Crystalline solids that exhibit the typical phase transition involve a change in the 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.

Scientists from Osaka Metropolitan University have uncovered a previously unrecognized phase transition in which crystals acquire amorphous properties while keeping their crystalline ones. Their research helps create hybrid materials used in harsh environments like space.

The structural phase change in the dielectric compound Ba1-xSrxAl2O4 is driven by a soft acoustic mode, whose atomic vibration pattern resembles sound waves. Ba/Sr atoms and an AlO4 tetrahedral network make up the molecule.

Scientists discovered that a highly disordered atomic arrangement is formed in the AlO4 network at chemical compositions near the structural quantum critical point, resulting in both crystalline and amorphous materials characteristics.

Ba1-xSrxAl2O4 is a crystalline solid. However, the scientists discovered that Ba1-xSrxAl2O4 exhibits the thermal characteristic of amorphous materials, i.e., low thermal conductivity equivalent to that of glass materials, at greater Sr concentrations than the structural quantum critical point (e.g., silica glass). They noticed that the incoherently stopped acoustic soft mode causes a portion of the atomic structure to lose periodicity. As a result, a periodic Ba arrangement combined with a glassy Al-O network is realized.

Scientists are the first to discover this hybrid state. It can be created simply by mixing raw materials uniformly and heating them.

Associate Professor Yui Ishii from the Graduate School of Engineering at Osaka Metropolitan University said, “In principle, the phenomenon revealed in this research can occur in materials exhibiting soft acoustic modes. Applying this technique to various materials will possibly help us create hybrid materials that combine the physical properties of crystals, such as optical properties and electrical conductivity, with the low thermal conductivity of amorphous materials. In addition, the high heat resistance of crystals can be utilized to develop insulation materials that can be used in harsh environments, such as outer space.”

Journal Reference:

  1. Y. Ishii et al. Partial breakdown of translation symmetry at a structural quantum critical point associated with a ferroelectric soft mode. Physical Review B. DOI: 10.1103/PhysRevB.106.134111