A novel isomorphic phase transition in β-pyrochlore oxide KOs2O6: a study using high resolution neutron powder diffraction.

We have carried out adiabatic calorimetric and neutron powder diffraction experiments on the β-pyrochlore oxide KOs(2)O(6), which has a superconducting transition at T(c) = 9.6 K and another novel transition at T(p) = 7.6 K. A characteristic feature of this compound is that the K ions exhibit rattling vibrations in the cages formed by O atoms even at very low temperatures. The temperature and entropy of the T(p) transition is in good agreement with previous data measured using a heat relaxation method, indicating that the present sample is of high purity and the transition entropy, 0.296 J K(-1) mol(-1), does not depend on the calorimetric method used. The neutron powder diffraction data show no peak splitting nor extra peaks over the temperature range between 2 and 295 K, suggesting that the T(p) transition is a rather unusual isomorphic transition. Rietveld analysis revealed an anomalous expansion of the lattice and a deformation of the O atom cage below 7.6 K. In the low-temperature phase, the distribution of scattering density corresponding to the K ions becomes broader whilst maintaining its maximum at the cage center. Based on these findings, we suggest that the T(p) transition is due to the expansion of the cage volume and cooperative condensation of the K ions into the ground state of the rattling motion.