Using the strain to control the properties of the oxynitride

Japanese scientists have come across a simple method to control the introduction of defects, called “vacant layers”, into perovskite oxynithrides, leading to changes in their physical properties. The approach, published in the journal Nature Communications, could help in the development of photocatalysts.

Oxynitrides are inorganic compounds made up of oxygen, nitrogen and other chemical elements. They have gained a lot of attention in recent years due to their interesting properties, with applications in optical and memory devices, and in photocatalytic reactions, for example.

In 2015, solid-state chemist Hiroshi Kageyama of Kyoto University’s Institute for Integrated Sciences of Cellular Materials (iCeMS) and his team reported that they found a way to fabricate oxynitrides using an ammonia treatment process. at a lower temperature than the conventional method which requires more than 1,000 ° C). The new process produced a polycrystalline powder with missing layers of oxygen atoms, known as no-oxygen planes.

The team wanted to examine the physical properties of this oxynitride, so they developed it as a single crystal thin film on a substrate. “But the oxygen-free layers in the resulting film were on a different plane than the original powder,” says Kageyama. They wondered if the underlying substrate affected the orientation of the oxygen-empty layers.

The team developed a vanadium strontium oxide (SrVO3) film on different substrates and treated it in ammonia at a low temperature of 600 ° C. The plane of the vacant oxygen layers and their periodicity – the frequency with which appear within the other layers of the film – they changed according to the degree of discrepancy between the “lattice deformations” in the substrate and the overlying film. Lattice deformation is a force applied by the substrate that causes atoms in a material to be slightly displaced from their normal position.

“Even though solid-state chemists know that defective oxygen planes play an important role in changing the properties of oxides, such as inducing superconductivity, we haven’t been able to control their formation before,” Kageyama says.

Oxides are typically synthesized using high temperature reactions, making it difficult to control their crystal structures. Using a lower temperature and lower voltage in this experiment was the key to success.

“Our team developed a method to create and control the direction and periodicity of the oxygen-free layers in thin-film oxides simply by applying effort,” says Kageyama. “Because the strain energy is enormously large, up to thousands of degrees Celsius, we are able to use it to stabilize new structures that would not otherwise form.”

Kageyama says it would be interesting to investigate how changes in oxide film thickness, or temperature and reaction time, could also affect the orientation and periodicity of the oxygen-free layers.

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DOI: 10.1038 / s41467-020-19217-7

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