Some materials can morph into multiple crystal structures with very different properties, such as squeezing a soft form of carbon to produce diamond. The Kurt Vonnegut novel “Cat’s Cradle” featured ice-nine, a fictional form of water with a much higher melting point than regular ice that threatened to irreversibly freeze all the water on Earth.

These are polymorphs – materials usually made by applying pressure. Materials engineers also seek to do the reverse, or apply negative pressure to stretch a material’s crystal structure into a new architecture. This approach has required negative pressures that are difficult to achieve in a laboratory setting, and it risks pulling the material apart.

A new route to realizing the equivalent of negative pressure involves mixing two materials together under the right conditions to make an alloy with an airier and entirely different crystal structure and unique properties. The alloy developed by researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) has more space between its atoms then either of its parent materials, as if it had been stretched out. Unlike its parents, the new material is piezoelectric, and can generate an electric charge in response to applied mechanical stress, a property of interest for use in sensors and actuators.

Continuing their recent work on mixing compounds with atomic structures that didn't match, the researchers theorized that mixing two different high-density structures would take more energy than forming a third low-density structure due to less competition for space between atoms in low-density structures. They proved this hypothesis by mixing the high-density forms of manganese selenide (MnSe) and manganese telluride (MnTe) that have different crystal structures in an approach known as heterostructural alloying. The result was a heterostructural alloy, Mn(Se,Te), with a third low-density type of crystal structure that would otherwise require impractically large negative pressure to achieve.

The research is published in Science Advances.

Researchers demonstrated a way to use negative pressure to combine two dense materials into an alloy (right) with a much lower density. The spheres represent atoms of manganese (blue), selenium (red) and tellurium (gold). Source: NREL