Raman phonons and ageing-related surface disorder in NaxCoO2

Abstract The polarized Raman spectra from ab and ac surfaces of single crystal Na x CoO 2 ( x ≈0.7), parent compound of recently discovered superconductor Na x CoO 2 · y H 2 O, are reported and discussed. The crystals were hexagon platelets of typical size 3 × 3 × 0.1 mm. Three of the five (A 1g +E 1g +3E 2g ) Raman active phonons were unambiguously identified at 458 (E 1g ), 494 (E 2g ) and 574 (A 1g ) cm −1 . The spectra from ab and ac surfaces differ significantly and provide evidence that within hours after preparation the ac surface, unlike the ab one, is strongly disordered. Within several days the disorder extends over the ab surface too.