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

作者：Sasai Kenzo^*; Kofu Maiko; Ibberson Richard M; Hirota Kazuma; Yamaura Jun ichi; Hiroi Zenji; Yamamuro Osamu

来源：Journal of Physics: Condensed Matter , 2010, 22(1): 015403.

DOI：10.1088/0953-8984/22/1/015403

We have carried out adiabatic calorimetric and neutron powder diffraction experiments on the beta-pyrochlore oxide KOs2O6, 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.

• 出版日期2010-1-13