New mechanism revealed on the anomalous thermal expansion behavior of NbO2F

Abstract

The near-zero thermal expansion of TaO₂F across a broad temperature range indicates its potential application in components requiring high thermal shock resistance. Nevertheless, the structurally analogous NbO₂F maintains low thermal expansion only at high temperatures and demonstrates significant positive thermal expansion at low temperatures, yet the underlying reasons for this anomalous behavior have remained elusive. In this study, we utilize neutron powder diffraction and synchrotron X-ray diffraction to uncover, for the first time, a temperature-induced structural phase transition in NbO₂F, shifting from a cubic to a rhombohedral phase. This discovery establishes a direct link between the phase transition and the anomalous thermal expansion behavior, resolving a long-standing scientific puzzle. Additionally, we construct a detailed pressure-temperature phase diagram, providing critical insights into the thermal properties of NbO₂F under varying conditions, offering valuable guidance for the design of materials with tailored thermal properties.