A crystalline material has atoms systematically arranged in repeating units, with this structure and the elements it contains determining the material’s properties. For example, silicon’s crystal structure allows it to be widely used in the semiconductor industry, whereas graphite’s soft, layered structure makes for great pencils. One class of crystalline materials that are critical for a wide range of applications, ranging from battery technology to electrolysis of water (i.e., splitting H₂O into its component hydrogen and oxygen), are crystalline metal oxides, which have repeating units of oxygen and metals. Researchers suspect that there is a significant number of crystalline metal oxides that could prove to be useful, but their number and the extent of their useful properties is unknown.

In “Discovery of complex oxides via automated experiments and data science”, a collaborative effort with partners at the Joint Center for Artificial Photosynthesis (JCAP), a Department of Energy (DOE) Energy Innovation Hub at Caltech, we present a systematic search for new complex crystalline metal oxides using a novel approach for rapid materials synthesis and characterization. Using a customized inkjet printer to print samples with different ratios of metals, we were able to generate more than 350k distinct compositions, a number of which we discovered had interesting properties. One example, based on cobalt, tantalum and tin, exhibited tunable transparency, catalytic activity, and stability in strong acid electrolytes, a rare combination of properties of importance for renewable energy technologies. To stimulate continued research in this field, we are releasing a database consisting of nine channels of optical absorption measurements, which can be used as an indicator of interesting properties, across 376,752 distinct compositions of 108 3-metal oxide systems, along with model results that identify the most promising compositions for a variety of technical applications.

Background
There are on the order of 100 properties of interest in materials science that are relevant to enhancing existing technologies and to creating new ones, ranging from electrical, optical, and magnetic to thermal and mechanical. Traditionally, exploring materials for a target technology involves considering only one or a few such properties at a time, resulting in many parallel efforts where the same materials are being evaluated. Machine learning (ML) for material properties prediction has been successfully deployed in many of these parallel efforts, but the models are inherently specialized and fail to capture the universality of the prediction problem. Instead of asking traditional questions of how ML can help find a suitable material for a particular property, we instead apply ML to find a short-list of materials that may be exceptional for any given property. This strategy combines high throughput materials experiments with a physics-aware data science workflow.

A challenge in realizing this strategy is that the search space for new crystalline metal oxides is enormous. For example, the Inorganic Crystal Structure Database (ICSD) lists 73 metals that exist in oxides composed of a single metal and oxygen. Generating novel compounds simply by making various combinations of these metals would yield 62,196 possible 3-metal oxide systems, some of which will contain several unique structures. If, in addition, one were to vary the relative quantities of each metal, the set of possible combinations would be orders of magnitude larger.

However, while this search space is large, only a small fraction of these novel compositions will form new crystalline structures, with the majority simply resulting in combinations of existing structures. While these combinations of structures may be interesting for some applications, the goal is to find the core single-structure compositions. Of the possible 3-metal oxide systems, the ICSD reports only 2,205 with experimentally confirmed compositions, indicating that the vast majority of possible compositions either have not been explored or have yielded negative results and have not been published. In the present work we do not directly measure the crystal structures of new materials, but instead use high throughput experiments to enable ML-based inferences of where new structures can be found.

Synthesis
Our goal was to explore a large swath of chemical space as quickly as possible. Whereas traditional synthesis techniques like physical vapor deposition can create high quality thin films, we decided to reuse an existing technology that was already optimized to mix and deposit small amounts of material very quickly: an inkjet printer. We made each metal element printable by dissolving a metal nitrate or metal chloride into an ink solution. We then printed a series of lines on glass plates, where the ratios of the elements used in the printing varied along each line according to our experiment design so that we could generate thousands of unique compositions per plate. Several such plates were then dried and baked together in a series of ovens to oxidize the metals. Due to the inherent variability in the printing, drying, and baking of the plates, we opted to print 10 duplicates of each composition. Even with this level of replication, we still were able to generate novel compositions 100x faster than traditional vapor deposition techniques.

The modified professional grade inkjet printer.

Top: A printed and baked plate that is 10 x 15 cm. Bottom: A close-up of a portion of the plate. Since the optical properties vary with composition, the gradient in composition appears as a color gradient along each line.

Characterization
When making samples at this rate, it is hard to find a characterization technique that can keep up. A traditional approach to design a material for a specific purpose would require significant time to measure the pertinent properties of each combination, but for the analysis to keep up with our high-throughput printing method, we needed something faster. So, we built a custom microscope capable of taking pictures at nine discrete wavelengths ranging from the ultraviolet (385 nm), through the visible, to the infrared (850 nm). This microscope produced over 20 TB of image data over the course of the project, which we used to calculate the optical absorption coefficients of each sample at each wavelength. While optical absorption itself is important for technologies such as solar energy harvesting, in our work we are interested in optical absorption vs. wavelength as a fingerprint of each material.

Analysis
After generating 376,752 distinct compositions, we needed to know which ones were actually interesting. We hypothesized that since the structure of a material determines its properties, when a material property (in this case, the optical absorption spectrum) changes in a nontrivial way, that could indicate a structural change. To test this, we built two ML models to identify potentially interesting compositions.

As the composition of metals changes in a metal oxide, the crystal structure of the resulting material may change. The map of the compositions that crystallize into the same structure, which we call the phase, is the “phase diagram”. The first model, the ‘phase diagram’ model, is a physics-based model that assumes thermodynamic equilibrium, which imposes limits on the number of phases that can coexist. Assuming that the optical properties of a combination of crystalline phases vary linearly with the ratio of each crystalline phase, the model generates a set of phases that best fit the optical absorption spectra. The phase diagram model involved a comprehensive search through the space of thermodynamically allowed phase diagrams. The second model seeks to identify “emergent properties” by identifying 3-metal oxide absorption spectra that can not be explained by a linear combination of 1-metal or 2-metal oxide signals.

Phase analysis of compounds with different relative fractions of the metals iron (Fe), tin (Sn) and yttrium (Y). Left: Panels showing the absorption coefficient at different wavelengths: a) 375 nm; b) 530 nm; c) 660 nm, d) 850 nm. Right: Based on the absorption, the phase diagram model identifies the boundaries at which changes in the relative composition in the compound lead to different optical properties and hence suggest compositions with potentially interesting behavior. In panels e), f) and g), red points are candidate phases, and vertices where blue lines meet indicate interesting phase behavior. Panel h) shows the emergent property model, where compositions are colored by the log-likelihood of their properties being explainable by lower-order compositions (darker colors are more likely to represent more interesting compounds).

Experimental Verification
In the end our systematic, combinatorial sweep of 108 3-metal oxide systems found 51 of these systems exhibited interesting behavior. Of these 108 systems, only 1 of them has an experimentally reported entry in the ICSD. We performed an in-depth experimental study of one unexplored system, the Co-Ta-Sn oxides. With guidance from the high throughput workflow, we validated the discovery of a new family of solid solutions by x-ray diffraction, successfully resynthesized the new materials using a common technique (physical vapor deposition), validated the surprisingly high transparency in compositions with up to 30% Co, and performed follow-up electrochemical testing that demonstrated electrocatalytic activity for water oxidation (a critical step in hydrogen fuel synthesis from water). Catalyst testing for water oxidation is far more expensive than the optical screening from our high throughput workflow, and even though there is no known connection between the optical properties and the catalytic properties, we use the analysis of optical properties to select a small number of compositions for catalyst testing, demonstrating our high level concept of using one high throughput workflow to down-select materials for practically any target technology.

Conclusions
The Co-Ta-Sn oxide example illustrates how finding new materials quickly is an important step in developing improved technologies, such as those critical for hydrogen production. We hope this work inspires the materials community — for the experimentalists, we hope to inspire creativity in aggressively scaling high-throughput techniques, and for computationalists, we hope to provide a rich dataset with plenty of negative results to better inform ML and other data science models.

Acknowledgements
It was a pleasure and a privilege to work with John Gregoire and Joel Haber at Caltech for this complex, long-running project. Additionally, we would like to thank Zan Armstrong, Sam Yang, Kevin Kan, Lan Zhou, Matthias Richter, Chris Roat, Nick Wagner, Marc Coram, Marc Berndl, Pat Riley, and Ted Baltz for their contributions.