Crystal structure of the pyrochlore oxide superconductor KOs2O6

Abstract

We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs 2 O 6 . The structure was identified as the b-pyrochlore structure with space group Fd3m and lattice constant a ¼ 10:089ð2ÞÅ at 300 K: the K atom is located at the 8b site, not at the 16d site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter U iso ¼ 0:0735ð8ÞÅ 2 at 300 K for the K cation, which suggests that the K cation weakly bound to an oversized Os 12 O 18 cage exhibits intensive rattling, as recently observed for clathrate compounds. The rattling of A cations is a common feature in the series of bpyrochlore oxide superconductors AOs 2 O 6 (A ¼ Cs, Rb and K), and is greatest for the smallest K cation. r 2005 Elsevier Inc. All rights reserved.

NMR Relaxation and Resistivity from Rattling Phonons in Pyrochlore Superconductors

“…Bandstructure calculations have indicated that the anomalies, in particular in KOs 2 O 6 , might be due to a highly anharmonic low frequency rattling motion of the alkali-ion inside an oversized cage formed by the Os and O ions [7,8]. This is consistent with X-ray observations of anomalously large atomic displacements for the K ions [9] and low frequency phonon structures seen in photoemission spectroscopy [10].Recent observations of NMR relaxation rates 1/T 1 T at the K site have been demonstrated to be entirely dominated by the vibrations of the K ion via coupling of the electric field gradient to the nuclear quadrupole moment [11]. Such a domination of phonons for nuclear relaxation usually occurs in diamagnetic insulators, but is extremely rare in metals and was attributed to the rattling motion of the K ions.…”

“…Bandstructure calculations have indicated that the anomalies, in particular in KOs 2 O 6 , might be due to a highly anharmonic low frequency rattling motion of the alkali-ion inside an oversized cage formed by the Os and O ions [7,8]. This is consistent with X-ray observations of anomalously large atomic displacements for the K ions [9] and low frequency phonon structures seen in photoemission spectroscopy [10].…”

Temperature-dependent Raman and x-ray studies of the spin-ice pyrochloreDy2Ti2O7and nonmagnetic pyrochlore

“…The most anomalous phonon is the taken from the single-crystal x-ray analysis of ref. [43] with that in the pyrochlore T b 2 T i 2 O 7 for which < u 2 >, obtained from neutron diffraction data, is available between 4.5 K and 600 K [44]. We find that the ratio of square-root of vibrational amplitude of oxygen to the bond length d A−O (A = K or Tb), for the two compounds, have nearly the same magnitude (∼ 3%).…”

Green's Function of Fully Anharmonic Lattice Vibration

“…As the radius of the guest ion gets smaller, it can move around in a larger area subject to weaker restoring force around its center and consequently shows larger atomic displacement. 1) Resistivity of KOs 2 O 6 shows concave-downward temperature dependence.…”