A novel isomorphic phase transition inβ-pyrochloreoxide KOs2O6: a study using high resolution neutron powder diffraction

Abstract

We have carried out adiabatic calorimetric and neutron powder diffraction experiments on theβ-pyrochloreoxide KOs2O6, which has a superconducting transition atTc = 9.6 K and anothernovel transition at Tp = 7.6 K. A characteristic feature of this compound is that the K ions exhibit rattling vibrations in thecages formed by O atoms even at very low temperatures. The temperature and entropy of theTp transition is in good agreement with previous data measured using a heat relaxationmethod, 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 nopeak splitting nor extra peaks over the temperature range between 2 and 295 K, suggesting that theTp transition is a rather unusual isomorphic transition. Rietveld analysis revealedan anomalous expansion of the lattice and a deformation of the O atom cagebelow 7.6 K. In the low-temperature phase, the distribution of scatteringdensity corresponding to the K ions becomes broader whilst maintaining itsmaximum at the cage center. Based on these findings, we suggest that theTp transition is due to the expansion of the cage volume and cooperative condensation of theK ions into the ground state of the rattling motion.