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Updated: Jun 3, 2026
08:43
Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles
Published on: October 27, 2018
A novel isomorphic phase transition in β-pyrochlore oxide KOs2O6: a study using high resolution neutron powder
Kenzo Sasai1, Maiko Kofu, Richard M Ibberson
1Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan. Department of Earth and Space Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
Summary
We discovered a novel transition in potassium osmium oxide (KOs2O6) at 7.6 K, distinct from its superconductivity. This transition involves unusual lattice expansion and K ion behavior, suggesting cooperative rattling motion condensation.
Area of Science:
- Condensed Matter Physics
- Materials Science
- Solid State Chemistry
Background:
- The β-pyrochlore oxide KOs2O6 exhibits superconductivity at 9.6 K.
- Potassium (K) ions in this material display rattling vibrations within oxygen (O) atom cages at low temperatures.
Purpose of the Study:
- To investigate a novel transition at 7.6 K in KOs2O6.
- To elucidate the nature of this low-temperature transition using calorimetric and diffraction methods.
Main Methods:
- Adiabatic calorimetry was employed to measure heat capacity.
- Neutron powder diffraction experiments were conducted to analyze crystal structure.
- Rietveld analysis was used to refine structural parameters.
Main Results:
- A distinct transition was observed at 7.6 K (T(p)) with an entropy of 0.296 J K(-1) mol(-1).
- Neutron diffraction revealed no structural phase transition, indicating an isomorphic transition.
- Anomalous lattice expansion and O cage deformation occurred below 7.6 K.
- K ion scattering density distribution broadened, yet remained centered.
Conclusions:
- The T(p) transition is an unusual isomorphic transition in KOs2O6.
- It is attributed to cage volume expansion and cooperative condensation of K ions into their rattling ground state.
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