements have been developed. The operating principle of these devices is that a voltage applied to a control electrode causes Cu+ to migrate through the solid electrolyte and deposit onto a channel, changing its resistivity as measured using two read terminals. It is important to understand the physical phenomena that underlie copper ion transport in this material in order to optimize properties of the devices such as the on/off ratio and the reliability. We have therefore developed an experimental design for transmission electron microscopy (TEM) to bias devices based on solid electrolytes in situ while imaging the structural changes that take place during operation. We fabricated electron-transparent thin film samples on commercial TEM window chips with a copper control electrode and various materials for a channel placed several hundred nm to one side. The chips are accommodated in a Hummingbird TEM holder with biasing capability. The entire active area of the device is visible and the structural changes both within the electrolyte and at the electrodes can be correlated with current flow. Movies recorded during these experiments show a variety of interlinked, dynamic phenomena. At the channel, copper is deposited as whiskers or nuclei, with morphology depending on the electrode material. Within the electrolyte, an unexpected structural modulation appears when the current density exceeds a certain threshold. Diffraction analysis reveals a phase transformation initiated at the electrode, converting the electrolyte from a polycrystalline structure to a nanocrystalline phase with different atomic spacings. A distinctive bright stripe becomes visible at the phase boundary with contrast that is consistent with the local reduction in ion concentration. We suggest that depletion of copper ions, initiated at the electrode, drives the conversion to a CuRbClI phase of different stoichiometry. Furthermore, we suggest that the lower ion conductivity of the new phase is responsible for the formation of the bright contrast: finite element and Monte Carlo simulations show that a drop in conductivity cause local depletion. The interlinked nature of the phase change, conductivity change, and ion concentration then alter the voltage and current flow at the next current pulse, allowing the structural modulation to propagate across the electrolyte in a series of bright stripes. We suggest that the insights achieved by TEM in terms of the structural stability and phase changes in the superionic conductor Cu16Rb4Cl13I7 help to assess its capabilities as an electrolyte for neuromorphic devices and provide useful guidance for its other applications. [1] Takahashi T., Journal of The Electrochemical Society 1979;126:1654. [2] Geller S, Akridge JR, Wilber SA., Physical Review B 1979;19:5396-402. [3] Kanno R, Ohno K, Kawamoto Y, Takeda Y, Yamamoto O, Kamiyama T, et al., Journal of Solid State Chemistry 1993;102:79-92. [4] Oikawa K, Kamiyama T, Kanno R, Izumi F, Ikeda T, Chakoumakos BC., Materials Science Forum 2004;443-444:337-40. [5] Teodor Todorov, Takashi Ando, Frances M Ross, John A. Ott, Jianshi Tang, Douglas Bishop, John Collins, Vijay Narayanan and John Rozen, ECS Meeting Abstracts 2019.