A novel isomorphic phase transition in β-pyrochlore oxide KOs ₂O₆: A study using high resolution neutron powder diffraction

We have carried out adiabatic calorimetric and neutron powder diffraction experiments on the β-pyrochlore oxide KOs₂O₆, which has a superconducting transition at T_c = 9.6K and another novel transition at T_p = 7.6K. 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, 0.296JK^-1mol^-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 295K, 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 7.6K. 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.