Discovery of a new quantum phase to develop hybrid materials

Scientists have discovered that in Ba_{1 time}sr_XAl₂O₄highly disordered atomic arrangement forms in AlO₄ lattice at chemical compositions close to the structural quantum critical point, resulting from both crystalline and amorphous material characteristics. Credit: Yui Ishii, Osaka Metropolitan University

If you’ve ever seen water turn to ice, you’ve witnessed what physicists call a “phase transition.” Scientists at Osaka Metropolitan University have discovered an unprecedented phase transition in which crystals attain amorphous characteristics while retaining their crystalline properties.

Their discoveries contribute to the development of hybrid materials for use in harsh environments, such as outer space. The results were published in Physical examination B.

A typical phase transition exhibited by crystalline solids involves a change in crystal structure. Such structural phase transitions usually occur at finite temperatures. However, controlling the chemical composition of the crystal can lower the transition temperature to absolute zero (-273°C). The transition point at absolute zero is called the structural quantum critical point.

In the dielectric compound Ba_{1 time}sr_XAl₂O₄, the structural phase transition is driven by an acoustic soft mode, whose atomic vibrational pattern is similar to that of sound waves. The compound includes an AlO₄ tetrahedral lattice and Ba/Sr atoms.

The research team led by Associate Professor Yui Ishii from the Graduate School of Engineering at Osaka Metropolitan University found that a highly disordered atomic arrangement forms in AlO₄ lattice at chemical compositions close to the structural quantum critical point, resulting from both crystalline and amorphous material characteristics.

Ba_{1 time}sr_XAl₂O₄ is a crystalline solid. However, the researchers found that at Sr concentrations higher than the structural quantum critical point, Ba_{1 time}sr_XAl₂O₄ exhibits the thermal characteristic of amorphous materials, i.e. low thermal conductivity comparable to that of glass materials (e.g., silica glass). They observed that part of the atomic structure loses its periodicity due to the incoherently stopped acoustic soft mode. As a result, a combination of a glassy Al-O lattice and a periodic Ba arrangement is realized.

This hybrid state, which the research team was the first to discover, can be created simply by uniformly mixing raw materials and heating them.

Professor Ishii concluded: “In principle, the phenomenon revealed in this research can occur in materials exhibiting soft acoustic modes. Applying this technique to various materials may help us to create hybrid materials that combine the physical properties of crystals, such as optical and electrical properties.conductivity, with the low thermal conductivity of amorphous materials.In addition, the high heat resistance of crystals can be used to develop insulating materials that can be used in environments difficult, such as outer space.